title: Estimation of organs doses and radiation-induced secondary cancer risk from scattered photons for conventional radiation therapy of nasopharynx: a Monte Carlo study.
abstract: OBJECTIVE
  We used Monte Carlo modeling to calculate the organs doses due to out-of field photons during radiation therapy of the nasopharynx.
  
  
  METHODS
  A medical internal radiation dose (MIRD)-based mathematical phantom resembling an adult man was modeled by MCNP4C MC code. Three validated models of a cobalt-60 machine, a 6-MV photon beam of a Varian 2300 C/D linac, and a 9-MV photon beam of a Neptun linac were used to simulate the isocentric irradiation of a mathematical phantom with two lateral fields of the nasopharynx. The organspecific dose, effective dose, and cancer risk estimates were obtained.
  
  
  RESULTS
  The effective doses for out-of-field radiation were 320, 295, and 248 mSv for the (60)Co beam, 6-MV beam, and 9-MV beam devices, respectively, for a 70-Gy tumor dose. The fatal cancer risks of 1.6%, 1.5%, and 1.2% were estimated for a 70-Gy tumor dose of (60)Co and the 6- and 9-MV photon beams, respectively.
  
  
  CONCLUSIONS
  Our results regarding the effective dose and cancer risk are in agreement with previously published experimental results on conventional radiation therapy. Further investigation on patients' out-of-field dose to provide more knowledge on various radiotherapy techniques is suggested.
authors.affiliation: Medical Physics Department, Tabriz University of Medical Sciences, Tabriz, Iran. asgharmesbahi@yahoo.com
authors.forename: Asghar
authors.initials: A
authors.lastname: Mesbahi
authors.forename: Farshad
authors.initials: F
authors.lastname: Seyednejad
authors.forename: Amir
authors.initials: A
authors.lastname: Gasemi-Jangjoo
mesh_terms: D008962:Models, Theoretical; D009010:Monte Carlo Method; D009303:Nasopharyngeal Neoplasms / Q000532:radiotherapy*; D009381:Neoplasms, Radiation-Induced / Q000209:etiology*; D017785:Photons; D011829:Radiation Dosage; D012542:Scattering, Radiation