title: Strengths and Weaknesses of a Planar Whole-Body Method of (153)Sm Dosimetry for Patients with Metastatic Osteosarcoma and Comparison with Three-Dimensional Dosimetry.
abstract: OBJECTIVE
  Dosimetric accuracy depends directly upon the accuracy of the activity measurements in tumors and organs. The authors present the methods and results of a retrospective tumor dosimetry analysis in 14 patients with a total of 28 tumors treated with high activities of (153)Sm-ethylenediaminetetramethylenephosphonate ((153)Sm-EDTMP) for therapy of metastatic osteosarcoma using planar images and compare the results with three-dimensional dosimetry.
  
  
  METHODS
  Analysis of phantom data provided a complete set of parameters for dosimetric calculations, including buildup factor, attenuation coefficient, and camera dead-time compensation. The latter was obtained using a previously developed methodology that accounts for the relative motion of the camera and patient during whole-body (WB) imaging. Tumor activity values calculated from the anterior and posterior views of WB planar images of patients treated with (153)Sm-EDTMP for pediatric osteosarcoma were compared with the geometric mean value. The mean activities were integrated over time and tumor-absorbed doses were calculated using the software package OLINDA/EXM.
  
  
  RESULTS
  The authors found that it was necessary to employ the dead-time correction algorithm to prevent measured tumor activity half-lives from often exceeding the physical decay half-life of (153)Sm. Measured half-lives so long are unquestionably in error. Tumor-absorbed doses varied between 0.0022 and 0.27 cGy/MBq with an average of 0.065 cGy/MBq; however, a comparison with absorbed dose values derived from a three-dimensional analysis for the same tumors showed no correlation; moreover, the ratio of three-dimensional absorbed dose value to planar absorbed dose value was 2.19. From the anterior and posterior activity comparisons, the order of clinical uncertainty for activity and dose calculations from WB planar images, with the present methodology, is hypothesized to be about 70%.
  
  
  CONCLUSIONS
  The dosimetric results from clinical patient data indicate that absolute planar dosimetry is unreliable and dosimetry using three-dimensional imaging is preferable, particularly for tumors, except perhaps for the most sophisticated planar methods. The relative activity and patient kinetics derived from planar imaging show a greater level of reliability than the dosimetry.
authors.affiliation: 1 Department of Radiation Oncology, Johns Hopkins University School of Medicine , Baltimore, Maryland.
authors.forename: Donika
authors.initials: D
authors.lastname: Plyku
authors.affiliation: 1 Department of Radiation Oncology, Johns Hopkins University School of Medicine , Baltimore, Maryland.
authors.forename: David M
authors.initials: DM
authors.lastname: Loeb
authors.affiliation: 1 Department of Radiation Oncology, Johns Hopkins University School of Medicine , Baltimore, Maryland.
authors.forename: Andrew R
authors.initials: AR
authors.lastname: Prideaux
authors.affiliation: 2 University Institute of Radiation Physics, University of Lausanne , Lausanne, Switzerland .
authors.forename: Sébastien
authors.initials: S
authors.lastname: Baechler
authors.affiliation: 1 Department of Radiation Oncology, Johns Hopkins University School of Medicine , Baltimore, Maryland.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Wahl
authors.affiliation: 1 Department of Radiation Oncology, Johns Hopkins University School of Medicine , Baltimore, Maryland.
authors.forename: George
authors.initials: G
authors.lastname: Sgouros
authors.affiliation: 1 Department of Radiation Oncology, Johns Hopkins University School of Medicine , Baltimore, Maryland.
authors.forename: Robert F
authors.initials: RF
authors.lastname: Hobbs
mesh_terms: D000293:Adolescent; D000328:Adult; D000465:Algorithms; D001859:Bone Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology; D002648:Child; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008297:Male; D009942:Organometallic Compounds / Q000493:pharmacokinetics*; D009943:Organophosphorus Compounds / Q000493:pharmacokinetics*; D012516:Osteosarcoma / Q000000981:diagnostic imaging* / Q000556:secondary; D019047:Phantoms, Imaging; D011874:Radiometry / Q000379:methods*; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000627:therapeutic use*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D015203:Reproducibility of Results; D012189:Retrospective Studies; D051598:Whole Body Imaging / Q000379:methods*; D055815:Young Adult
chemical_list: D009942:Organometallic Compounds; D009943:Organophosphorus Compounds; D019275:Radiopharmaceuticals; C061972:samarium Sm-153 lexidronam
keywords: dosimetry; imaging; osteosarcoma; radionuclide therapy; radiopharmaceuticals