Radiation dose to the testes after I-131 therapy for ablation of postsurgical thyroid remnants in patients with differentiated thyroid cancer

Radioiodine-131 is used in differentiated thyroid cancer (DTC) for ablation of postsurgical thyroid remnants and destruction of metastases. The questi on may be raised of whether I-131 treatment of DTC in male patients may giv e an irradiation dose to the testes that could impair fertility. Few data i n the literature concern the dose absorbed by the testes after I-131 therap y for DTC. Because I-131 kinetics may be altered by the hypothyroid conditi on commonly present at the time of treatment and by the radioiodinated iodo proteins released by the damaged thyroid tissue, the dose values reported i n the International Commission on Radiological Protection (ICRP) tables for euthyroid men may not be appropriate. To clarify this problem, three male subjects undergoing I-131 therapy for ablation of thyroid remnants shortly after thyroidectomy for DTC were studied. Methods: The mean administered ac tivity was 1256 MBq, and the duration of the Study was 2 wk. The gamma dose was measured by thermoluminescent dosimeters (TLDs) applied to the lower p oles of the testes. Correction factors were calculated for the distance of the TLD from the center of the testes and for attenuation by the testes of the gamma rays reaching the TLD. After correction, the gamma dose to the te stes ranged from 21 to 29 mGy. The gamma dose calculated by the Medical Int ernal Radiation Dose (MIRD) method from blood and urine samples was similar (18-20 mGy) to that measured by TLDs. The beta dose was estimated by the M IRD method from blood activity and testicular volume and ranged between 14 and 31 mGy. Results: The total (beta and gamma) doses to testes were 30, 33 and 43 mu Gy/MBq in the three subjects. Conclusion: These values are close to those derived from the ICRP tables (26-37 mu Gy/MBq I-131) for euthyroi d subjects. The present data indicate that significant irradiation is deliv ered to the testes after the administration of the I-131 ablative dose to t hyroidectomized patients. The relevance of the radiation absorbed by testes on fertility remains to be established.