title: Limitations of the TG-43 formalism for skin high-dose-rate brachytherapy dose calculations.
abstract: OBJECTIVE
  In skin high-dose-rate (HDR) brachytherapy, sources are located outside, in contact with, or implanted at some depth below the skin surface. Most treatment planning systems use the TG-43 formalism, which is based on single-source dose superposition within an infinite water medium without accounting for the true geometry in which conditions for scattered radiation are altered by the presence of air. The purpose of this study is to evaluate the dosimetric limitations of the TG-43 formalism in HDR skin brachytherapy and the potential clinical impact.
  
  
  METHODS
  Dose rate distributions of typical configurations used in skin brachytherapy were obtained: a 5 cm × 5 cm superficial mould; a source inside a catheter located at the skin surface with and without backscatter bolus; and a typical interstitial implant consisting of an HDR source in a catheter located at a depth of 0.5 cm. Commercially available HDR(60)Co and (192)Ir sources and a hypothetical (169)Yb source were considered. The Geant4 Monte Carlo radiation transport code was used to estimate dose rate distributions for the configurations considered. These results were then compared to those obtained with the TG-43 dose calculation formalism. In particular, the influence of adding bolus material over the implant was studied.
  
  
  RESULTS
  For a 5 cm × 5 cm(192)Ir superficial mould and 0.5 cm prescription depth, dose differences in comparison to the TG-43 method were about -3%. When the source was positioned at the skin surface, dose differences were smaller than -1% for (60)Co and (192)Ir, yet -3% for (169)Yb. For the interstitial implant, dose differences at the skin surface were -7% for (60)Co, -0.6% for (192)Ir, and -2.5% for (169)Yb.
  
  
  CONCLUSIONS
  This study indicates the following: (i) for the superficial mould, no bolus is needed; (ii) when the source is in contact with the skin surface, no bolus is needed for either (60)Co and (192)Ir. For lower energy radionuclides like (169)Yb, bolus may be needed; and (iii) for the interstitial case, at least a 0.1 cm bolus is advised for (60)Co to avoid underdosing superficial target layers. For (192)Ir and (169)Yb, no bolus is needed. For those cases where no bolus is needed, its use might be detrimental as the lack of radiation scatter may be beneficial to the patient, although the 2% tolerance for dose calculation accuracy recommended in the AAPM TG-56 report is not fulfilled.
authors: Department of Radiation Physics, ERESA, Hospital General Universitario, 46014 Valencia, Spain.
authors: Domingo
authors: D
authors: Granero
authors: Radiotherapy Department, La Fe University and Polytechnic Hospital, Valencia 46026, Spain.
authors: Jose
authors: J
authors: Perez-Calatayud
authors: Department of Atomic, Molecular and Nuclear Physics, University of Valencia, Burjassot 46100, Spain and IFIC (UV-CSIC), Paterna 46980, Spain.
authors: Javier
authors: J
authors: Vijande
authors: Department of Atomic, Molecular and Nuclear Physics, University of Valencia, Burjassot 46100, Spain.
authors: Facundo
authors: F
authors: Ballester
authors: Department of Radiation Oncology, Tufts University School of Medicine, Boston, Massachusetts 02111.
authors: Mark J
authors: MJ
authors: Rivard
mesh_terms: D001918:Brachytherapy / Q000379:methods*; D006801:Humans; D009010:Monte Carlo Method; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D012867:Skin / Q000528:radiation effects*