Decreased uptake after fractionated ablative doses of iodine-131

In an attempt to obviate the necessity for hospitalisation, the ablative dose of 131I in the treatment of thyroid cancer is divided into two or three fractions at weekly intervals in some hospitals with no special bed for 131I treatment. Thyroid stunning has been observed in patients receiving a 131I dose between 74 and 370 MBq (2-10 mCi). However, the influence of 131I uptake after administration of a higher dose, such as 1,110-1,850 MBq of 131I, has never been reported. In this study, we evaluated the degree of reduction in 131I uptake after patients received 1,480 MBq of 131I and evaluated the clinical value of fractionated ablative doses of 131I.

Thirty-five patients with functional thyroid cancer received a total of 4,440 MBq (120 mCi) of 131I which was divided into three fractions administered at weekly intervals. In all patients two 131I whole-body scans were performed. The first scan was performed directly prior to the second dose of 131I (7 days after the first administration of 131I), and the second scan was performed 7 days after the second administration of 131I and directly prior to the third administration. Regions of interest including the neck and lungs were drawn to calculate the uptake of 131I in the thyroid remnant and possible cervical lymph node and lung metastases.

The mean uptake of 131I was 2.73% 7 days after the first administration, and decreased significantly to 0.26% 7 days after the second administration. The mean decrease was as high as 80.7%. The decrease in 131I uptake was significant in all patients except the two with lung metastases. In the two patients with lung metastases, no definite evidence of decreased uptake was noted; the uptake of 131I in the lung metastases even increased on the second 131I image in one of these patients. After administration of 1,480 MBq of 131I, the decreased uptake was significant in all neck lesions but not in lung metastases.

The use of fractionated ablative doses of 131I is not to be recommended in patients without lung metastases. However, the influence of fractionated ablative doses of 131I in patients with lung metastases is worthy of further study.