Dose evaluation in iodine-131 therapy of hyperthyroidism with heterogeneous thyroid iodine distribution using SPECT images

Introduction: For I-131 therapy for hyperthyroidism, the Marinelli-Quimby formula is used to estimate the absorbed dose. This formula is a method for calculating the mean absorbed dose by the thyroid gland using thyroid volume, 24-h radioiodine uptake (RIU), and effective half-life (EHL), assuming a homogeneous distribution of iodine-131 (I-131). In Plummer's disease, I-131 uptake is high only in the autonomic nodules, resulting in heterogeneous I-131 uptake in the thyroid gland. Heterogeneous accumulation of I-131 in the thyroid gland results in over- or under-absorbption in parts of the thyroid compared with the absorbed dose of the entire thyroid gland predicted by the Marinelli–Quimby formula, which may affect treatment efficacy. In this study, we investigated the error when the mean absorbed dose was calculated for the entire thyroid gland or when the absorbed dose was calculated for the affected and unaffected sides to evaluate the inhomogeneity of the absorbed-dose distribution in the thyroid gland.

Methods: Thirteen patients with Plummer's disease and 32 patients with Graves’ disease were recruited and retrospectively evaluated. Prior to dosimetry, a thyroid phantom containing I-131 (520 MBq) was routinely imaged for two months using single-photon emission computed tomography (SPECT). The results were used to obtain a conversion equation for calculating radioactivity based on SPECT counts. We calculated the thyroid volume, 24-h RIU, and EHL to obtain the absorbed doses of the entire thyroid, right lobe, and left lobe using the Marinelli–Quimby formula. The volumes of the entire thyroid gland, right lobe, and left lobe were calculated using X-ray computed tomography (CT) images. SPECT images taken 24 h after I-131 administration were used to calculate the 24-h RIU. The regions of interest were the entire thyroid, left lobe, and right lobe of the SPECT images, and the total counts obtained were converted to radioactivity and divided by the administered radioactivity to calculate the 24-h RIU. EHL was calculated by approximating the RIUs calculated from SPECT images taken at 24 and 168 h as a mono-exponential function. In patients with Plummer's disease with an autonomous nodule in one lobe, the mean absorbed dose on the affected and unaffected sides was compared with that of the entire thyroid gland, and errors were calculated. In patients with Graves' disease who did not have a nodule, the mean absorbed dose in the right and left lobes was compared to that in the entire thyroid gland, and the error was calculated.

Results: In Plummer's disease, the mean error of the absorbed dose on the affected side was 24.1 Gy and −55.4 Gy on the unaffected side compared with the mean absorbed dose for the entire thyroid gland. In contrast, in patients with Graves' disease, the mean error of the absorbed dose on the right lobe was 0.56 Gy and that on the left lobe was −0.53 Gy compared to the mean absorbed dose of the entire thyroid gland. Compared to patients with Graves’ disease, patients with Plummer’s disease showed a large error, with a maximum error of 87.7 Gy on the affected side and −162 Gy on the unaffected side.

Conclusions: In Plummer's disease with autonomic nodules, the mean error of the absorbed dose on the affected side was 24.1 Gy and −55.4 Gy on the unaffected side compared with the mean absorbed dose for the entire thyroid gland. In cases with heterogeneous uptake of I-131 in the thyroid gland, the conventional method of calculating the mean absorbed dose for the entire thyroid gland results in large errors; thus, it is necessary to calculate the absorbed dose considering the heterogeneous I-131 uptake.