title: VMC++ validation for photon beams in the energy range of 20-1000 keV.
abstract: OBJECTIVE
  In high energy teletherapy, VMC++ is known to be a very accurate and efficient Monte Carlo (MC) code. In principle, the MC method is also a powerful dose calculation tool in other areas in radiation oncology, e.g., brachytherapy or orthovoltage radiotherapy. However, VMC++ is not validated for the low-energy range of such applications. This work aims in the validation of the VMC++ MC code for photon beams in the energy range between 20 and 1000 keV.
  
  
  METHODS
  Dose calculations were performed in different 40 x 40 x 40 cm3 phantoms of different materials. Dose distributions of monoenergetic (ranging from 20 to 1000 keV) 10 x 10 and 2 x 2 cm2 parallel beams were calculated. Voxel sizes of 4 x 4 x 4 and 1 x 1 x 1 mm3 were used for the dose calculations. The resulting dose distributions were compared to those calculated using EGSnrc, which is used as a golden standard in this work.
  
  
  RESULTS
  At energies between 100 and 1000 keV, EGSnrc and VMC++ calculated dose distributions agree within the statistical uncertainty of about 1% (1sigma). At energies < or = 50 keV, dose differences of up to 1.6% (in % of D(max)) occur when VMC++ and EGSnrc are compared. Turning off Rayleigh scattering, binding effects for Compton scattering, and the atomic relaxation after photoelectric absorption in EGSnrc (all not implemented in VMC++) leads to an agreement between both MC codes within statistical uncertainty. Further, using the KERMA approximation feature implemented in VMC++ leads to very efficient simulations in the energy range between 20 and 1000 keV.
  
  
  CONCLUSIONS
  Further improvements for very low energies in accuracy of VMC++ could be achieved by implementing Rayleigh scattering, binding effects for Compton scattering, and the atomic relaxation after photoelectric absorption. Implementation into VMC++ of KERMA approximation has been validated.
authors.affiliation: Division of Medical Radiation Physics, Inselspital and University of Berne, CH-3010 Berne, Switzerland. dario.terribilini@insel.ch
authors.forename: Dario
authors.initials: D
authors.lastname: Terribilini
authors.forename: Michael K
authors.initials: MK
authors.lastname: Fix
authors.forename: Daniel
authors.initials: D
authors.lastname: Frei
authors.forename: Werner
authors.initials: W
authors.lastname: Volken
authors.forename: Peter
authors.initials: P
authors.lastname: Manser
mesh_terms: D000465:Algorithms; D055592:Biophysical Phenomena; D003198:Computer Simulation; D006801:Humans; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons / Q000627:therapeutic use*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000706:statistics & numerical data*; D012542:Scattering, Radiation; D012878:Skin Neoplasms / Q000532:radiotherapy