Purpose: To estimate personalized dose from radioiodine therapy for thyroid cancer patients with pulmonary metastases and enrolled in a multicentric clinical trial. This trial aims at evaluating the efficacy of re-differentiation therapy on BRAF-V600E-mutant thyroid cancers, applied prior to administration of iodine-131 (5.5 GBq). This work reports methods and results for lesion dosimetry.
Methods: Biokinetics of I-131 in lesions was evaluated using whole-body planar images acquired daily from day 1 to 4 after I-131 administration. Activity in lesions was quantified at day 4 using the SPECT/CT image and calculated at days 1 to 3 by planar-to-SPECT calibration at day 4. Time-activity curves were obtained by mono-exponential regression to compute the cumulated activity. Absorbed dose was estimated using a Monte Carlo radiation transport code in a realistic model of the patient’s body. This model was generated using 3DSlicer, by segmentation from the diagnostic CT acquired a few days before I-131 administration, of volumes of interest (lesions, lungs, and external contours) ; and imported as a mask to our in-house dosimetry software. This tool enabled (i) creation of a voxelized phantom of the patient’s anatomy, (ii) attribution of cumulated activity in lesions, (iii) generation of input file to the Monte Carlo code, and (iv) extraction of dose results from the input file. S-values were calculated and compared with those from IDAC-Iodide.
Results: Absorbed doses obtained for 5 patients and 19 lesions varied from 20 to 2900 Gy. S-values were comparable with those from IDAC-Dose2.1, while remaining slightly smaller. Differences tended to increase when lesion volume decreases.
Conclusion: These dose estimates can be used to evaluate the correlation with the response to treatment and the method can be extended to the calculation of heterogeneous dose distribution in lungs. Results may contribute to a better understanding of the effects of radiopharmaceutical therapy.
Funding Support, Disclosures, and Conflict of Interest: The French Ministry of Health financed the clinical trial. Dabrafenib and Trametinib were provided by Novartis, rhTSH was provided by Sanofi Genzyme.