title: Comparison of TG-43 and TG-186 in breast irradiation using a low energy electronic brachytherapy source.
abstract: OBJECTIVE
  The recently updated guidelines for dosimetry in brachytherapy in TG-186 have recommended the use of model-based dosimetry calculations as a replacement for TG-43. TG-186 highlights shortcomings in the water-based approach in TG-43, particularly for low energy brachytherapy sources. The Xoft Axxent is a low energy (<50 kV) brachytherapy system used in accelerated partial breast irradiation (APBI). Breast tissue is a heterogeneous tissue in terms of density and composition. Dosimetric calculations of seven APBI patients treated with Axxent were made using a model-based Monte Carlo platform for a number of tissue models and dose reporting methods and compared to TG-43 based plans.
  
  
  METHODS
  A model of the Axxent source, the S700, was created and validated against experimental data. CT scans of the patients were used to create realistic multi-tissue/heterogeneous models with breast tissue segmented using a published technique. Alternative water models were used to isolate the influence of tissue heterogeneity and backscatter on the dose distribution. Dose calculations were performed using Geant4 according to the original treatment parameters. The effect of the Axxent balloon applicator used in APBI which could not be modeled in the CT-based model, was modeled using a novel technique that utilizes CAD-based geometries. These techniques were validated experimentally. Results were calculated using two dose reporting methods, dose to water (Dw,m) and dose to medium (Dm,m), for the heterogeneous simulations. All results were compared against TG-43-based dose distributions and evaluated using dose ratio maps and DVH metrics. Changes in skin and PTV dose were highlighted.
  
  
  RESULTS
  All simulated heterogeneous models showed a reduced dose to the DVH metrics that is dependent on the method of dose reporting and patient geometry. Based on a prescription dose of 34 Gy, the average D90 to PTV was reduced by between ~4% and ~40%, depending on the scoring method, compared to the TG-43 result. Peak skin dose is also reduced by 10%-15% due to the absence of backscatter not accounted for in TG-43. The balloon applicator also contributed to the reduced dose. Other ROIs showed a difference depending on the method of dose reporting.
  
  
  CONCLUSIONS
  TG-186-based calculations produce results that are different from TG-43 for the Axxent source. The differences depend strongly on the method of dose reporting. This study highlights the importance of backscatter to peak skin dose. Tissue heterogeneities, applicator, and patient geometries demonstrate the need for a more robust dose calculation method for low energy brachytherapy sources.
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology, Maastricht University Medical Center (MUMC), Maastricht 6201 BN, The Netherlands.
authors.forename: Shane A
authors.initials: SA
authors.lastname: White
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology, Maastricht University Medical Center (MUMC), Maastricht 6201 BN, The Netherlands.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Landry
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology, Maastricht University Medical Center (MUMC), Maastricht 6201 BN, The Netherlands and Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP, São Paulo CP 11049, 05422-970 Brazil.
authors.forename: Gabriel Paiva
authors.initials: GP
authors.lastname: Fonseca
authors.affiliation: Xoft, A Subsidiary of iCAD, Sunnyvale, California 94085-4115.
authors.forename: Randy
authors.initials: R
authors.lastname: Holt
authors.affiliation: Xoft, A Subsidiary of iCAD, Sunnyvale, California 94085-4115.
authors.forename: Thomas
authors.initials: T
authors.lastname: Rusch
authors.affiliation: Centre Hospitalier Universitaire de Québec Université Laval, Radio-Oncologie et Centre de Recherche en Cancérologie de l'Université Laval, Québec, Québec G1R 2J6 Canada.
authors.forename: Luc
authors.initials: L
authors.lastname: Beaulieu
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology, Maastricht University Medical Center (MUMC), Maastricht 6201 BN, The Netherlands and Department of Oncology, McGill University, Montreal, Quebec H3G 1A4, Canada.
authors.forename: Frank
authors.initials: F
authors.lastname: Verhaegen
authors.affiliation: Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology, Maastricht University Medical Center (MUMC), Maastricht 6201 BN, The Netherlands.
authors.forename: Brigitte
authors.initials: B
authors.lastname: Reniers
mesh_terms: D000388:Air; D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods*; D001940:Breast / Q000528:radiation effects; D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D003198:Computer Simulation; D006801:Humans; D008327:Mammography; D008954:Models, Biological; D009010:Monte Carlo Method; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012867:Skin / Q000528:radiation effects; D014057:Tomography, X-Ray Computed; D014867:Water
chemical_list: D014867:Water