Response Assessment of Bone Metastases from Differentiated Thyroid Cancer Patients in the Initial Radioiodine Treatment Using Iodine-124 PET Imaging

Iodine-positive bone metastases (BMs) are often resistant after initial radioiodine therapy applying the standard activity approach. A comprehensive lesion-based response study for BMs has not yet been performed. Pre-therapy and follow-up 124I PET/CT data on BMs from differentiated thyroid cancer patients were retrospectively analyzed to assess the relationship between absorbed radiation dose (AD) and response after initial radioiodine treatment. Methods: Before and after initial radioiodine therapy, patients received serial PET/CT scans after administration of 20–40 MBq 124I. The pre-therapy PET data were used to segment BM volumes and to predict the average ADs after administrations of dosimetry-guided 131I activities. The lower volume limit of determinability of the applied segmentation method was a sphere volume of 0.16 mL. This volume limit classified the BMs into known-volume and fixed-volume groups with their respective average and minimum ADs. Follow-up 124I and 18F-FDG PET/CT data after treatment were analyzed to assess lesion-based therapy response. Response rates at different AD thresholds were calculated and were expressed as the percentage of completely responding BMs above the respective AD threshold. BMs with maximum extensions greater than twice the PET spatial resolution were visually scored for nonuniformity. Results: A total of 61 BMs in 10 patients were included, of which 46 and 15 comprised the known-volume group and the fixed-volume group, respectively. The median follow-up time was 5.7 mo (range, 3.7–23.2 mo). The median average and median minimum ADs in therapy were 183 Gy (range, 39–3,600 Gy) and 270 Gy (range, 63–1,300 Gy), respectively. A range of response rate of 70-80% was achieved at an AD threshold range of 350–650 Gy. There were 26 BMs that were amenable to visual assessment of nonuniformity, of which two-thirds (17/26) were scored as clearly nonuniform, and the majority (11/17) of these nonuniform BMs responded incompletely. Conclusion: Both the high AD threshold associated with high response rates and the low median AD per unit 131I activity elucidate the difficulty in achieving a therapeutic efficacy for BMs when administering a single standard activity. The relatively high AD threshold range is possibly a result of distinct levels of spatial nonuniformity of ADs. by on August 27, 2017. For personal use only. jnm.snmjournals.org Downloaded from