REVIEW 
Annals of Nuclear Medicine Vol. 
15, No. 
6, 471~L86, 2001 
From tumor biology to clinical PET: A review of positron emission tomography (PET) in oncology* 
Kazuo KUBOTA 
Department of Nuclear Medicine and Radiology, Institute of Development, Aging and Cancer (IDAC), Tohoku University 
Cancer cells show increased metabolism of both glucose and amino acids, which can be monitored with 18F-2-deoxy-2-fluoro-D-glucose (FDG), a glucose analogue, and 11C-L-methionine (Met), respectively. FDG uptake is higher in fast-growing than in slow-growing tumors. FDG uptake is considered to be a good marker of the grade of malignancy. Several studies have indicated that the degree of FDG uptake in primary lung cancer can be used as a prognostic indicator. Differential diagnosis of lung tumors has been studied extensively with both computed tomography (CT) and positron emission tomography (PET). It has been established that FDG-PET is clinically very useful and that its diagnostic accuracy is higher than that of CT. Detection of lymph node or distant metastases in known cancer patients using a whole-body imaging technique with FDG-PET has become a good indication for PET. FDG uptake may be seen in a variety of tissues due to physiological glucose consumption. Also FDG uptake is not specific for cancer. Various types of active inflammation showed FDG uptake to a certain high level. Understanding of the physiological and benign causes of FDG uptake is important for accurate interpretation of FDG-PET. In monitoring radio/chemotherapy, changes in FDG uptake correlate with the number of viable cancer cells, whereas Met is a marker of proliferation. Reduction of FDG uptake is a sensitive marker of viable tissue, preceding necrotic extension and volumetric shrinkage. FDG-PET is useful for the detection of recurrence and for monitoring the therapeutic response of tumor tissues in various cancers, including those of the lung, colon, and head and neck. Thus, PET, particularly with FDG, is effective in monitoring cancer cell viability, and is clinically very useful for the diagnosis and detection of recurrence of lung and other cancers. 
Key words: positron emission tomography, tumor diagnosis, lung cancer, autoradiography, 18F-fluorodeoxyglucose, 11C-methionine, radiotherapy monitoring, tumor hypoxia, 18F-fluoromisonidazole 
INTRODUCTION 
POSITRON EMISSION TOMOGRAPHY (PET), especially with 18F-2-deoxy-2-fluoro-D-glucose (FDG) has been used for the diagnosis of cancers in various organs. Its diagnostic accuracy is generally higher than that of the conventional imaging technique of CT/magnetic resonance imaging 
* This review is based on an invited lecture at the 39th Annual Meeting of the Japanese Society of Nuclear Medicine in Akita City, October 1999. Received August 10, 2001. For reprint contact: Kazuo Kubota, M.D., Ph.D., Department of Nuclear Medicine and Radiology, Institute of Development, Aging and Cancer (IDAO, Tohoku University, Seiryoucho 4-1 , Aoba-ku, Sendai 980-8575, JAPAN. E-mail: kkubota@idac.tohoku,ac.jp 
(MRI), and PET results may facilitate more appropriate therapeutic planning for cancer patients. Because PET visualizes molecular events in living human tissue, an understanding of the biological characteristics of tumor tissue, and its correlation to tracer uptake, is essential for oncological application of PET. In this review, I focus on the principles of cancer cell metabolism and its correlation with tracer uptake, the distribution of tracers in whole-body imaging and examples of clinical studies, the structure of tumor tissue, effects of therapeutic intervention, and detection of hypoxia. This review is not intended to cover cancers in all organs, but will aid in understanding the physiological and pathological background of tracer uptake and its clinical application. After reading this review and gaining a degree of knowledge about PET in oncology, I recommend to the reader several other reviews that have listed clinical data for a wide variety of cancers.1-8 
Cancer Cell Metabolism, Proliferation and Tracers 
Glucose metabolism of cancer cells 
An enhanced glycolytic rate in cancer cells was first demonstrated more than 70 years ago. Originally, decreased respiration and an increase in both aerobic and anaerobic glycolysis were considered to be the most important and specific characteristics of cancer cells.9 Considerable efforts have been devoted to elucidating the role of increased glycolysis in malignant cell proliferation. Studies using Morris hepatoma cell lines revealed that the degree of increased glycolysis and the activity of key enzymes in glycolysis, such as hexokinase, correlated with the rate of tumor growth.10 But none have conclusively determined whether a high glycolytic rate is essential for cancer cells or is a consequence of other metabolic processes. It was later demonstrated that many, but not all, tumor cells and some proliferating normal cells exhibited high rates of glycolysis, and that increased glycolysis was neither an essential property of proliferating cells nor a distinction between malignancy and benignancy.11 More recently, increased glucose transport has been studied in malignant transformed cells.12 Malignant transformation by oncogenes coincides with high levels of glucose transporter messenger RNA.13,14 Analyses of resected human cancer tissues have demonstrated increased expression of glucose transporters in brain tumors15 and various abdominal malignancies 16 2-deoxyglucose (2DG) was originally studied as an anticancer drug to inhibit glucose metabolism of tumors 17,18 but this aim was abandoned due to significant side-effects associated with high phannacological doses.19 It has since been used as a 14C-labeled tracer for glucose metabolism in research.20 After successful labeling with 18F to form 18F-2-deoxy-2-fluoro-D-glucose (FDG, Fig. 1),21 FDG has been used for tumor imaging.22-24 FDG is transported into the cell through the glucose transporter protein at the cell membrane, depending on the concentration gradient of glucose from outside to inside the cell. This process does not require energy (ATP). In the cell, FDG is phosphorylated to FDG-6-phosphate by hexokinase and ATP, a rate-limiting step in glycolysis. FDG-6-phosphate is not metabolized further in the glycolytic pathway. The cell membrane is not permeable to intermediate phosphorylated products, so that FDG-6-phosphate remains trapped inside the cell, in a process called "metabolic trapping." The reverse reaction from FDG-6-phos-phate to FDG, mediated by glucose-6-phosphatase, is possible only in the liver and epithelia of the renal tubules and small intestine (Fig. 2). Accumulation of FDG by trapping is observed in the brain, myocardium and tumor tissues. FDG uptake reflects accelerated transport of glucose and increased activity of hexokinase.25 Various metabolic substrates labeled with 11C, 18F, or 13N have been tested for tumor imaging, and, of these. FDG has become the most important and useful tracer for that indication. In vitro studies of various human cancer cell lines have revealed a strong correlation between FDG uptake and glucose transporter expression, but no correlation between 26,27 FDG uptake and the level of hexokinase activity A strong correlation between glucose transporter expression and FDG uptake has also been reported in breast 28,29 cancer and in non-small cell lung cancer (NSCLC) These findings suggest that glucose transport may be important in characterizing the tumor uptake of FDG. This mechanism in cancer cells appears to differ from that in the brain, where the rate-limiting step for FDG uptake is hexokinase activity. 
Amino acid metabolism of cancer cells 
In addition to glucose, amino acids are important metabolic substrates for cancer cells. Incorporation of amino acids into the protein fraction has been correlated with tumor growth rate 30 Increased transport of amino acids by viral transformed malignant cells was demonstrated in vitro.31,32 Many cancer cell lines are dependent on methionine in vitro culture media due to increased transmethylation.33 PET studies have demonstrated that 11C-L-methionine (Met) is useful for imaging lung cancers,34,35 breast cancers36 and malignant melanoma.37 Met uptake reflects amino acid transport, trans-methylation and protein synthesis in tumor tissue.38 Met uptake by tumor tissue plateaus early, at about 15-30 min after injection. In the early phase, a significant amount of free Met is pooled inside the tumor tissue. After this early phase, time-related incorporation of the cellular pool of free Met into macromolecules, protein, Iipids, RNA and DNA continues until 1 hr or more after injection (Fig. 3).39 Protein-bound Met also appears in the blood from about 0.5 hr after injection, and its percentages varies from patient to patient. Subtraction of the protein-bound Met activity from the total blood activity is important to ascertain the true input function for quantitative evaluation of Met uptake by means of a Patlak plot.40,41 Proliferation and FDG or Met Both FDG and Met are good markers of tumor viability. Among tumors with different growth rates, FDG and Met uptake is higher in faster-growing than in slow-growing tumors.39,42 The uptake difference between the faster- and slow-growing tumors is largest with FDG, and therefore FDG uptake is considered to be a good marker of the grade of malignancy (Fig. 4).39,43,44 In proliferating cancer cells in vivo, FDG uptake exhibit cell cycle dependency. Higher uptake is observed in the cells of G0/G1 and G2 phases than in the S and M phases. Cycling cells may consume glucose during the G0/G1 phase in preparation for the S (DNA synthesis) phase, and during the G2 (gap 2) phase in preparation for the M phase (mitosis).45 Among tumors with the same growth rate, FDG uptake may be higher in those exhibiting undifferentiated histology than in well-differentiated tumors.46 Met or thymidine uptake has been reported to correlate more directly with proliferation. Cancer cells had the highest uptake of Met or thymidine in the early exponential growth phase, but low uptake in the plateau phase. FDG uptake had a different pattern and was more closely correlated with the number of viable cells.47,48 
Thymidine and its analogues 
Thymidine is incorporated into DNA synthesis, and it has been labeled with 11C for PET imaging. But the short half-life (20 min) and rapid in vivo degradation of 11C have restricted the use and quantitative evaluation of DNA synthesis rate with 11C-thymidine. [18F]-3'-deoxy-3'-fluorothymidine (18F-FLT) has been developed with the aim of metabolically trapping the substrate within the cell after phosphorylation by thymidine kinase 1 (TK). Preliminary in vivo evaluation of 18F-FLT with PET gave promising results, both in animal studies and in cancer patients.49 Further evaluation in a large numbers of patients and a comparison with FDG are necessary. 
Whole-Body Distribution of FDG and Met 
Whole-body distribution FDG is transported, phosphorylated, and trapped as FDG-6-phosphate in the brain, heart and tumor tissues. In the liver, FDG-6-phosphate is dephosphorylated by glucose-6-phosphatase, and excreted.22 Liver FDG uptake reduces with time, and high tumor to liver contrast is expected at 1 hr after injection 24 Tumors in the abdomen may be better visualized with a lower background by using FDG rather than Met. Glucose is not excreted in urine, but its analogue FDG is renally excreted, and therefore bladder activity may disturb the imaging of pelvic tumors.50 The high FDG uptake of the normal brain may also interfere with detection of small metastatic tumors in the brain (Fig. 5). High physiological accumulation of Met is observed in the liver and pancreas due to synthesis of digestive and other enzymes. Moderate uptake may be observed in salivary glands. Generally speaking, Met uptake by normal tissues in the head and chest is low. Therefore, high contrast imaging of tumors in the brain, neck and chest is expected with Met.35-37,51 Because of the low Met uptake by normal brain, Met-PET is particularly useful in delineating the extent of invasion of brain tumors.52 Peak time of uptake and delayed imaging The tracer uptake and clearance of Met are faster than those of FDG. Tumor uptake of Met plateaus at about 15-30 min after injection, while that of FDG plateaus after more than I hr has elapsed. PET analysis of breast cancer revealed that the tumor to non-tumor ratio was significantly higher in 3 hr images than in 1.5 hr images 53 A dynamic FDG-PET study of lung cancer demonstrated the peak of FDG uptake at around 2-2.5 hr.54 Extrapolation from dynamic FDG-PET studies of lung cancer showed that tumor uptake reached a peak level at around 4-6 hr 55 In our study, we compared whole-body FDG-PET images at 1 and 2 hr after injection, in patients with lung cancer and malignant lymphoma. We found that, (a) all malignant tumors exhibited a higher FDG uptake at 2 hr than at 1 hr, (b) most normal tissues exhibited a lower FDG uptake at 2 hr than at I hr, and (c) consequently, the tumor to background contrast was enhanced at 2 hrs, and the sensitivity was improved (Figs. 6, 7).56 Delayed imaging may suffer from higher noise due to the radioactivity decay of 18F, and high sensitivity of the PET scanner detector is therefore extremely important. High sensitivity with 3D data acquisition may be helpful for delayed tumor imaging. Effects of blood glucose Distribution of FDG is affected by blood glucose levels, because of the competition with glucose as a metabolic substrate. In hyperglycemia (after feeding or in diabetics) FDG uptake by tumor tissue is reduced,57 but FDG uptake by myocardium and skeletal muscle is increased.58,59 FDG uptake by the brain is also decreased in hyperglycemia 57 With glucose loading, a greater reduction in uptake by normal brain than by tumor resulted in enhancement of tumor to normal cortex contrast.60 But increased FDG uptake by skeletal muscle and decreased tumor uptake resulted in lower tumor to muscle contrast in body tumors in hyperglycemic patients.61,62 Hyperglycemia could be responsible for false-negative FDG uptake by tumors in the body. Tumor uptake of Met is also affected by amino acid levels and by ingestion of food.63 A fasting protocol is generally recommended for oncology applications of PET with FDG or Met. If the patient is diabetic, FDG-PET should be re-scheduled after the hyperglycemia has been controlled. But injection of insulin simultaneously with FDG should be avoided as it leads to increased accumulation of FDG in skeletal muscle, and thus less FDG is available for accumulation in the tumor.64 
Clinical Diagnosis of Lung Cancer with PET 
The CT scan has played an important role in the diagnosis and staging of lung tumors. Since CT can detect calcification, an important sign of a benign tumor, if the tumor has soft-tissue attenuation without calcification, the differential diagnosis of cancer from a benign lesion is difficult. CT can provide excellent anatomic information, but not metabolic or pathophysiologic information about the lesion. To predict the nature of non-calcified lung tumors, we performed a prospective study of 70 patients by means of FDG or Met and PET. A pathological diagnosis was obtained at biopsy or surgery. Lesions with a no-malignancy result from biopsy, or lesions responded to antibiotic treatment were clinically diagnosed as abscesses, and lesions that were followed up for more than one year without a change in size or nature were also clinically diagnosed as benign. The tumor to muscle radioactivity ratio was used for evaluation of tumor uptake of tracers. Tumors less than 1 cm in diameter were difficult to evaluate accurately due to the limitations of PET resolution. Compared to the final diagnosis, Met studies exhibited a sensitivity of 90%, specificity of 67% and accuracy of 83%. FDG studies exhibited a sensitivity of 89%, specificity of 92% and accuracy of 90% (Fig. 8).65 This study suggested that PET, especially with FDG, might be clinically very useful for differential diagnosis of lung tumors (Fig. 9). The original version of this report on 46 patients presented at the Society of Nuclear Medicine meeting in 1989, was the first report of actual clinical use of PET for diagnostic oncology. Subsequently a large number of clinical studies from the US and Europe were reported and all agreed that PET was superior to CT for 66 this purpose. After the introduction of the whole-body imaging technique with FDG-PET, detection of lymph node or distant metastases in known cancer patients has become another common application of PET (Fig. 10).67 With CT/MRI, the diagnosis of lymph node metastasis has been based on a size criterion (>1 cm in diameter), but small lymph nodes could still have metastases and large lymph nodes could be benign due to inflammatory reactions. The diagnosis of metastasis by using metabolic criteria with PET may be superior to the conventional imaging techniques of CT scan or MRI. Our results of whole-body PET for the detection of metastases yielded a sensitivity of 93%, specificity of 63% and accuracy of 88%.68 Other institutions have reported even better results with over 90% accuracy.66,69 Whole-body PET enables very sensitive detection of whole-body spread of disease with a single examination, that may change the strategy of patient therapy, and may be cost-effective. Recently, FDG-PET for the diagnosis of lung cancer was approved for medical insurance reimbursement by the Health Care Financing Administration (HCFA) in the USA (Medicare), and by medical insurance funds in Germany and the UK, but is unfortunately still pending in Japan at this moment (October 2001). In a recent report on stage diagnosis of lung cancer from Guy's Hospital in London, whole-body FDG-PET studies of 97 patients scheduled for operation were compared for clinical stage as assessed by CT and follow-up, and with the result of operation or biopsy.70 The sensitivity and specificity for diagnosis of N2 or N3 mediastinal lymph nodes were CT: 20% and 89.9%, and PET: 70.6% and 97% (sensitivity and specificity, respectively). Changes in the staging of disease by PET occurred in 26.8%, the N factors were changed in 13.4% and changes in the M factor (that is, discovery of unexpected metastasis) occurred in 16.5% of patients. There were changes in treatment policies in 37% of patients based on the PET findings. Conversely, PET failed to detect 7 of 10 brain metastases. The researchers concluded that the degree (SUV) of tumor FDG accumulation detected by PET was the best prognostic factor with the exception of the operative stage. A report from Duke University, USA, compared FDG-PET and conventional imaging with CT, bone scintigraphy, brain CT or MRI for staging of 100 patients with NSCLC with reference to the pathological stage.71 PET staging was accurate in 83% of patients, compared with an accuracy of 65% for conventional methods (p < 0.005). Staging of mediastinal lymph nodes was correct by PET in 85% and by CT in 58% (p < 0.001). Nine patients had metastases detected by PET that were not found by conventional imaging, and 10 patients suspected of metastases by conventional imaging were correctly diagnosed as no-metastasis by PET. The authors concluded that whole-body FDG-PET was more accurate than conventional imaging (chest CT, bone scan, brain CT or MRI) in staging lung cancer. FDG-PET for evaluation of lung cancer is the most extensively studied and most widely accepted clinical application of PET in oncology. Only a few studies have reported the use of 11C-methionine (Met) for lung cancer.35,65 Miyazawa et al.72 studied Met-PET of 24 patients with NSCLC and analyzed resected tumor tissue by DNA flow cytometry. The Met uptake rate in tumors correlated well with proliferation indices (S phase fraction, S + G2/M phase fraction). The authors concluded that the tumor uptake rate of Met reflected the tumor growth rate in NSCLC. Yasukawa et al.73 compared Met-PET and CT for the diagnosis of lymph node metastases in 41 patients with lung cancer. The diagnostic indices of Met-PET vs. CT were, sensitivity (86.1%, 52.8%), specificity (91.1%, 84.4%), and accuracy (89.7%, 75.4%) (Met-PET, CT, respectively). They concluded that Met-PET was superior to CT for diagnosis of lymph node metastasis of lung cancer. Met-PET appears to be as useful as FDG-PET for lung cancer, but the short half-life of 11C (20 min) is a major drawback for the widespread use of Met. Met uptake is more selective for cancer cells than is FDG,39 and its normal brain uptake is low, so that Met-PET appears to have an advantage over FDG-PET for imaging brain tumors.52 Further studies with Met and 18F-labeled amino acid analogues are necessary to establish the role of non-FDG PET in clinical oncology PET.74-77 
FDG-PET, Proliferation and Survival 
Ahuja et al.78 reported that the degree of FDG uptake in primary lung cancer correlated with survival. On hundred and fifty-five NSCLC patients were studied with FDG-PET. The stage at presentation, cell type, tumor size and survival data were recorded. A standardized uptake ratio (SUR) was calculated for FDG uptake by the primary lesion and was correlated with clinical information to determine prognostic significance. Multivariate analysis demonstrated that an SUR > 10 provided prognostic information independent of the clinical stage and lesion size.78 Higashi et al.79 demonstrated that FDG uptake correlated with cell proliferation rather than with the cellular density of NSCLC. They studied 31 patients with NSCLC, and tumor FDG uptake was evaluated with SUV. Cell proliferation as assayed by the PCNA labeling index was evaluated by the immunohistochemistry of tumor tissues resected at thoracotomy. Cellular density was also evaluated. FDG uptake correlated significantly with the PCNA labeling index, but only weakly with cellular density. Bronchoalveolar carcinoma (BAC), which is a well-differentiated tumor known to have an FDG uptake as low as benign lesions 80 had a significantly lower SUV and PCNA labeling index than tumors of other histology, but no significant differences in cellular density were evident between BAC and tumors of other histology. This suggested that FDG accumulation in tumor tissue would be a useful index of proliferation, of the degree of differentiation, and of patient survival. A similar study has been reported in breast cancer by Oshida et al.81 who examined FDG uptake and various known prognostic indicators by multivariate analysis, in 70 patients with breast cancer, to assess their contribution to overall and relapse-free survival. FDG uptake evaluated with DAR was an independent prognostic factor for relapse-free survival of breast cancer patients. Other independent prognostic indicators included the number of positive lymph nodes and histologic grade. In head and neck cancer, Minn et al.82 conducted a univariate analysis and reported that FDG uptake was clearly correlated with survival, but in their multivariate analysis, the only independent predictors of survival were the mitotic count and stage of disease. 
Non-Pathological Functional Uptake of FDG 
Whole-body glucose metabolism can be monitored with FDG. Therefore, high FDG accumulation may be observed in various tissues, such as muscle, due to physiological requirements. Accurate knowledge of such physiological accumulations is necessary to diagnose tumors in whole-body FDG-PET. FDG uptake by the limb muscles due to running83 and other exercises that consume glucose as the energy source, can easily be recognized. Confusion may arise in the distinction between normal shoulder and neck muscle uptake and lymph node metastases. Movement of the shoulder or neck after FDG injection and tension associated with anxiety may be causes of increased FDG uptake. Muscle uptake may be longitudinal and symmetrical. It was reported that administration of diazepam prevented muscle uptake 84 Speech during the phase of FDG uptake increases FDG activity in the laryngeal muscles85 and tongue movement or sucking may increase FDG uptake in pharyngeal muscles. Chewing may increase FDG uptake in the muscles of mastication (Fig. 11).86 Eye movement will induce FDG uptake in ocular muscles that then appear like two "V" letters. Non-pathological FDG uptake by the palatine tonsil was observed in about 50% of patients 87 Inflammatory bowel disease is another cause of FDG uptake 88 but the normal colon and small intestine also often exhibit increased FDG uptake. This can be partially explained by smooth muscle in the colon, but details of this mechanism are unknown. FDG distribution in liver and kidney was described in the previous section. Several reviews of functional and non-malignant FDG uptake have been published previously.89-93 
Tumor Structure and Distribution of FDG and Methionine 
False-positive FDG or methionine uptake has been reported in active inflammation such as abscesses 94 tuberculosis 65,95 aspergillosis,65 sarcoidosis and other lesions.90,96 Because of this limitation, the accuracy of PET cannot reach 100%. In addition, early post-operative scarring and the early inflammatory reaction after radiotherapy exhibited increased FDG uptake. In order to clarify the mechanisms of FDG uptake by inflammation and tumor tissues, we have performed autoradiographic studies. Tumor tissue is comprised of cancer cells and non-neoplastic tissues (stroma). Stroma includes macrophages, neutrophils and lymphocytes that infiltrate from capillaries due to the host-tumor immune reaction, and also includes granulation tissue comprised of fibroblasts, collagen fibers, and capillaries. Capillary growth is enhanced by angiogenic factors in tumor tissue. All these tissues consume glucose, at various levels. In addition to cancer cells, high FDG uptake is seen in activated macrophages and young granulation tissue (Fig. 12).97 The latter two may cause false-positive FDG uptake in inflammation, sarcoidosis and early post-operative scarring. FDG uptake was also studied in an experimental model of inflammation, and silver grains were observed by FDG in infiltrating immune cells and in granulation tissue by autoradiography .98 Immune-deficient nude mice are not an appropriate model to study this problem, as nude mice have no active immune system, and therefore both their lymphocytes and macrophages exhibit very low glucose metabolism.99 Studies with a syngeneic animal tumor model and an inflammation model, and PET-pathology correlation, have all demonstrated this phenomenon. Compared to FDG, Met distribution in tumor tissue is more specific for viable cancer cells. Granulation tissue and macrophages exhibited uptake of Met at levels lower than those of FDG. Tumor uptake of Met is apparently 39 predominantly by viable cancer cells (Fig. 13). 
Why PET for Treatment Evaluation? 
Tumor size measurement by X-ray or CT has been the standard method of the treatment evaluation of the cancer. But tumors containing non-active tissues, such as fibrosis, necrosis and injured cells about to die, produce a diagnostic dilemma in that the residual mass after treatment does not always equate with residual disease. Monitoring the viability of the tumor and evaluation of the, residual mass by means of PET would therefore be beneficial for cancer patients. 
Tumor Uptake Response to Radiotherapy 
Tumor tissue response to therapy and its correlation to tracer uptake must be evaluated carefully. We have compared tumor volume, amount of viable tumor tissue, and tracer uptake, after experimental radiotherapy in a rat tumor model.100-103 3H-Thymidine and 14C-Methionine uptake exhibited a rapid and sensitive response to irradiation, preceding both volumetric shrinkage and necrotic extension. FDG uptake almost paralleled the necrotic extension that preceded volumetric shrinkage (Fig. 14). FDG exhibited a wide range of uptake changes and a steady response to irradiation. FDG uptake was linearly correlated with the percentage of viable tissue in vivo. Thymidine, a marker of proliferation, underestimated the amount of viable tissue. Results with Met more closely resembled those obtained with thymidine than with FDG.47,104 Radiotherapy (and also chemotherapy) results in injury to DNA, RNA, protein and membranes of cancer cells, so that altered metabolism and cell death are reflected in a reduction in Met or Thd uptake, which precedes the autolysis of cells observed as necrosis. The reduction of viable tumor tissue is reflected by the decrease in FDG uptake. No visible reduction of tumor volume is evident until a large part of the necrotic tissue has been removed. PET with FDG enables functional evaluation of tumor viability to assess the therapeutic effects on tumor tissue, earlier than morphologic evaluation of tumor volume reduction by CT scan. Long after the therapy, if necrosis is replaced by fibrosis, differential diagnosis of recurrent viable cancer cells from fibrosis (scar) may be another useful indication for PET (Fig. 15).105 Our studies and the above considerations on applications for PET, have dealt with FDG uptake changes at least one day or later after radiotherapy. These observations differ from the acute reaction described in the next paragraph. The acute reaction of FDG uptake by cancer cells to radiotherapy or chemotherapy is more complicated. Fujibayashi et al.106 reported that FDG uptake by cultured human cancer cells increased from I hr after 30 Gy of radiotherapy and peaked at 3 hr, after which time it decreased. Both glucose transporter- 1 mRNA expression and hexokinase activity were significantly increased. After the inhibition experiments, the authors concluded that the transient increase in glucose metabolism occurred via a process at the level of gene expression. The same group reported a FDG-PET study of brain tumor patients before and 4 hr after a 24-32 Gy single dose of stereotactic radiosurgery.107 The influx constants, Ki, of FDG in irradiated tumors exhibited a 30 +- 14% increase after radiotherapy. In metastatic tumors, the percentage volume decrease was positively correlated with the percentage change in Ki ratio. The authors suggested that hyperacute changes in glucose metabolism could predict a therapeutic response. In a study on human tumor xenografts, rapid increase in FDG uptake after radiotherapy was observed only in the most radiosensitive tumor line and was accompanied by apoptosis in the tissue.108 A similar transient increase in FDG uptake by cancer cells was reported soon after experimental chemotherapy by Slossman et al.109 These early reactions of glucose metabolism to therapy may be related to cellular stress responses. This could involve a phenomenon similar to the mechanism of heat shock proteins, but this has not been well studied to date. 
Treatment Evaluation with Met-PET 
In a long-term follow-up study of lung cancer, changes in Met uptake were superior to changes in tumor volume detected by CT scan in assessing the effects of radio-therapy, and detecting recurrence.110 We have compared the early treatment response of Met uptake and tumor volume in order to predict the final outcome of the treatment results in 19 lung cancer patients. In the Met uptake data, the early-recurrence group was clearly distinguishable from the no-recurrence group. But the late-recurrence group was indistinguishable from the no-recurrence group, probably because the number of residual viable cancer cells at the end of radiotherapy was not large enough to detect with PET. When the reduction in Met uptake was combined with the reduction in tumor volume, the differentiation of each group became easier than when a single parameter was used. When a residual mass is visible on CT, PET appears to be useful in evaluating tumor viability.111 Effects of radiotherapy on head and neck cancer were studied with Met-PET by Nuutinen et al,112 Met uptake by tumors exhibited a significant decrease during the first 2-3 weeks of radiotherapy. The rate of decrease in Met uptake was comparable in relapsing patients and those who remained locally-controlled, so that the use of Met-PET for prediction of response to radiotherapy appears to be limited. Effects of chemotherapy or hormonal therapy on breast cancer metastases were also evaluated with MetPET by Huovinen et al.113 Met uptake by tumors decreased when clinical objective regression of the tumor was later attained, and increased in patients where progressive disease was observed during treatment. They concluded that changes in amino acid metabolism as detected by Met-PET precede the clinical response, and may be of clinical value in predicting the treatment response. Met uptake by a variety of tumors appears to decrease earlier than the regression of tumor volume or other clinical indicators, but it may remain difficult to predict whether the tumor will finally relapse. 
Treatment Evaluation with FDG-PET 
Excellent results have been reported in the detection of lung cancer recurrence after various treatments.114,115 FDG-PET is superior to MRI and CT for differentiation of recurrence from post-operative scarring in colon cancer.116 Nevertheless, several false-positive studies have been reported after radiotherapy in lung, colon, and head and neck cancer, where cancer recurrence was denied after biopsy or clinical follow-up despite the increased FDG uptake. FDG uptake by normal chest wall within the radiation field has also been reported, which peaked at 6 months after radiotherapy.117 Attention must be paid to this increased FDG uptake by post-radiotherapy inflammatory tissue. An autoradiography study demonstrated that the macrophage layer and granulation tissue at the rim of necrosis after fractionated radiotherapy exhibited high FDG uptake despite the absence of viable cancer cells. High FDG uptake was also observed in the muscle within the radiation field adjacent to the tumor.118 Bury et al.119 reported FDG-PET studies of 129 patients in detecting residual or recurrent NSCLC after surgery, or radiotherapy and/or chemotherapy. PET revealed increased FDG uptake in all cases (n = 60) of persistent or recurrent tumor, whereas CT was nonspecific in 17 cases. There were 5 false-positives with PET and 3 with CT. In detecting residual or recurrent NSCLC, PET and CT showed sensitivities of 100% and 71%, specificities of 92% and 95%, and accuracies of 96% and 84%, respectively. PET correctly identified response to therapy in 96% of patients. PET appeared to be more accurate than conventional imaging in distinguishing residual or recurrent tumor from fibrotic scar in patients undergoing treatment for NSCLC (Fig. 16). Detection of recurrence in patients with previously treated head and neck cancer is difficult. CT/MRI findings are often equivocal or inconclusive because of distortion of anatomic structures and the presence of diffuse soft-tissue swelling caused by previous treatment. Contrast enhancement can be detected with CT/MRI in both recurrent tumors and post-surgical or post-radiation changes. Anzai et al.120 reported a comparison of FDG-PET and CT/MRI to detect recurrence of head and neck cancer in 12 patients. Sensitivity (77%, 25%) and specificity ( 100%, 75%; PET and CT/MRI, respectively) were assessed. FDG-PET exhibited significantly better diagnostic accuracy than CT/MRI. Fischbein121 reported similar results in 35 patients with squamous cell carcinoma of the head and neck, and sensitivity and specificity for residual/recurrent disease at the primary site were 100% and 64% respectively, and for nodal diseases 93% and 77%. Kao et al.122 studied 36 nasopharyngeal carcinomas (NPC) with FDG-PET and CT, 4 months after radiotherapy. The diagnostic results for FDG vs. CT were sensitivities of 100% and 72%, specificities of 96% and 88%, and accuracies of 97% and 83%, respectively. FDG-PET was superior to CT for detection of recurrent or persistent NPC. In our preliminary study to detect recurrence of head and neck cancer after radio-chemo-therapy, FDG-PET was compared with CT/MRI. Twenty-nine lesions in 26 patients were examined and compared to the results of biopsy, surgery, or clinical follow-up. Sensitivity (78%, 67%; PET and CT/MRI, respectively), specificity (70%, 20%), and accuracy (72%, 34%) were assessed. There were two false-negative, and 6 false-positive results with FDG-PET, the latter including 3 cases of post-radio-therapy inflammation. In this study. FDG-PET exhibited significant superiority to CT/MRI in the detection of recurrent head and neck cancer. In particular, the specificity of PET (70%) was vastly superior to that of CT/MRI (20%) (see Fig. 11c).123 
Detection of Hypoxic Tumor Tissue 
Prediction of the sensitivity or resistance of a tumor to radio- or chemo-therapy would be of great benefit to the patient and physician. The imbalance between cancer cell growth and capillary growth produces heterogeneous perfusion patterns in tumor tissue, which eventually lead to regional hypoxia or tissue necrosis. The presence of hypoxic cells in a tumor is very relevant to radioresistance, and can also contribute to chemoresistance. Non-invasive assessment of tumor hypoxia has been studied with several radio-tracers to predict radioresistance of tumors. Misonidazole and its derivatives are metabolically trapped in cells that are alive but hypoxic, and are used as markers of hypoxic tissues 18F-fluoromisonidazole (FMISO) has been developed and studied for imaging hypoxic tissue 124,125 We studied FMISO uptake in a rat tumor model of hypoxia and in controls, and investigated the correlation between intra-tumoral distributions of FMISO, 14C-2-deoxyglucose (2DG) and 14C-methionine (Met). Double-tracer autoradiography of the tumor demonstrated that the areas of high FMISO uptake had low uptake of Met, whereas low FMISO uptake areas had high Met uptake. FMISO showed high grain density in the tumor rim surrounding the necrotic area. 2DG showed a more uniform distribution over the entire area of viable cells. The mean uptake of FMISO by hypoxic, radioresistant tumors was significantly higher than by control tumors (p < 0.05), and both 2DG and Met uptake by the control tumors was higher than by hypoxic tumors. When individual tumors were examined, FMISO uptake was inversely correlated with that of Met (r = -0.507, p < 0.02), but 2DG exhibited almost uniform uptake with no significant correlation with FMISO. We concluded that there was a large overlap in the distribution of FMISO and 2DG within the tumor, but only a small overlap in the distribution of FMISO and Met (Fig. 17). 126 In our previous study we used micro-autoradiography to demonstrate that uptake of FDG was increased in prenecrotic (hypoxic) cells at the peripheral rim of necrosis 45 But in double-tracer autoradiography at the macro level, increased FDG uptake at the periphery of necrosis was not clear. The combination of FMISO and other tracers in a PET study would possibly be more helpful than a single-tracer study in predicting the response of tumor tissues to radiotherapy. The influence of hypoxia on FDG, methionine and leucine accumulation in cultured human cancer cells was reported by Clavo et al.127 and Minn et al.128 3H-FDG accumulation is increased in hypoxic cancer cells, in part due to increased membrane expression of the GLUT1 glucose transporter. Hypoxia was associated with decreased cellular uptake of thymidine. The decrease in acid-precipitable 3H-leucine in hypoxic conditions may indicate a decline in protein synthesis, whereas the unchanged 3H-methionine uptake probably reflects unaltered amino acid transport and slow trans-methylation 129 Recently, the mechanism of increased FDG uptake by hypoxic cells was further studied by Burgman et al. 130 A two-fold increase in 3H-FDG uptake was reported under hypoxic conditions, but no changes in the cellular levels of glucose transporter proteins or hexokinase were observed. That study, with reducing or oxidizing agents, suggested that hypoxia-induced modification (reduction) of the cysteine residues in GLUT may be the mechanism underlying the hypoxia-induced increase in 3H-FDG uptake. Enhancement of transmembrane 3H-FDG transport without translocation of GLUTS to the plasma membrane may result either from increased affinity of GLUTS or activation of dormant GLUTS that pre-exist within the plasma membrane. This is inconsistent with the findings of Clavo et al. 127 whose study suggested increased membrane expression of the GLUT1. The finding of Clavo et al.127 were consistent with those of Waki et al.26 in that GLUT activity, and not hexokinase activity, was rate-limiting for 3H-FDG uptake in cancer cells. 18F-fluoroerythronitroimidazole (FETNIM) has been reported as an agent for detecting tumor hypoxia by imaging. The tumor to blood distribution ratio of FETNIM at 4 hr after injection was significantly higher than that of FMISO. Autoradiographs indicated that both agents could help differentiate hypoxic from necrotic regions in tumors. FETNIM is reported to be easier to prepare, less costly, and more hydrophilic than FMISO.131 PET studies using FETNIM in cancer patients have been performed and will be reported from the University of Turku. 
CONCLUSION 
PET with FDG can effectively monitor molecular events involved in glucose transport and phosphorylation in vivo, parameters that usually reflect the viability of cancer cells in tumor tissue. FDG-PET is clinically very useful for differential diagnosis, detection of metastasis in the whole-body, and for detection of recurrence of lung and other cancers. Further investigations are required to clarify the usefulness of PET in predicting sensitivity or resistance of tumors to chemo/radio therapy. 
ACKNOWLEDGMENTS 
The author thank Drs. Roko Kubota, Susumu Yamada, Takehiko Fujiwara, Hiroshi Fukuda, Masatoshi Itoh. Manabu Tashiro, Shuichi Ono, Keiichiro Yamaguchi, Takashi Akaizawa, Kenji Yamada, Jyutaro Takahashi, Hiromichi Ohira, Michael J. Reinhardt, Muhammad Babar Imran, Ahmad Qureshy, Kiichi Ishiwata. Ren Iwata, Tatsuo Ido. Masao Tada and the staff of the Cyclotron Radioisotope Center and the Institute of Development, Aging and Cancer, Tohoku University, particulary Mr. Yasuhiro Sugawara, Mr. Tachio Sato, Mr. Shouichi Watanuki, and Mr. Masahiro Miyake for their excellent technical assistance. The author thank Drs. Jyunkichi Yokoyama and Hisashi Rikimaru, for their excellent support in the interpretation of head and neck images, and kind supply of images. This work was supported by Grants-in-Aids No. 12470183 from the Ministry of Education, Science, Sports and Culture Japan. 
REFERENCES 
1. Bar-Shalom R, Valdivia AY, Blaufox MD. PET imaging in oncology. Semin Nucl Med 2000; 30: 150-185. 
2. O'Doherty MJ. PET in oncology 1-lung, breast, soft tissue sarcoma. Nucl Med Comm 2000; 21 : 224-229. 
3. Nunan TO, Hain SF. PET in oncology 2-0ther tumors. Nucl Med Comm 2000; 21 : 229-233. 
4. Ruhlmann J, Oehr P, Biersack HJ (eds.). PET in oncology; basics and clinical application. Heidelberg; Springer, 1999. 
5. Martin WH, Sandler MP. Positron imaging in oncology: Present and future. In: Nuclear Medicine Annual, Freeman LM, ed. Philadelphia; Lippincott-Raven, 1998: 1-50. 
6. Biersack HJ, Bender H, Ruhlmann J. Schomburg A, Grunwald F. FDG-PET in clinical oncology: review and evaluation of results of a private clinical PET center. In: Nuclear Medicine Annual, Freeman LM, ed. Philadelphia; Lippincott-Raven, 1997: 1-29. 
7. Brock CS, Meikle SR, Price P. Does fluorine-18 fluoro-deoxyglucose metabolic imaging of tumours benefit oncology? Eur J Nucl Med 1997; 24: 691-705. 
8. Conti PS, Lilien DL, Hawley K, Keppler J, Graffton ST, Bading JR. PET and [18F]-FDG in oncology: a clinical update. Nucl Med Biol 1996; 23: 717-735. 
9. Warburg O. On the origin of cancer cells. Science 1956; 123: 309-314. 
10. Sweeney MJ, Ashmore J, Morris HP, Weber G. Comparative biochemistry of hepatomas IV. Isotope studies of glucose and fructose metabolism in liver tumors of different growth rates. Cancer Res 1963; 23: 995-1002. 
11. Pauwels EKJ, Ribeiro MJ, Stoot JHMB, McCready VR, Bourguignon M, Maziere B. FDG accumulation and tumor biology. Mini-review. Nucl Med Biol 1998; 25: 3 17-322. 
12. Hatanaka M. Transport of sugars in tumor cell membranes. Biochim Biophys Acta 1974; 355: 77-l04. 
13. Flier JS, Mueckler MM, Usher P, Lodish HF. Elevated levels of glucose transporter and transporter messenger RNA are induced by ras or sac oncogenes. Science 1987; 235: 1492-1495. 
14. Birnbaum MJ, Haspel HC, Rosen OM. Transformation of rat fibroblasts by FSV rapidly increases glucose transporter gene transcription. Science 1987; 235: 1495-1498. 
15. Nishioka T, Oda Y, Seino Y, Yamamoto T, Inagaki N, Yano H, et al. Distribution of glucose transporters in human brain tumors. Cancer Res 1992; 52: 3972-3979. 
16. Yamamoto T, Seino Y, Fukumoto H, Koh G, Yano H, Inagaki N, et al. Over-expression of facilitative glucose transporter genes in human cancer. Biochem Biophys Res Commun 1990; 170: 223-230. 
17. Woodward GE, Hudson MT. The effect of 2-deoxy-D-glucose on glycolysis and respiration of tumor and normal tissues. Cancer Res 1954; 14: 599-605. 
18. Nirenberg MW, Hogg JE. Inhibition of anaerobic glycolysis in ehrlich ascites tumor cells by 2-deoxy-D-glucose. Cancer Res 1958; 18: 518-521. 
19. Landau BR, Laszlo J, Stengle J, Burk D. Certain metabolic and pharmacologic effects in cancer patients given infusion of 2-deoxy-D-glucose. J Natl Cancer Inst 1958; 21 : 485-494. 
20. Sokoloff L, Reivich M, Kennedy C, Des Rosiers MH, Patlak CS, Pettygrew KD, et al. The [14C]deoxyglucose method for the measurement of local cerebral glucose utilization: theory, procedure, and normal values in the conscious and anesthetized albino rat. J Neurochem 1977; 28: 897-916. 
21. Ido T, Wan CN, Fowler JS, et al. Fluorination with F2. A convenient synthesis of 2-deoxy-2-fluoro-D-glucose. J Org Chem 1977; 42: 2341-2342. 
22. Gallagher BM. Fowler JS, Gutterson NI, MacGregor RR, Wan CN, Wolf AP. Metabolic trapping as a principle of radiopharmaceutical design: some factors responsible for the biodistribution of [18F]2-deoxy-2-fiuoro-D-glucose. J Nucl Med 1978; 19: 1151-1161. 
23. Som P, Atkins HL. Bandoypadhyay D, Fowler JS, MacGregor RR. Matsui K, et al. A fluorinated glucose analog, 2-fluoro-2-deoxy-D-glucose (18F): nontoxic tracer for rapid tumor detection. J Nucl Med 1980; 21 : 670-675. 
24. Yonekura Y. Benua RS. Brill AB. Som P, Yeh SDJ, Kemeny NE, et al. Increased accumulation of 2-deoxy-2-[18F]fluoro-D-glucose in liver metastasis from colon carcinoma. J Nucl Med 1982; 23: 1133-1137. 
25. Harberkorn U. FDG uptake, tumor proliferation and expression of glycolysis associated genes in animal tumor models. Nucl Med Biol 1994; 21 : 827-834. 
26. Waki A, Kato H, Yano R, Sadato N, Yokoyama A, Ishii Y, et al. The importance of glucose transport activity as the rate-limiting step of 2-deoxyglucose uptake in tumor cells in vivo. Nucl Med Biol 1998; 25: 593-597. 
27. Waki A, Fujibayashi Y, Yokoyama A. Recent advances in the analyses of the characteristics of tumors on FDG uptake. Nucl Med Biol 1998; 25: 589-592. 
28. Brown RS, Leung JY. Fisher SJ, Frey KA, Ethier SP. Wahl RL. Intratumoral distribution of tritiated-FDG in breast carcinoma: correlation between Glut-1 expression and FDG uptake. J Nucl Med 1996; 37: 1042-1047. 
29. Brown RS, Leung JY, Kison PV, Zasadny KR, Flint A, Wahl RL. Glucose transport and FDG uptake in untreated primary human non-small cell lung cancer. J Nucl Med 1999; 40: 556-565. 
30. Wagle SR, Morris HP, Weber G. Comparative biochemistry of hepatoma V. Studies of amino acid incorporation in liver tumors of different growth rates. Cancer Res 1963; 23: 1003-1007. 
31. Foster DO, Pardee AB. Transport of amino acid by confluent and nonconfluent 3T3 and polyoma virus-transformed 3T3 cells growing on glass cover slips. J Biol Chem 1969; 244: 2675-2681. 
32. Isselbacker KJ. Increased uptake of amino acids and 2-deoxy-D-glucose by virus-transformed cells in culture. Proc NatAcad Sci USA 1972; 69: 585-589. 
33. Hoffman RM. Methionine dependence in cancer cells-a review. In Vitro 1982; 18: 421-428. 
34. Kubota K, Ito M. Fukuda H, Abe Y, Ito K, Fujiwara T, et al. Cancer diagnosis with positron computed tomography and carbon-11-labeled I_-methionine. Lancet 1983; 2: 1192. 
35. Kubota K. Matsuzawa T, Ito M, Ito K, Fujiwara T, Abe Y, et al. Lung tumor imaging by positron emission tomography using C-11-L-Methionine. J Nucl Med 1985; 26: 37-42. 
36. Leskinen-Kallio S, Nagren K, Lehikoinen P, Ruotsalainen U, Joensuu H. Uptake of C-11 methionine in breast cancer studied by PET: and association with the size of S-phase fraction. Br J Cancer 1991 ; 64: 1121-1124. 
37. Lindholm P. Leskinen S, Nagren K, et al. Carbon-11-methionine PET imaging of malignant melanoma. J Nucl Med 1995; 36: 1805-1810. 
38. Ishiwata K, Kubota K, Murakami M, Kubota R, Sasaki T, Ishii S, Senda M. Re-evaluation of amino acid PET studies: Can the protein synthesis rates in brain and tumor tissues be measured in vivo? J Nucl Med 1993; 34: 1936-1943. 
39. Kubota R, Kubota K, Yamada S, Tada M. Takahashi T, Iwata R, Tamahashi N. Methionine uptake by tumor tissue: a microautoradiographic comparison with FDG. J Nucl Med 1995; 36: 484-492. 
40. Ishiwata K, Hatazawa J, Kubota K, Kameyama M, Itoh M. Matsuzawa T, et al. Metabolic fate of L-[methyl-11C]methionine in human plasma. Eur J Nucl Med 1989; 15: 665-669. 
41. Hatazawa J, Ishiwata K, Itoh M, Kameyama M, Kubota K, Ido T, et al. Quantitative evaluation of L-[methyl-11C]methionine uptake in tumor using positron emission tomography. J Nucl Med 1989; 30: 1809-1813. 
42. Minn H, Clavo AC, Grenman R, Wahl RL. In vitro comparison of cell proliferation kinetics and uptake of tritiated fluorodeoxyglucose and L-methionine in squamous-cell carcinoma of the head and neck. J Nucl Med 1995; 36: 252-258. 
43. Kubota R, Kubota K, Yamada S, Tada M, Ido T, Tamahashi N. Microautoradiographic study for the differentiation of intratumoral macrophages, granulation tissues and cancer cells by the dynamics of fluorine-18-fluorodeoxyglucose uptake. J Nucl Med 1994; 35: 104-112. 
44. Ohira H. Kubota K, Ohuchi N, Harada Y. Fukuda H, Satomi S. Comparison of intratumoral distribution of 99mTc-MIBI and deoxyglucose in mouse breast cancer models. J Nucl Med 2000; 41 : 1561-1568. 
45. Kubota R, Kubota K, Yamada S, Tada M, Ido T, Tamahashi N. Active and passive mechanisms of [fiuorine-18]fluorodeoxyglucose uptake by proliferating and prenecrotic cancer cells in vivo: a microautoradiographic study. J Nucl Med 1994; 35: 1067-1075. 
46. Yoshioka T, Takahashi H, Oikawa H, Maeda S. Wakui A, Watanabe T, et al. Accumulation of 2-Deopxy-2[18F]fluoro-D-glucose in human cancers heterotransplanted in nude mice:comparison between histology and glycolytic status. J Nucl Med 1994; 35: 97-103. 
47. Higashi K. Clavo AC. Wahl RL. Dose FDG uptake measure proliferative activity of human cancer cells? In vitro comparison with DNA flow cytometry and tritiated thymidine uptake. J Nucl Med 1993; 34: 414-419. 
48. Higashi K. Clavo AC, Wahl RL. In vitro assessment of 2-fluoro-2-deoxy-D-glucose, L-methionine and thymidine as agents to monitor the early response of a human adenocarcinoma cell line to radiotherapy. J Nucl Med 1993; 34: 773-779. 
49. Shields AF, Grierson JR, Dohmen BM, Machulla HJ, Stayanoff JC. Lawhorn-Crews JM, et al. Imaging proliferation in vivo with [F-18]FLT and positron emission tomography. Nature Med 1998; 4: 1334-1336. 
50. Strauss LG, Clorius JH, Schlag P, et al. Recurrence of colorectal tumors: PET evalutaion. Radiology 1989; 170: 329-332. 
51. Lindholm P, Leskinen S, Lapela M. Carbon-11-methio-nine uptake in squamous cell head and neck cancer. J Nucl Med 1998; 39: 1393-1397. 
52. Ogawa T. Shishido F, Kanno I, et al. Cerebral gliomas: evaluation with methionine-PET. Radiology 1993; 186: 45-53. 
53. Boemer AR, Weckesser M, Herzog H, et al. Optimal scan time for fluorine- 18-fluorodeoxyglucose positron emission tomography in breast cancer. Eur J Nucl Med 1999; 26: 226-230. 
54. Lowe VJ, Delong DM, Hoffman JM, Coleman RE. Optimum scanning protocol for FDG-PET evaluation of pulmonary malignancy. J Nucl Med 1995; 36: 883-887. 
55, Hamberg LM, Hunter GJ, Alpert NM, Choi NC, Babich JW, Fischman AJ. The dose uptake ratio as index of glucose metabolism: useful parameter or over simplification J Nucl Med 1994; 35: 1308-1312. 
56. Kubota K, Itoh M, Ozaki K, Ono S, Tashiro M, Yamaguchi K, et al. Advantage of delayed imaging of whole-body FDG-PET for tumor detection. Eur J Nucl Med 2001 ; 28: 696-703. 
57. Wahl RL, Henry CA, Ethier SP. Serum glucose: effects on tumor and normal tissue accumulation of 2-[F18]-fluoro-2-deoxy-D-glucose in rodents with mammary carcinoma. Radiology 1992; 183: 643-647. 
58. Yamada K, Endo S, Fukuda H, Abe Y, Yoshioka S, Kubota K, et al. Experimental studies on myocardial glucose metabolism of rats with 18F-2-fluoro-2-deoxy-1)-glucose. Eur J Nucl Med 1985; 10: 341-345. 
59. Kubota K, Kubota R, Yamada S, Tada M, Takahashi T, lwata R. Re-evaluation of myocardial FDG uptake in hyperglycemia. J Nucl Med 1996; 37: 1713-1717. 
60. Ishizu K, Sadato N, Yonekura Y, et al. Enhanced detection of brain tumors by [18F]fluorodeoxyglucose PET with glucose loading. J Comput Assist Tomogr 1994; 18: 12-15. 
61. Langen KJ. Braun U, Rota Kops E, et al. The influence of plasma glucose levels on fluorine-18-fluorodeoxyglucose uptake in bronchial carcinomas. J Nucl Med 1993; 34: 355-359. 
62. Lindholm P, Minn H, Leskinen KS, Bergman J, Ruotsalainen U. Joensuu H. Influence of the blood glucose concentration on FDG uptake in cancer-a PET study. J Nucl Med 1993; 34: 1-6. 
63. Lindholm P, Leskinen-Kallio S, Kirvela O, et al. Head and neck cancer: effect of food ingestion on uptake of C-11 methionine. Radiology 1994; 190: 863-867. 
64. Torizuka T, Fisher SJ, Wahl RL. Insulin-induced hypoglycemia decreases uptake of 2-[F-18]fluoro-2-deoxy-D-glucose into experimental mammary carcinoma. Radiology 1997; 203: 169-172. 
65. Kubota K, Matsuzawa T, Fujiwara T, Ito M, Hatazawa J, Ishiwata K, et al. Differential diagnosis of lung tumor with positron emission tomography: a prospective study. J Nucl Med 1990; 31: 1927-1933. 
66. Coleman RE. PET in lung cancer. J Nucl Med 1999; 40: 814-820. 
67. Hoh CK, Hawkins RA, Glaspy JA, et al. Cancer detection with whole-body positron emission tomography using fluorodeoxyglucose. J Comput Assist Tomogra 1993; 17: 582-589. 
68. Kubota K, Imran MB, Ono S, Akaizawa T. Gotoh R, Ohira H, et al. Diagnostic value of whole-body positron emission tomography using fluorine-18-fluorodeoxyglucose for lung cancer. Jpn J Clin Radiol 2000; 45: 199-208. 
69. Pieterman RM, van Putten JWG, Meuzelaar JJ, Mooyaart EL, Vaarburg W, Koeteer GH, et al. Preoperative staging of non-small cell lung cancer with positron emission tomography. N Engl J Med 2000; 343: 254-261. 
70. Saunders CAB, Dussek JE, O'Doherty MJ, Maissey MN. Evaluation of fluorine-18-fluorodeoxyglucose whole body positron emission tomography imaging in the staging of lung cancer. Ann Thorac Surg 1999; 67: 790-797. 
71. Marom EM, McAdams HP, Goodman PC, Culhane DL, Coleman RE, Herndon JE, Pats EF. Staging non-small cell lung cancer with whole-body PET. Radiology 1999; 212: 803-809. 
72. Miyazawa H, Arai T, Iio M, Hara T. PET imaging of non-small-cell lung carcinoma with carbon-11-methionine: relationship between radioactivity uptake and flow-cytometric parameters. J Nucl Med 1993; 34: 1886-1891. 
73. Yasukawa T, Yoshikawa K, Aoyagi H, Yamamoto N, Tamura K, Suzuki K, et al. Usefulness of PET with 11C-methionine for the detection of hilar and mediastinal lymph node metastasis of lung cancer. J Nucl Med 2000; 41: 283-290. 
74. Kubota K, Ishiwata K, Kubota R, Yamada S, Takahashi J, Abe Y, et al. Feasibility of fluorine-18 fluorophenylalanine for body tumor imaging compared with L-methionine. J Nucl Med 1996; 37: 320-325. 
75. Inoue T, Shibasaki T. Oriuchi N, Aoyagi K, Tomiyoshi K, Amano S, et al. 18F alpha-methyltyrosine PET studies in patients with brain tumors. J Nucl Med 1999; 40: 399-405. 
76. Inoue T, Koyama K. Oriuchi N, Alyafei S, Yuan Z, Suzuki H, et al. Detection of malignant tumors: whole-body PET with fluorine 18 a-methyl tyrosine versus FDG-preliminary study. Radiology 2001 ; 220: 54-62. 
77. Hoegerle S, Altehoefer C, Ghanem N, Koehler G, Waller FC, Scheruebl H, et al. Whole-body 18F DOPA PET for detection of gastrointestinal carcinoid tumors. Radiology 2001 ; 220: 373-380. 
78. Ahuja V, Coleman RE, Herndon J, Patz EF. The prognostic significance of fluorodeoxyglucose positron emission tomography imaging for patients with nonsmall cell lung carcinoma. Cancer 1998; 83: 918-924. 
79. Higashi K, Ueda Y, Yagishita M, Arisaka Y, Sakurai A, Oguchi M, et al. FDG PET measurement of the proliferative potentional of non-small cell lung cancer. J Nucl Med 2000; 41 : 85-92. 
80. Higashi K, Ueda Y, Seki H, Yuasa K, Oguchi M, Noguchi T, et al. Fluorine-18-FDG imaging is negative in bronchioloalveolar lung carcinoma. J Nucl Med 1 998; 39: 1016-1020. 
81. Oshida M, Uno K, Suzuki M, Nagashima T, Hashimoto H, Yagata H, et al.. Predicting the prognosis of breast carcinoma patients with positron emission tomography using 2-deoxy-2-fluoro[18F]-D-glucose. Cancer 1998; 82: 2227-2234. 
82. Minn H, Lapela M, Klemi PJ, Grenman R, Leskinen S, Lindholm P, et al. Prediction of survival with fluorine-18-fluorodeoxyglucose and PET in head and neck cancer. J Nucl Med 1997; 38: 1907-1911. 
83. Tashiro M, Fijimoto T, Itoh M, Kubota K, Fujiwara T, Miyake M, et al, 18F-FDG PET Imaging of muscle activity in runners. J Nucl Med 1999; 40: 70-76. 
84. Barrington SF, Maisey MN. Skeletal muscle uptake of fluorine-18-FDG: Effect of oral diazepam. J Nucl Med 1996; 37: 1127-1129. 
85. Kostakoglu L, Wong JCH, Barrington SF, Cronin BF, Dynes AM, Maisey MN. Speech-related visualization of laryngeal muscles with fluorine-18-FDG. J Nucl Med 1996; 37: 1771-1773. 
86. Rikimaru H, Kikuchi M, Itoh M. Tashiro M. Watanabe M. Mapping energy metabolism in Haw and tongue muscles during chewing. J Dental Res 2001 ; 80; 1849-1 853. 
87. Kawabe J, Okamura T, Shakudo M, Koyama K, Sakamoto H, Ohachi Y, et al. Physiological FDG uptake in the palatine tonsils. Ann Nucl Med 2001 ; 15: 297-300. 
88. Meyer MA. Diffusely increased colonic F-18-FDG uptake in acute enterocolitis. Clin Nucl Med 1995; 20: 434-435. 
89. Cook GJR, Fogelman I, Maisey MN. Normal physiological and benign pathological variants of 18-fluoro-2-deoxyglucose positron-emission tomography scanning: potential for error in interpretation. Semin Nucl Med 1996; 26: 308-314. 
90. Bakheet SM, Powe J. Gamut. Benign causes of 18-FDG uptake on whole body imaging. Semin Nucl Med 1998; 28: 352-358. 
91. Vesselle HJ, Miraldi FD. FDG PET of the retroperitoneum: normal anatomy, variants, pathologic conditions, and strategies to avoid diagnostic pitfalls. RadioGraphics 1998; 18: 805-823. 
92. Cook GJR. Maisey MN, Fogelman I. Normal variants, artefacts and interpretative pitfalls in PET imaging with 18-fluoro-2-deoxyglucose and carbon-11-methionine. Eur J Nucl Med 1999; 26: 1363-1378. 
93. Shreve PD, Anzai Y, Wahl RL. Pitfalls in oncologic diagnosis with FDG PET imaging: physiologic and benign variants. RadioGraphics 1999; 19: 61-77. 
94. Tahara T, Ichiya Y, Kuwabara U, et al. High [18F]-fluoro-deoxyglucose uptake in abdominal abscessed: a PET study. J ComputAssist Tomogr 1989; 13: 829-831. 
95. Knopp MV, Bischoff HG. Evaluation of pulmonary lesions with positron emission tomography. Radiologe 1994; 34: 588-591. 
96. Lewis PJ. Salama A. Uptake of fluorine-18-fluorodeoxy-glucose in sarcoidosis. J Nucl Med 1994; 35: 1647-1649. 
97. Kubota R, Yamada S, Kubota K, Ishiwata K, Tamahashi N, Ido T. Intratumoral distribution of fluorine-18-fluoro-deoxyglucose in vivo: high accumulation in macrophages and granulation tissues studied by microautoradiography. J Nucl Med 1992; 33: 1972-1980. 
98. Yamada S, Kubota K, Kubota R, Ido T, Tamahashi N. High accumulation of fluorine-18-fluorodeoxyglucose in turpentine-induced inflammatory tissue. J Nucl Med 1995; 36: 1301-1306. 
99. Brown RS, Wahl RL. Autoradiographic evaluation of the intra-tumoral distribution of 2-deoxy-D-glucose and monoclonal antibodies in xenografts of human ovarian adenocarcinoma. J Nucl Med 1993; 34: 75-82. 
100. Kubota K, Kubota R, Yamada S. FDG accumulation in tumor tissue. J Nucl Med 1993; 34: 419-421. 
101. Kubota K, Matsuzawa T, Takahashi T. Fujiwara T, Kinomura T, Ido T, et al. Rapid and sensitive response of 11C-L-methionine tumor uptake to irradiation. J Nucl Med 1989; 30: 2012-2016. 
102. Kubota K. Ishiwata K, Kubota R, Yamada S, Tada M, Sato T. Ido T. Tracer feasibility for monitoring tumor radiotherapy: A quadruple tracer study with fluorine-18-fluorodeoxyglucose, fluorine-18-fluorodeoxyuridine, L-[methyl 14C]methionine, [6-3H]thymidine and gallium-67. J Nucl Med 1991 ; 32: 2118-2123. 
103. Kubota K, Ishiwata K, Yamada S. Kubota R, Sato T, Takahashi J, et al. Dose-responsive effect of radiotherapy on the tumor uptake of L-[methyl 11C]methionine; feasibility for monitoring recurrence of tumor. Nucl Med Biol 1992; 19: 27-32. 
104. Slosman DO, Pittet N, Donath A, Polla BS. Fluoro-deoxyglucose cell incorporation as an index of cell proliferation: evaluation of accuracy in cell culture. Eur J Nucl Med 1993; 20: 1084-1088. 
105. Kubota K, Kubota R, Yamada S, Ishiwata K. Feasibility of 11C-L-Methionine and 18F-Fluorodeoxyglucose for the treatment evaluation of cancer with PET. In: Matsuzawa T (ed), Clinical PET in Oncology, Singapore; World Scientific, 1994: 47-53. 
106. Fujibayashi Y, Waki A, Sakahara H, Konishi J, Yonekura Y, Ishii Y, Yokoyama A. Transient increase in glycolytic metabolism in cultured tumor cells immediately after exposure to ionizing radiation: from gene expression to deoxyglucose uptake. Radiat Res 1997; 147: 729-734. 
107. Maruyama I, Sadato N, Waki A, Tsuchida T, Yoshida M, Fujibayashi Y, et al. Hyperacute changes in glucose metabolism of brain tumors after stereotactic radiosurgery: a PET study. J Nucl Med 1999; 40: 1085-1090. 
108. Furuta M, Hasegawa M, Hayakawa K, Yamakawa M, Ishikawa H, Nonaka T, et al. Rapid rise in FDG uptake in an irradiated human tumour xenograft. Eur J Nucl Med 1997; 24: 435-438. 
109. Slosman DO, Pugin J. Lack of correlation between tritiated deoxyglucose, thallium-201 and technetium-99m-MIBI cell incorporation under various cell stresses. J Nucl Med 1994; 35: 120-126. 
110. Kubota K, Yamada S, Ishiwata K, Ito M, Ido T. Positron emission tomography for treatment evaluation and recurrence detection compared with CT in long follow-up cases of lung cancer. Clin Nucl Med 1992; 17: 877-881. 
111. Kubota K, Yamada S, Ishiwata K, Ito M, Fujiwara T, Fukuda H, et al. Evaluation of the treatment response of lung cancer with PET and L-[methyl 11C]methionine: a preliminary study. Eur J Nucl Med 1993; 20: 495-501. 
112. Nuutinen J, Jyrkkio S, Lehikoinen P, Lindholm P, Minn H. Evaluation of early response to radiotherapy in head and neck cancer measured with [11C]methionine-positron emission tomography. Radioth Oncol 1999; 52: 225-232. 
113. Huovinen R, Leskinen-Kallio S, Nagren K, Lehikoinen P, Ruotsalainen U, Teras M. Carbon-11-methionine and PET in evaluation of treatment response of breast cancer. Brit J Cancer 1993; 67: 787-791. 
114. Patz EF, Lowe VJ, Hoffman JM, Paine SS, Harris LK, Goodman PC. Persistent or recurrent bronchogenic carcinoma: detection with PET and 2-[18F]-2-deoxy-D-glucose. Radiology 1994; 191 : 379-382. 
115. Inoue T, Kim E, Komaki R, Wong FCL, Basa P, Wong WH, et al. Detecting recurrent or residual lung cancer with FDG-PET. J Nucl Med 1995; 36: 788-793. 
116. Strauss LG, Clorius JH, Schlag P, Lehner B, Kimming B, Engenhart R, et al. Recurrence of colorectal tumors: PET evaluation. Radiology 1989; 170: 329-332.
117. Lowe VJ, Herbert ME, Hawk TC, Ansher MS, Coleman RE. Chest wall FDG accumulation in serial FDG-PET images in patients being treated for bronchogenic carci-noma with radiation (abstract). J Nucl Med 1994; 35: 76p. 
118. Reinhardt M. Kubota K, Yamada S. Iwata R, Yaegashi H. Assessment of cancer recurrence in residual tumors after fractionated radiotherapy: a comparison of fluorodeoxy-glucose. L-methionine and thymidine. J Nucl Med 1997; 38: 280-287. 
119. Bury Th, Corhay JL, Duysinx B, Daenen F, Ghaye B, Barthelemy N, et al. Value of FDG-PET in detecting residual or recurrent nonsmall cell lung cancer. Eur Respir J 1999; 14: 1376-1380. 
120. Anzai Y, Carroll WR, Quint DJ, Bradford CR, Minoshima S, Wolf GT, Wahl RL. Recurrence of head and neck cancer after surgery or irradiation: prospective comparison of 2-deoxy-2-[F-18]fluoro-l)-glucose PET and MR imaging diagnosis. Radiology 1996; 200: 135-141. 
121. Fischbein NJ, Assar S, Caputo GR, Kaplan MJ, Singer MI, Price DC, et al. Clinical utility of positron emission tomography with 18F-fluorodeoxyglucose in detecting residual/recurrent squamous cell carcinoma of the head and neck. Am J Neuroradiol 1998; 19: 1189-1196. 
122. Kao CH, ChangLai SP. Cheing PU, Yen RF, Yen TC. Detection of recurrent or persistent nasopharyngeal carcinomas after radiotherapy with 18-fluoro-2-deoxyglucose positron emission tomography and comparison with computed tomography. J Clin Oncol 1998; 16: 3550-3555. 
123. Kubota K. Yokoyama J, Ono S, Fukuda H, Yamaguchi K, Itoh M. Comparison of FDG-PET and MRI/CT in detecting recurrence of head and neck cancer. J Nucl Med 2001 ; 42: 290P. 
124. Rasey JS. Grunbaum Z, Magee S, Nelson HJ, Olive PL, Durand RE, Krohn KA. Characterization of radiolabeled fluoromisonidazole as a probe for hypoxic cells. Radiat Res 1987; 111 : 292-304. 
125. Rasey JS, Koh WJ, Evans ML, Peterson LM, Lewellen TK, Graham MM, Krohn KA. Quantifying regional hypoxia in human tumors with positron emission tomography of [F-18]fluoromisonidazole. Int J Radiat Oncol Biol Phys 1996; 36: 417-428. 
126. Kubota K, Tada M, Yamada S, Hori K, Saito S, Iwata R, et al. Comparison of 18F-fluoromisonidazole, deoxyglucose and methionine in tumour tissue distribution. Eur J Nucl Med 1999; 26: 750-757. 
127. Clavo AC. Brown RS, Wahl RL. Fluorodeoxyglucose uptake in human cancer cell lines is increased by hypoxia. J Nucl Med 1995; 36: 1625-1632. 
128. Minn H, Clavo AC, Wahl RL. Influence of hypoxia on tracer accumulation in squamous-cell carcinoma: in vitro evaluation for PET imaging. Nucl Med Biol 1996; 23: 941-946. 
129. Clavo AC, Wahl RL. Effects of hypoxia on the uptake of tritiated thymidine, L-leucine, L-methionine and FDG in cultured cancer cells. J Nucl Med 1996; 37: 502-506. 
130. Burgman P, O'Donoghue JA, Humm JL, Ling CC. Hypoxia-induced increase in FDG uptake in MCF7 cells. J Nucl Med 2001 ; 42: 170-175. 
131. Yang DJ, Wallace S, Cherif A, Li C. Gretzer MB, Kim EE, Podoloff DA. Development of F-18-labeled fluoro-erythronitroimidazole as a PET agent for imaging tumor hypoxia. Radiology 1995; 194: 795-800.