title: Monte Carlo dosimetric evaluation of high energy vs low energy photon beams in low density tissues.
abstract: OBJECTIVE
  Low megavoltage photon beams are often the treatment choice in radiotherapy when low density heterogeneities are involved, because higher energies show some undesirable dosimetric effects. This work is aimed at investigating the effects of different energy selection for low density tissues.
  
  
  METHODS
  BEAMnrc was used to simulate simple treatment set-ups in a simple and a CT reconstructed lung phantom and an air-channel phantom. The dose distribution of 6, 15 and 20 MV photon beams was studied using single, AP/PA and three-field arrangements.
  
  
  RESULTS
  Our results showed no significant changes in the penumbra width in lung when a pair of opposed fields were used. The underdosage at the anterior/posterior tumor edge caused by the dose build-up at the lung-tumor interface reached 7% for a 5 x 5 cm AP/PA set-up. Shrinkage of the 90% isodose volume was noticed for the same set-up, which could be rectified by adding a lateral field. For the CT reconstructed phantom, the AP/PA set-up offered better tumor coverage when lower energies were used but for the three field set-up, higher energies resulted to better sparing of the lung tissue. For the air-channel set-up, adding an opposed field reduced the penumbra width. Using higher energies resulted in a 7% cold spot around the air-tissue interface for a 5 x 5 cm field.
  
  
  CONCLUSIONS
  The choice of energy for treatment in the low density areas is not a straightforward decision but depends on a number of parameters such as the beam set-up and the dosimetric criteria. Updated calculation algorithms should be used in order to be confident for the choice of energy of treatment.
authors.affiliation: Medical Physics Department, Medical School, University of Thessalia, Larissa, Hellas, Greece. tsiakalos@med.uth.gr
authors.forename: Miltiadis F
authors.initials: MF
authors.lastname: Tsiakalos
authors.forename: Sotirios
authors.initials: S
authors.lastname: Stathakis
authors.forename: George A
authors.initials: GA
authors.lastname: Plataniotis
authors.forename: Constantin
authors.initials: C
authors.lastname: Kappas
authors.forename: Kiki
authors.initials: K
authors.lastname: Theodorou
mesh_terms: D000465:Algorithms; D006801:Humans; D008175:Lung Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D020266:Radiotherapy, Conformal; D011882:Radiotherapy, High-Energy