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 the S-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: Department of Medical Radiation Physics, Lund University, Lund 22185, Sweden and Department of Medical Physics, Gurutzeta/Cruces University Hospital, Barakaldo 48903, Spain.
authors: Pablo
authors: P
authors: Mínguez
authors: Joint Department of Physics, Royal Marsden NHS Foundation Trust and Institute of Cancer Research, Sutton SM2 5PT, United Kingdom.
authors: Glenn
authors: G
authors: Flux
authors: Department of Nuclear Medicine, Gurutzeta/Cruces University Hospital, Barakaldo 48903, Spain.
authors: José
authors: J
authors: Genollá
authors: Department of Nuclear Medicine, Gurutzeta/Cruces University Hospital, Barakaldo 48903, Spain.
authors: Alejandro
authors: A
authors: Delgado
authors: Department of Nuclear Medicine, Gurutzeta/Cruces University Hospital, Barakaldo 48903, Spain.
authors: Emilia
authors: E
authors: Rodeño
authors: Department of Medical Radiation Physics, Lund University, Lund 22185, Sweden.
authors: Katarina
authors: K
authors: Sjögreen Gleisner
mesh_terms: D001692:Biological Transport; D005260:Female; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use*; D008297:Male; D019047:Phantoms, Imaging; D011786:Quality Control; D011874:Radiometry; D000072098:Single Photon Emission Computed Tomography Computed Tomography; D012974:Sodium Iodide / Q000378:metabolism / Q000627:therapeutic use*; D013964:Thyroid Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*
chemical_list: D007457:Iodine Radioisotopes; D012974:Sodium Iodide