TECHNICAL NOTES 
Annals of Nuclear Medicine Vol. 12, No. 5, 303-306. 1998 
Determining the breast-feeding interruption schedule after administration of 123I-iodide 
Seiichiro MORITA,* Noriyoshi UMEZAKI,** Masatoshi ISHIBASHI,*Seiji KAWAMURA,*** Chizuko INADA**** and Naofumi HAYABUCHI* 
*Department of Radiology, **Radioisotope Institute for Basic and Clinical Medicine and ***Center for Diagnostic Imaging, Kurume University Hospital ****Fourth Department of Medicine, Kurume University School of Medicine 

Radioactivity after administration of 123I-sodium iodide was measured in breast milk samples obtained from a patient with postpartum thyroiditis. The breast milk was collected over 93 h during the infant's regular feeding times. The radioactivity in the breast milk was calculated with a 123I capsule of the same lot number as the standard source. 123I was excreted exponentially with an effective half-life of 5.5 h; 2.5% of the total radioactivity administered was excreted in the breast milk over the 93 h, 95% of which was excreted within the first 24 h, and 98,2% within 36 h. 
The first milk sample collected at 7 h after administration of the radiopharmaceutical contained 48.5% of the total radioactivity excreted. We estimated the potential absorption of radioactivity to an infant's thyroid in uninterrupted breast-feeding to be 30.3 mGy. With a 24-hour interruption, the 
absorbed radioactivity would be 1.25 mGy; with a 36-hour interruption, it would be 0.24 mGy. According to our calculations, breast feeding should be curtailed for 36 h to reduce the infant's exposure to 123I radioactivity. By using a correction factor based on maximum radioactivity from another 123I capsule of the same lot, we were able to ascertain the appropriate protocol for our patient and establish a measurement method that can be applied in similar clinical situations. 
Key words:  123I-sodium iodide, breast-feeding, calculation method, thyroid uptake, radiation exposure
INTRODUCTION 
IT is GENERALLY ACCEPTED that administration of radiopharmaceuticals to nursing mothers should be avoided if possible. When, however, nuclear medicine investigations must be performed, countermeasures must be taken to reduce exposure of the nursing child. Clinicians therefore advise the interruption of breast-feeding, and most measure the radioactivity excreted in the breast milk to determine when breast-feeding can be resumed. In the literature to date, only case reports of one or a few patients have addressed the issue of radioiodine exposure 
Received April 27, 1998, revision accepted August 26, 1998. For reprint contact: Seiichiro Morita, M.D., Department of Radiology, Kurume University School of Medicine, 67 Asahi-machi, Kurume 830-0003, JAPAN. 
E-mail: smorita@med.kurume-u.ac.jp 
in breast milk.1-6 Many reports are about 131I or 125I, and there is one case report about 123I sodium iodide.7 We measured radioactivity in the breast milk of a patient who underwent thyroid gland imaging and an uptake test with 123I sodium iodide to diagnose postpartum thyroiditis. The breast milk was collected for 5 days after administration of the radiopharmaceutical. To avoid exposing the infant to radioactivity, breast-feeding was interrupted during the collection period. We calculated the radioactivity in the excreted milk by using a correction factor based on 123I capsule of the same lot number as the standard source. 
PATIENT 
A 23-year-old woman with suspected hyperthyroidism was referred for a thyroid scan and uptake test. The patient noticed swelling of the anterior collar region when she was about 17 years of age, but she did not have it examined. On January 20, 1995, she delivered a healthy girl by natural childbirth. The patient was aware of general fatigue, dullness, palpitation, tachycardia and excessive sweating right after childbirth, and these symptoms progressed. 
On February 22, she noticed pressure-pain in the right thyroid gland, and she visited the Fourth Department of Medicine at Kurume University Hospital on February 24. Physical examination revealed a diffusely enlarged, soft and tender thyroid gland but no exophthalmos. Thyroid function tests were performed on an outpatient basis with 
the following results: FT3, 17.5 pg/ml (normal 2.7-4.5 pg/ml); FT4, 6.8 ng/dl (normal 1.08-1.86 ng/dl); TSH, < 0.1 uU/ml (normal 0.3-3.9 uU/ml); T.Chol, 111 mg/dl (normal 100-220 mg/dl); antimicrosomal antibodies, 1 : 400 (-100). Basedow's disease was suspected. The patient was hospitalized on March 9 for detailed examination and treatment. She was 48 days postpartum and was breast-feeding. Test results on admission were as follows: FT3, 12.0 pg/ml; FT4, 5.44 ng/dl; TSH, < 0.1 uU/ml; T.Chol, 102 mg/dl; Tg, 35 ng/ml; antimicrosomal antibodies, 1 : 400; TBII 3.7% (< 10); TSAb 131% (< 180). Thyroid gland scintigraphy with I-123 (9.2 MBq) sodium iodide was performed. The 5-h and 24-h uptake values were low, 1.74% and 0.22%, respectively, and the patient's thyroid gland could not be imaged. The thyroid hormone level decreased during bed rest; hence, temporary thyrotoxicosis after childbirth (postpartum thyroiditis) was suspected. Biopsy of the thyroid gland was performed on March 30 and revealed Hashimoto's disease but no subacute thyroiditis or acute exacerbation of Hashimoto's disease, so that the diagnosis of postpartum thyroiditis was confirmed . 
METHODS 
The breast milk was collected twice on the first day, with the first milk sample collected 7 h after administration of the radiopharmaceutical. Beginning on the second day, breast milk was obtained five times per day at the infant's regular feeding times of 7:00, 10:00, 13:00, 16:00, and 20:00. The last milk sample was obtained at 7:00 (93 h after administration) on day 5. As for the collection of breast milk, the patient manually expressed as much milk as possible herself, without an apparatus. The collection of breast milk and the measurement of radioactivity were done as follows. 1. Nursing was prohibited after taking of 2 capsules (total 9.2 MBq) of 123I sodium iodide. 
2. Breast milk was collected at the infant's regular feeding times for 5 days (until 93 h after 123I administration). a. The volume of each milk sample was measured, and the radioactivity per ml of milk was measured in a gamma-ray counter. 
b. A 3.9 MBq 123I capsule with the same lot number was dissolved in 100 ml of hot water in order to calculate the maximum radioactivity of a 1 ml milk sample and establish a correction factor for clinical measurement of excreted radioactivity. 
c. At the time of collection, excreted activity was calculated by multiplying the corrected radioactivity per ml by the volume of each milk sample. 
d. The excretion ratio was determined by dividing the calculated radioactivity in the milk sample by the total radioactivity administered. 
RESULTS 
Figure 1 gure shows the radioactivity levels in the breast milk at each collection point, indicating a steadily decreasing fraction per ml of milk. 123I was excreted expo-nentially with an effective excretion half-life of 5.5 h. Over the 5 days, 233 kBq was excreted in the breast milk, which was equivalent to about 2.5% of the total activity (9.2 MBq) administered to the mother: 2.1% on the first day, 0.368% on the second, 0.0191% on the third, 0.0011% on fourth, and 0.000055% on the fifth. Most importantly, 125.2 kBq of the 198.01 kBq in the breast milk collected on the first day was excreted in the first sample. This amount was equivalent to 48.5% of the total radioactivity for the 5 days (Table 1 ). The potential dose absorbed dose by the infant's thyroid gland was calculated according to the method of Hedrick.7 If the nursing had been continued over the 5 days, the absorbed dose could have been 30.3 mGy. Resumption after a 24-h interruption would have resulted in a 1.25 mGy absorbed dose. With 36-h and 48-h with-drawals, the absorbed dose would have been 0.24 mGy and 0.0059 mGy, respectively (Table 2). 
DISCUSSION 
The best possible approach should be taken to avoid exposure of a child to breast milk containing radioactivity after radiopharmaceutical administration to a lactating mother. With that purpose in mind, the quantity and time-course of radioactivity excreted in breast milk should be measured. In the literature, many authors do not describe in detail the method for measuring radioactivity. Hedrick et al.7 measured the radioactivity in breast milk with a calculation system calibrated to a 57Co standard by applying a correction for different emission frequencies, and the measured activity was corrected for decay in order to determine the activity at the time of collection. We, however, used a 123I capsule of the same lot number as that administered to the patient as the standard. The differences between radioactivity in the 123I capsule and that shown on the label of the radiopharmaceutical should be examined. The 123I capsule administered to the patient was marketed with about a +5.4% difference between actual radioactivity in the capsule and that specified on the label.8 And the difference in radioactivity between capsules was +-0.2%.8 We therefore considered that any capsule with the same lot number can be used as the standard radiation source. This is very convenient. As to the study of radiopharmaceutical defluxion in breast milk, there are many case reports. Comparing the reports to date regarding defiuxion in breast milk raises some distressing concerns because the collection methods and the activity determination methods vary across reports. In addition, even with similar administration and collection methods, the results differ widely. Therefore, establishing a clinical protocol for preventing infant exposure is difficult. In 1989, Mountford and Coakley4 established four interruption protocols for lactating mothers based on their quantitative analysis of radioactivity in breast milk: interruption not essential, interruption for a defined period, interruption with measurement, and total cessation. Rubow et al.5 recommended a fifth protocol: restriction of close contact between the mother and infant. 123I-sodium iodide is classed in the interruption with measurement protocol. In other reports, the total excreted radioactivity in the milk was usually calculated by using the activity per ml and assuming the standard breast milk output to be 850 ml per day,10 but we calculated the total radioactivity by using the total volume of the collected milk samples (about 400 ml/day on average in this study). A large difference in total excreted milk volume was noted between Hedrick's7 study and ours, but the percentage of total excreted activity was very similar: 2.6% in Hedrick's and 2.5% in ours. The effective excretion half-life was also similar: 5.8 h and 5.5 h respectively. These findings suggest that excreted activity correlates with administered radioactivity rather than excreted milk volume.5 
As for the absorption of 123I, the thyroid gland is the most sensitive organ, followed by the stomach wall; absorption is quite decreased in other organs. In a 1-year-old child, thyroid absorption is 56 mGy/MBq, and stomach wall absorption is 0.36 mGy/MBq.11 Accordingly, exposure to the thyroid gland is of chief concern. The potential absorbed doses for 123I excretion were calculated in our study as if the tracer was pure, but contaminations of 124I and 125I have been reported, and the relative absorption must be taken into account.2,4,12 Between 23 and 106 days' interruption of breast-feeding has 123I4 been recommended after administration of 3.7 MBq 123I,14 1.5 to 3 days' interruption after administration of cur-rently available 123I that contains contaminating 124I and 125I.13 
The 123I-sodium iodide administered to our patient was made by decay of 123Cs and 123Xe, which are produced by irradiating 124Xe with a proton beam.13 The preparation produced by this method contains 99.7% 123I and 0.3% or less 121Te, and does not contain 124I, 126I, 130I, 131I or 24Na.11 Because the target organ of 121Te is not the thyroid, the dose absorbed by the thyroid gland does not increase significantly. Even if this radionuclide is disseminated through the whole body, the exposure dose can be ignored, but if 123I produced by proton-irradiated-127I is administered, the level of 125I contamination should be measured.15 
Mountford4 recommended that for 123I, in which the fractional ingestion is less, although mothers can be advised before injection that feeding can be resumed 
within a few days, each patient should be instructed to interrupt feeding while milk samples are expressed and analyzed. Because the acquisition of milk samples is difficult with out-patients, a list of interruption periods for various radiopharmaceuticals has been used,3 but 1231 is not listed. Because 123I-sodium iodide was administered before assay time in our case, the patient absorbed more radioactivity (9.2 MBq) than usual. The thyroid gland could not be imaged scintigraphically. The 5-h and 24-h uptake ratios were 1.74% and 0.22%, respectively. Accordingly, the decrease in thyroid gland uptake adversely increased the excreted radioactivity in breast milk.7 Nevertheless, the dose absorbed by the thyroid gland of the infant after a 36-h interruption in feeding would have been acceptably low (0.24 mGy). Based on our calculations in this case, it appears that breast-feeding should be curtailed for 36 h with the administration of 123I sodium iodide. Further group studies should be performed to confirm this as the appropriate 123I protocol. 
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