title: 124I PET Assessment of Response of Bone Metastases to Initial Radioiodine Treatment of Differentiated Thyroid Cancer.
abstract: 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-up124I 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 of124I. The pretherapy PET data were used to segment BM volumes and to predict the average ADs after administration of dosimetry-guided131I 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-up124I and18F-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.
  
  
  CONCLUSIONS
  Both the high AD threshold associated with high response rates and the low median AD per unit of131I 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.
authors.affiliation: Klinik für Nuklearmedizin, Universität Duisburg-Essen, Essen, Germany walter.jentzen@uni-duisburg-essen.de.
authors.forename: Walter
authors.initials: W
authors.lastname: Jentzen
authors.affiliation: Fontys University of Applied Sciences, Eindhoven, The Netherlands; and.
authors.forename: Femke
authors.initials: F
authors.lastname: Verschure
authors.affiliation: Fontys University of Applied Sciences, Eindhoven, The Netherlands; and.
authors.forename: Annelie
authors.initials: A
authors.lastname: van Zon
authors.affiliation: Fontys University of Applied Sciences, Eindhoven, The Netherlands; and.
authors.forename: Rudie
authors.initials: R
authors.lastname: van de Kolk
authors.affiliation: Maastricht University Medical Centre, Maastricht, The Netherlands.
authors.forename: Roel
authors.initials: R
authors.lastname: Wierts
authors.affiliation: Klinik für Nuklearmedizin, Universität Duisburg-Essen, Essen, Germany.
authors.forename: Jochen
authors.initials: J
authors.lastname: Schmitz
authors.affiliation: Klinik für Nuklearmedizin, Universität Duisburg-Essen, Essen, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Bockisch
authors.affiliation: Klinik für Nuklearmedizin, Universität Duisburg-Essen, Essen, Germany.
authors.forename: Ina
authors.initials: I
authors.lastname: Binse
mesh_terms: D000328:Adult; D000368:Aged; D001859:Bone Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy* / Q000556:secondary; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use*; D008297:Male; D008875:Middle Aged; D000072078:Positron Emission Tomography Computed Tomography; D012189:Retrospective Studies; D013964:Thyroid Neoplasms / Q000473:pathology*; D016896:Treatment Outcome
chemical_list: D007457:Iodine Radioisotopes
keywords: 124I; bone metastases; dosimetry; radioiodine therapy; thyroid carcinoma