title: Large-scale Retrospective Monte Carlo Dosimetric Study for Permanent Implant Prostate Brachytherapy.
abstract: To retrospectively compare water-based and full tissue model Monte Carlo dose calculations in a large cohort of patients undergoing125I permanent implant prostate brachytherapy.
  
  
  
  For 613 patients, EGSnrc BrachyDose dose calculations were performed in 2 virtual patient models: TG43sim (simulated American Association of Physicists in Medicine Task Group Report 43 conditions) and MCref (computed tomography-derived heterogeneous tissue model with interseed effects). A sensitivity analysis was performed in a patient subset (25 with and 25 without prostatic calcifications) to explore dose calculation dependence on organ-at-risk (OAR) and calcification tissue elemental compositions and modelling approach.
  
  
  
  In the target volume, the minimum radiation dose delivered to 90% of prostate (D90) (volume of prostate receiving at least 100% of prescription dose [V100]) was lower with MCref than with TG43sim by 5.9% ± 1.6% (2.6% ± 1.7%), on average. Patients with prostatic calcifications can have substantial underdosed volumes due to calcification shielding, lowering the D90by ≤25%. In the urethra, the average D5(D30) was lower with MCref than with TG43sim by 4.4% ± 1.8% (4.7% ± 1.9%). In the rectum (bladder), the minimum dose to the hottest 0.1 cm3(D_0.1cm3) of the contoured organ was lower (higher) with MCref than with TG43sim by 5.2% ± 1.8% (1.3% ± 1.8%). Doses to the target and OARs can increase or decrease by several percentages, depending on the assumed tissue elemental composition. In patients with calcifications, differences between approaches to model calcifications can change the target and OAR dose metrics by upward of 10%.
  
  
  
  TG43sim typically overestimates the target and OAR doses by several percentages, on average, compared with MCref. The considerable variation in the relative TG43sim and MCref doses between patients, and the larger dose differences for patients with calcification, suggests that clinical adoption of Monte Carlo dose calculations for permanent implant prostate brachytherapy should be pursued. The substantial sensitivity of the Monte Carlo dose calculations to the patient modelling approach supports the adoption of a consensus modelling scheme, such as MCref described in the present study, to ensure consistency of practice.
authors.affiliation: Carleton Laboratory for Radiotherapy Physics, Department of Physics, Carleton University, Ottawa, Ontario, Canada.
authors.forename: Nelson
authors.initials: N
authors.lastname: Miksys
authors.affiliation: Département de Physique et Centre de recherche sur le cancer, Québec, Québec, Canada; Département de Radio-oncologie et Centre de recherche du CHU de Québec, Québec, Québec, Canada.
authors.forename: Eric
authors.initials: E
authors.lastname: Vigneault
authors.affiliation: Département de Physique et Centre de recherche sur le cancer, Québec, Québec, Canada; Département de Radio-oncologie et Centre de recherche du CHU de Québec, Québec, Québec, Canada.
authors.forename: Andre-Guy
authors.initials: AG
authors.lastname: Martin
authors.affiliation: Département de Physique et Centre de recherche sur le cancer, Québec, Québec, Canada; Département de Radio-oncologie et Centre de recherche du CHU de Québec, Québec, Québec, Canada.
authors.forename: Luc
authors.initials: L
authors.lastname: Beaulieu
authors.affiliation: Carleton Laboratory for Radiotherapy Physics, Department of Physics, Carleton University, Ottawa, Ontario, Canada. Electronic address: rthomson@physics.carleton.ca.
authors.forename: Rowan M
authors.initials: RM
authors.lastname: Thomson
mesh_terms: D001918:Brachytherapy / Q000379:methods*; D002114:Calcinosis / Q000000981:diagnostic imaging; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use; D008297:Male; D009010:Monte Carlo Method; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D012007:Rectum / Q000000981:diagnostic imaging / Q000528:radiation effects; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D014521:Urethra / Q000000981:diagnostic imaging / Q000528:radiation effects; D001743:Urinary Bladder / Q000000981:diagnostic imaging / Q000187:drug effects
chemical_list: D007457:Iodine Radioisotopes