title: Commissioning of a grid-based Boltzmann solver for cervical cancer brachytherapy treatment planning with shielded colpostats.
abstract: OBJECTIVE
  We sought to commission a gynecologic shielded colpostat analytic model provided from a treatment planning system (TPS) library. We have reported retrospectively the dosimetric impact of this applicator model in a cohort of patients.
  
  
  METHODS
  A commercial TPS with a grid-based Boltzmann solver (GBBS) was commissioned for (192)Ir high-dose-rate (HDR) brachytherapy for cervical cancer with stainless steel-shielded colpostats. Verification of the colpostat analytic model was verified using a radiograph and vendor schematics. MCNPX v2.6 Monte Carlo simulations were performed to compare dose distributions around the applicator in water with the TPS GBBS dose predictions. Retrospectively, the dosimetric impact was assessed over 24 cervical cancer patients' HDR plans.
  
  
  RESULTS
  Applicator (TPS ID #AL13122005) shield dimensions were within 0.4 mm of the independent shield dimensions verification. GBBS profiles in planes bisecting the cap around the applicator agreed with Monte Carlo simulations within 2% at most locations; differing screw representations resulted in differences of up to 9%. For the retrospective study, the GBBS doses differed from TG-43 as follows (mean value ± standard deviation [min, max]): International Commission on Radiation units [ICRU]rectum (-8.4 ± 2.5% [-14.1, -4.1%]), ICRUbladder (-7.2 ± 3.6% [-15.7, -2.1%]), D2cc-rectum (-6.2 ± 2.6% [-11.9, -0.8%]), D2cc-sigmoid (-5.6 ± 2.6% [-9.3, -2.0%]), and D2cc-bladder (-3.4 ± 1.9% [-7.2, -1.1%]).
  
  
  CONCLUSIONS
  As brachytherapy TPSs implement advanced model-based dose calculations, the analytic applicator models stored in TPSs should be independently validated before clinical use. For this cohort, clinically meaningful differences (>5%) from TG-43 were observed. Accurate dosimetric modeling of shielded applicators may help to refine organ toxicity studies.
authors.affiliation: The University of Texas Graduate School of Biomedical Sciences at Houston, Houston, TX; Department of Radiation Physics, The University of Texas MD Anderson Cancer, Houston, TX.
authors.forename: Justin K
authors.initials: JK
authors.lastname: Mikell
authors.forename: Ann H
authors.initials: AH
authors.lastname: Klopp
authors.forename: Michael
authors.initials: M
authors.lastname: Price
authors.forename: Firas
authors.initials: F
authors.lastname: Mourtada
mesh_terms: D001918:Brachytherapy / Q000295:instrumentation*; D012809:Colon, Sigmoid / Q000528:radiation effects; D004867:Equipment Design; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008962:Models, Theoretical; D011832:Radiation Injuries / Q000517:prevention & control; D011874:Radiometry / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012007:Rectum / Q000528:radiation effects; D012189:Retrospective Studies; D001743:Urinary Bladder / Q000528:radiation effects; D002583:Uterine Cervical Neoplasms / Q000532:radiotherapy*
keywords: (192)Ir; Acuros; Cervical cancer; Dose calculation; Shielded applicator