Dosimetry Study in Patients with Autonomous Thyroid Nodule Who Are Candidates for Radioiodine Therapy

A dosimetry study was performed on 26 patients with an autonomous thyroid nodule and suppressed serum thyroid-stimulating hormone, to determine the dose to extranodular tissue when the nodule receives 300 Gy for 131I therapy. Parameters of radioiodine turnover to be used in the dosimetry formula were separately obtained for the nodule and the contralateral lobe, as a measurable example of the extranodular tissue, using 55 MBq 123I and a computer-assisted gamma camera. The biologic half-life of 123I was then converted into the effective half-life of 131I, and the volumes of the nodule and the lobe were obtained by scintigraphy or sonography. The mean dose to the contralateral lobe from uptake and irradiation by the nodule was calculated to be 32 Gy, and that to the ipsilateral lobe was estimated to be 34 Gy. During radioiodine therapy for autonomous thyroid nodules, the extranodular tissue receives a higher dose than is generally assumed, which explains the relatively high rate of post-treatment hypothyroidism reported in the literature.