title: Model-based dose calculations for (125)I lung brachytherapy.
abstract: OBJECTIVE
  Model-baseddose calculations (MBDCs) are performed using patient computed tomography (CT) data for patients treated with intraoperative (125)I lung brachytherapy at the Mayo Clinic Rochester. Various metallic artifact correction and tissue assignment schemes are considered and their effects on dose distributions are studied. Dose distributions are compared to those calculated under TG-43 assumptions.
  
  
  METHODS
  Dose distributions for six patients are calculated using phantoms derived from patient CT data and the EGSnrc user-code BrachyDose. (125)I (GE Healthcare/Oncura model 6711) seeds are fully modeled. Four metallic artifact correction schemes are applied to the CT data phantoms: (1) no correction, (2) a filtered back-projection on a modified virtual sinogram, (3) the reassignment of CT numbers above a threshold in the vicinity of the seeds, and (4) a combination of (2) and (3). Tissue assignment is based on voxel CT number and mass density is assigned using a CT number to mass density calibration. Three tissue assignment schemes with varying levels of detail (20, 11, and 5 tissues) are applied to metallic artifact corrected phantoms. Simulations are also performed under TG-43 assumptions, i.e., seeds in homogeneous water with no interseed attenuation.
  
  
  RESULTS
  Significant dose differences (up to 40% for D(90)) are observed between uncorrected and metallic artifact corrected phantoms. For phantoms created with metallic artifact correction schemes (3) and (4), dose volume metrics are generally in good agreement (less than 2% differences for all patients) although there are significant local dose differences. The application of the three tissue assignment schemes results in differences of up to 8% for D(90); these differences vary between patients. Significant dose differences are seen between fully modeled and TG-43 calculations with TG-43 underestimating the dose (up to 36% in D(90)) for larger volumes containing higher proportions of healthy lung tissue.
  
  
  CONCLUSIONS
  Metallic artifact correction is necessary for accurate application of MBDCs for lung brachytherapy; simpler threshold replacement methods may be sufficient for early adopters concerned with clinical dose metrics. Rigorous determination of voxel tissue parameters and tissue assignment is required for accurate dose calculations as different tissue assignment schemes can result in significantly different dose distributions. Significant differences are seen between MBDCs and TG-43 dose distributions with TG-43 underestimating dose in volumes containing healthy lung tissue.
authors: Department of Physics, Carleton University, Ottawa, Ontario K1S 5B6, Canada. jsuther@physics.carleton.ca
authors: J G H
authors: JG
authors: Sutherland
authors: K M
authors: KM
authors: Furutani
authors: Y I
authors: YI
authors: Garces
authors: R M
authors: RM
authors: Thomson
mesh_terms: D001918:Brachytherapy / Q000379:methods*; D003198:Computer Simulation; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use; D008175:Lung Neoplasms / Q000532:radiotherapy*; D008954:Models, Biological; D015233:Models, Statistical; D011874:Radiometry / Q000379:methods*; D019275:Radiopharmaceuticals / Q000627:therapeutic use; D011878:Radiotherapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D016896:Treatment Outcome
chemical_list: D007457:Iodine Radioisotopes; D019275:Radiopharmaceuticals