title: A Monte Carlo approach to lung dose calculation in small fields used in intensity modulated radiation therapy and stereotactic body radiation therapy.
abstract: OBJECTIVE
  In the current study, the effect of electronic disequilibrium on lung dose with small photon beams was verified.
  
  
  METHODS
  The central axis absorbed dose in lung phantom was calculated by Monte Carlo (MC) method. The 6 and 18 MV photon beams of Varian Clinac 2100EX were simulated using MCNPX MC Code (Los Alamos national lab, USA). The MC model was used to calculate the depth doses water and low density water resembling the soft-tissue and lung, respectively. Four small field sizes including 0.5 cm2×0.5 cm2, 1 cm2×1 cm2, 2 cm2×2 cm2, and 3 cm2×3 cm2 were used in this study.
  
  
  RESULTS
  Percentage of dose reduction in lung region relative to homogenous phantom for 6 MV photon beam were 44.6%, 39%, 13%, and 7% for 0.5 cm2×0.5 cm2, 1 cm2×1 cm2, 2 cm2×2 cm2, and 3 cm2×3 cm2 fields, respectively. For 18 MV photon beam, the results were found to be 82%, 69%, 46%, and 25.8% for the same field sizes, respectively. The solid tumor dose inside lung was reduced considerably between 17% and 35% for 18 MV beam, while there was only 9% dose reduction for tumor dose for 0.5 and 1 cm field sizes.
  
  
  CONCLUSIONS
  Our study showed that the dose reduction with small fields in the lung was very enormous. Thus, inaccurate prediction of absorbed dose inside lung and also lung soft-tissue interfaces with small photon beams may lead to critical consequences for treatment outcome.
authors.affiliation: Department of Medical Physics, Medical School, Tabriz University of Medical Sciences; Department of Radiation Oncology, Imam Hospital, Tabriz, Iran.
authors.forename: Asghar
authors.initials: A
authors.lastname: Mesbahi
authors.forename: Habib
authors.initials: H
authors.lastname: Dadgar
authors.forename: Nahideh
authors.initials: N
authors.lastname: Ghareh-Aghaji
authors.forename: Mohammad
authors.initials: M
authors.lastname: Mohammadzadeh
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D004583:Electrons; D006801:Humans; D008168:Lung / Q000473:pathology / Q000528:radiation effects; D008175:Lung Neoplasms / Q000532:radiotherapy*; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D015203:Reproducibility of Results; D014867:Water
chemical_list: D014867:Water