87. Lesion dosimetry in metastatic thyroid cancer treated with 131I: Method and preliminary results

Purpose 131 I treatment of metastatic thyroid cancer should take into account doses to metastasis for an optimized approach as highlighted by EU 2013/59 Directive. A SPECT-TC dose calculation method is proposed and verified to standardize the operating protocol; patients’ preliminary results are shown. Methods SPECT-TC was calibrated with a phantom (spheres 11.5, 5.6, 1.1 ml, cylinder 130 ml) filled with liquid 131 I (13 MBq/ml). Acquisition (64views, circular-orbit, 256 × 256, Iterative SCAC) on successive days allowed partial volume effects, dead-time and detector radius (25 ÷ 33 cm) corrections to calculate the dose, whose accuracy was verified. For 8 patients after the therapeutic administration and for 2 patients also in pre-therapy phase 4 SPECT-TC (4 ÷ 6, 24, 48, 96 ÷ 144 h) were performed to calculate the dose to 30 lesions (24 bone,7 lymph node,6 pulmonary) with the MIRD spheres model. Results Calibration factors (cylinder 1.4 kcts/MBq) confirmed partial volume effects (130 ml:100%, 11 ml:75%, 5.5 ml:58%, 1.1 ml:6%) and radius dependence ( ∼ 10%). Dead-time strongly influences counts in early acquisition (up to 17 %). Mean accuracy on activity −15% [−9% ÷ −24%] and dose −13 % [−9% ÷ −18%] for volumes larger than 2.5 cc validates the dosimetric method, according to literature (MIRD24). Dose to lesions per activity unit varies among patients (mean ± 1dev.st 26  ± 38, range 0.1 ÷ 189 Gy/GBq) and within the same patient (up to +583 %) and uptake seems to decrease in following treatments: individualized dosimetry is needed. CT-based method is recommended for VOIs segmentation (mean – 4.9% compared to the isocontour threshold one). In-therapy dose values show high differences (up to −126%) compared to pre-therapy results, whose efficacy must be further investigated with more patients also in terms of optimal activity to be administered to simulate the treatment. Conclusions Lesion dosimetry of metastatic thyroid cancer treated with radioiodine is accurate with SPECT-TC and provides to the physician a fundamental tool to optimize the treatment. Download high-res image (119KB) Download full-size image Download high-res image (118KB) Download full-size image