124I-PET dosimetry in advanced differentiated thyroid cancer:: Therapeutic impact

Purpose: This study evaluated the impact of (124)vertical bar-positron emission tomography (PET) dosimetry on post-primary surgery therapy in radioiodine-naive patients with advanced differentiated thyroid cancer (DTC). Patients, material, methods: In each of 28 thyroidectomized patients with high-risk DTC (one or more of pT4, pN1 or pM1), we gave 23-50 MBq of (124)vertical bar as on oral capsule and performed PET dosimetry to calculate the individualized therapeutic (131)vertical bar activity that would, insofar as possible, achieve a radio-iodine dose >= 100 Gyto all metastases without exceeding 2 Gy to the blood (a surrogate for bone morrow toxicity). We thus determined the absorbed lesion dose per GBq of administered (131)vertical bar activity (LDpA) based on serial PET (4, 24, 48, 72 and 96 h after oral (124)vertical bar intake) and PET/computed tomography (25 h after (124)vertical bar intake) and the critical blood activity (CBA) based on blood and whole-body radiation counting (2, 4, 24, 48, 72, 96 h after (124)vertical bar intake). We compared the dosimetry-based interventions with our standard empirical protocol. Results: 25 patients had a total of 126 iodine-positive metastases. 18 (72%) of the 25 had solely iodine-avid metastases, while seven (28%) had both iodine-avid and -non-avid metastases. In two patients (8%), none of the iodine-avid metastases could have been practically treated with a sufficient radiation dose. Relative to the empirical protocol, (124)vertical bar-PET dosimetry findings changed management in 7 (25%) patients, e. g. allowing application of activities > 11GBq (131)vertical bar. Further changes included implementation of hematological back-up in a patient found to be at risk of life-threatening morrow toxicity, and early multimodal therapy in 9 (32%) patients. Conclusion: (124)vertical bar-PET dosimetry is a useful routine procedure in advanced DTC and may allow safer or more effective radio-iodine activities and earlier multimodal interventions than do standard empirical protocols.