title: A general model for stray dose calculation of static and intensity-modulated photon radiation.
abstract: OBJECTIVE
  There is an increasing number of cancer survivors who are at risk of developing late effects caused by ionizing radiation such as induction of second tumors. Hence, the determination of out-of-field dose for a particular treatment plan in the patient's anatomy is of great importance. The purpose of this study was to analytically model the stray dose according to its three major components.
  
  
  METHODS
  For patient scatter, a mechanistic model was developed. For collimator scatter and head leakage, an empirical approach was used. The models utilize a nominal beam energy of 6 MeV to describe two linear accelerator types of a single vendor. The parameters of the models were adjusted using ionization chamber measurements registering total absorbed dose in simple geometries. Whole-body dose measurements using thermoluminescent dosimeters in an anthropomorphic phantom for static and intensity-modulated treatment plans were compared to the 3D out-of-field dose distributions calculated by a combined model.
  
  
  RESULTS
  The absolute mean difference between the whole-body predicted and the measured out-of-field dose of four different plans was 11% with a maximum difference below 44%. Computation time of 36 000 dose points for one field was around 30 s. By combining the model-calculated stray dose with the treatment planning system dose, the whole-body dose distribution can be viewed in the treatment planning system.
  
  
  CONCLUSIONS
  The results suggest that the model is accurate, fast and can be used for a wide range of treatment modalities to calculate the whole-body dose distribution for clinical analysis. For similar energy spectra, the mechanistic patient scatter model can be used independently of treatment machine or beam orientation.
authors.affiliation: Faculty of Science, University of Zurich, Zurich 8057, Switzerland and Radiotherapy Hirslanden, Hirslanden Medical Center, Aarau 5000, Switzerland.
authors.forename: Pascal
authors.initials: P
authors.lastname: Hauri
authors.affiliation: Radiotherapy Hirslanden, Hirslanden Medical Center, Aarau 5000, Switzerland.
authors.forename: Roger A
authors.initials: RA
authors.lastname: Hälg
authors.affiliation: Radiotherapy Hirslanden, Hirslanden Medical Center, Aarau 5000, Switzerland.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Besserer
authors.affiliation: Faculty of Science, University of Zurich, Zurich 8057, Switzerland and Radiotherapy Hirslanden, Hirslanden Medical Center, Aarau 5000, Switzerland.
authors.forename: Uwe
authors.initials: U
authors.lastname: Schneider
mesh_terms: D006801:Humans; D008954:Models, Biological; D009381:Neoplasms, Radiation-Induced; D019047:Phantoms, Imaging; D017785:Photons / Q000009:adverse effects / Q000627:therapeutic use*; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated; D012542:Scattering, Radiation