title: Whole-remnant and maximum-voxel SPECT/CT dosimetry in131I-NaI treatments of differentiated thyroid cancer.
abstract: OBJECTIVE
  To investigate the possible differences between SPECT/CT based whole-remnant and maximum-voxel dosimetry in patients receiving radio-iodine ablation treatment of differentiated thyroid cancer (DTC).
  
  
  METHODS
  Eighteen DTC patients were administered 1.11 GBq of131I-NaI after near-total thyroidectomy and rhTSH stimulation. Two patients had two remnants, so in total dosimetry was performed for 20 sites. Three SPECT/CT scans were performed for each patient at 1, 2, and 3-7 days after administration. The activity, the remnant mass, and the maximum-voxel activity were determined from these images and from a recovery-coefficient curve derived from experimental phantom measurements. The cumulated activity was estimated using trapezoidal-exponential integration. Finally, the absorbed dose was calculated using S-values for unit-density spheres in whole-remnant dosimetry and S-values for voxels in maximum-voxel dosimetry.
  
  
  RESULTS
  The mean absorbed dose obtained from whole-remnant dosimetry was 40 Gy (range 2-176 Gy) and from maximum-voxel dosimetry 34 Gy (range 2-145 Gy). For any given patient, the activity concentrations for each of the three time-points were approximately the same for the two methods. The effective half-lives varied (R = 0.865), mainly due to discrepancies in estimation of the longer effective half-lives. On average, absorbed doses obtained from whole-remnant dosimetry were 1.2 ± 0.2 (1 SD) higher than for maximum-voxel dosimetry, mainly due to differences in theS-values. The method-related differences were however small in comparison to the wide range of absorbed doses obtained in patients.
  
  
  CONCLUSIONS
  Simple and consistent procedures for SPECT/CT based whole-volume and maximum-voxel dosimetry have been described, both based on experimentally determined recovery coefficients. Generally the results from the two approaches are consistent, although there is a small, systematic difference in the absorbed dose due to differences in the S-values, and some variability due to differences in the estimated effective half-lives, especially when the effective half-life is long. Irrespective of the method used, the patient absorbed doses obtained span over two orders of magnitude.
authors.affiliation: Department of Medical Radiation Physics, LundUniversity, Lund 22185, Sweden and Department of Medical Physics, Gurutzeta/Cruces University Hospital, Barakaldo 48903, Spain.
authors.forename: Pablo
authors.initials: P
authors.lastname: Mínguez
authors.affiliation: Joint Department of Physics, Royal Marsden NHS Foundation Trust andInstitute of Cancer Research, Sutton SM2 5PT, United Kingdom.
authors.forename: Glenn
authors.initials: G
authors.lastname: Flux
authors.affiliation: Department of Nuclear Medicine, Gurutzeta/Cruces UniversityHospital, Barakaldo 48903, Spain.
authors.forename: José
authors.initials: J
authors.lastname: Genollá
authors.affiliation: Department of Nuclear Medicine, Gurutzeta/Cruces UniversityHospital, Barakaldo 48903, Spain.
authors.forename: Alejandro
authors.initials: A
authors.lastname: Delgado
authors.affiliation: Department of Nuclear Medicine, Gurutzeta/Cruces UniversityHospital, Barakaldo 48903, Spain.
authors.forename: Emilia
authors.initials: E
authors.lastname: Rodeño
authors.affiliation: Department of Medical Radiation Physics, LundUniversity, Lund 22185, Sweden.
authors.forename: Katarina
authors.initials: K
authors.lastname: Sjögreen Gleisner
mesh_terms: D002138:Calibration; D005260:Female; D006801:Humans; D007457:Iodine Radioisotopes / Q000008:administration & dosage*; D008297:Male; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D018714:Radiotherapy, Adjuvant / Q000379:methods; D000072098:Single Photon Emission Computed Tomography Computed Tomography / Q000295:instrumentation / Q000379:methods*; D012974:Sodium Iodide / Q000008:administration & dosage; D013964:Thyroid Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy* / Q000601:surgery; D013965:Thyroidectomy; D013997:Time Factors; D016896:Treatment Outcome
chemical_list: D007457:Iodine Radioisotopes; D012974:Sodium Iodide
keywords: 131I-NaI; Biological material, e.g. blood, urine; Haemocytometers; Calibration; Cameras; Cancer; Dose-volume analysis; Dosimetry; Dosimetry/exposure assessment; Germanium; Measuring radioactive content of objects, e.g. contamination (whole-body counters G01T011/63); Medical image spatial resolution; Numerical differentiation and integration; Radiation therapy; Scintigraphy; Single photon emission computed tomography; Single photon emission computed tomography
  (SPECT); Spatial resolution; Therapeutic applications, including brachytherapy; Tissues; biological organs; cancer; differentiated thyroid carcinoma; dosimetry; integration; iodine; phantoms; radiation therapy; single photon emission computed tomography; sodium compounds; thyroid bed remnants; tumours