title: Lowest effective 131I activity for thyroid remnant ablation of differentiated thyroid cancer patients. Dosimetry-based model for estimation.
abstract: OBJECTIVE
  A theoretical dosimetry-based model was applied to estimate the lowest effective radioiodine activity for thyroid remnant ablation of low-risk differentiated thyroid cancer patients.
  
  
  METHODS
  The model is based on the distribution of the absorbed (radiation) dose per administered radioiodine activity and the absorbed dose threshold of 300 Gy for thyroid remnants, the level believed to destroy most thyroid remnants. For this purpose, ¹²⁴I PET/CT images of 49 thyroidectomised patients were retrospectively analysed to measure the distribution of the (average) absorbed doses to thyroid remnant per administered ¹³¹I activity. The fraction of thyroid remnants that received at least 300 Gy was determined for standard activities between 0.37 and 5.55 GBq. The lower activity was considered to be equally effective to that obtained with higher activity if the (absolute) fraction difference was below 5%.
  
  
  RESULTS
  A total of 62 thyroid remnants were included. The medians and ranges (in parentheses) for the absorbed dose per unit 131I activity were 359 Gy/GBq (34 to 1825 Gy/GBq). The fractions of thyroid remnants receiving more than 300 Gy at different therapy activities (within parentheses) were 60% (1.11 GBq), 76% (1.85 GBq), 79% (2.22 GBq), and 81-82% for activities between 2.59 and 3.70 GBq. The therapy activity of 1.11 GBq is considerably less effective than that of 1.85 or 2.22 GBq; therapy activities were equally effective in the range between 2.22 to 3.70 GBq.
  
  
  CONCLUSIONS
  On the basis of the model and the patients' data included, the lowest effective therapy activity appears to be approximately 2.2 GBq to ablate thyroid remnants. The results of this study may help to guide the design of prospective clinical studies.
authors.affiliation: Walter Jentzen, Klinik für Nuklearmedizin, Universität Duisburg-Essen, Hufelandstrasse 55, 45122 Essen, Germany, Tel. +49/(0)201/723 41 73, Fax + 49/(0)201/723 41 43, E-mail: walter.jentzen@uni-duisburg-essen.de.
authors.forename: W
authors.initials: W
authors.lastname: Jentzen
authors.forename: A-S
authors.initials: AS
authors.lastname: Moldovan
authors.forename: M
authors.initials: M
authors.lastname: Ruhlmann
authors.forename: R
authors.initials: R
authors.lastname: Görges
authors.forename: A
authors.initials: A
authors.lastname: Bockisch
authors.forename: S
authors.initials: S
authors.lastname: Rosenbaum-Krumme
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D003198:Computer Simulation; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D007457:Iodine Radioisotopes / Q000008:administration & dosage*; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D018365:Neoplasm, Residual; D019275:Radiopharmaceuticals / Q000008:administration & dosage; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D013964:Thyroid Neoplasms / Q000532:radiotherapy*; D016896:Treatment Outcome; D055815:Young Adult
chemical_list: D007457:Iodine Radioisotopes; D019275:Radiopharmaceuticals
keywords: Differentiated thyroid carcinoma; dosimetry; iodine-124; radioiodine therapy; thyroid remnant ablation