Lung absorbed doses in radioiodine therapy of thyroid cancer patients with diffuse pulmonary metastases

Objectives The lungs bearing diffuse metastases are the important dose-limiting organ in radioiodine(I-131) therapy for thyroid cancer. The objective of the study was to estimate the absorbed doses to the lungs in I-131 therapy for thyroid cancer patients with diffuse lung metastases.

Methods Ten consecutive thyroid cancer patients with diffuse lung metastases undergoing I-131 therapy were prospectively recruited. Whole body planar quantitative scintigrams were acquired serially after administration of 185 MBq of I-131 and the time-activity curves of the lungs and the-remainder-of-body were derived. Assumed that β-electron emissions from I-131 deposited in the lungs were completely absorbed by diffuse metastatic lesions,and γ-photon emissions from I-131 deposited in the lungs and the-remainder-of-body irradiated normal lung parenchyma, and based on Medical Internal Radiation Dosimetry formalism of the Society of Nuclear Medicine, the absorbed doses to the lungs was calculated.

Results The median 24-h I-131 uptake in the lungs was 5.0% (1.0 % - 37.1%), and the median effective I-131 half-lives in the lungs and the-remainder-of-body were 33 h (15 h-107 h) and 16 h (8-25 h), respectively. The resulting lung absorbed doses ranged from 0.03 mGy/MBq to 1.11 mGy/MBq.

Conclusions Based on the empiric-fixed-activity method, limiting single treatment to 7.4 GBq, severe radiation-induced lung toxicity expected at normal lung absorbed doses of 25-27 Gy can be avoided in I-131 therapy for thyroid cancer patients with diffuse lung metastases. While the risk of lung fibrosis increases with higher cumulative amounts of I-131 given over an extended period.

Research Support National Natural Science Fund of China (grants 30870724 and 81071184