RT Journal Article SR Electronic T1 124I PET Assessment of Response of Bone Metastases to Initial Radioiodine Treatment of Differentiated Thyroid Cancer JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 1499 OP 1504 DO 10.2967/jnumed.115.170571 VO 57 IS 10 A1 Walter Jentzen A1 Femke Verschure A1 Annelie van Zon A1 Rudie van de Kolk A1 Roel Wierts A1 Jochen Schmitz A1 Andreas Bockisch A1 Ina Binse YR 2016 UL http://jnm.snmjournals.org/content/57/10/1499.abstract AB 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, to our knowledge, yet been performed. In this study, pretherapy and follow-up 124I PET/CT data on BMs from differentiated thyroid cancer patients were retrospectively analyzed to assess the relationship between absorbed dose (AD) of radiation and response after initial radioiodine treatment. Methods: Before and after initial radioiodine therapy, patients underwent serial PET/CT scanning after administration of 20–40 MBq of 124I. The pretherapy PET data were used to segment BM volumes and to predict the average ADs after administration of dosimetry-guided 131I activity. 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 a maximum extent greater than twice the PET spatial resolution were visually scored for nonuniformity. Results: In total, 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 5.6 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 of 131I activity elucidate the difficulty in achieving therapeutic efficacy for BMs when a single standard activity is administered. The relatively high AD threshold range is possibly a result of distinct levels of spatial nonuniformity in ADs.