title: Dosimetric variation due to the photon beam energy in the small-animal irradiation: a Monte Carlo study.
abstract: OBJECTIVE
  The impact of photon beam energy and tissue heterogeneities on dose distributions and dosimetric characteristics such as point dose, mean dose, and maximum dose was investigated in the context of small-animal irradiation using Monte Carlo simulations based on the EGSnrc code.
  
  
  METHODS
  Three Monte Carlo mouse phantoms, namely, heterogeneous, homogeneous, and bone homogeneous were generated based on the same mouse computed tomography image set. These phantoms were generated by overriding the tissue type of none of the voxels (heterogeneous), all voxels (homogeneous), and only the bone voxels (bone homogeneous) to that of soft tissue. Phase space files of the 100 and 225 kVp photon beams based on a small-animal irradiator (XRad225Cx, Precision X-Ray Inc., North Branford, CT) were generated using BEAMnrc. A 360 degrees photon arc was simulated and three-dimensional (3D) dose calculations were carried out using the DOSXYZnrc code through DOSCTP in the above three phantoms. For comparison, the 3D dose distributions, dose profiles, mean, maximum, and point doses at different locations such as the isocenter, lung, rib, and spine were determined in the three phantoms.
  
  
  RESULTS
  The dose gradient resulting from the 225 kVp arc was found to be steeper than for the 100 kVp arc. The mean dose was found to be 1.29 and 1.14 times higher for the heterogeneous phantom when compared to the mean dose in the homogeneous phantom using the 100 and 225 kVp photon arcs, respectively. The bone doses (rib and spine) in the heterogeneous mouse phantom were about five (100 kVp) and three (225 kVp) times higher when compared to the homogeneous phantom. However, the lung dose did not vary significantly between the heterogeneous, homogeneous, and bone homogeneous phantom for the 225 kVp compared to the 100 kVp photon beams.
  
  
  CONCLUSIONS
  A significant bone dose enhancement was found when the 100 and 225 kVp photon beams were used in small-animal irradiation. This dosimetric effect, due to the presence of the bone heterogeneity, was more significant than that due to the lung heterogeneity. Hence, for kV photon energies of the range used in small-animal irradiation, the increase of the mean and bone dose due to the photoelectric effect could be a dosimetric concern.
authors.affiliation: Radiation Medicine Program, Princess Margaret Hospital, University of Toronto, Toronto, Ontario M5G 2M9, Canada. james.chow@rmp.uhn.on.ca
authors.forename: James C L
authors.initials: JC
authors.lastname: Chow
authors.forename: Michael K K
authors.initials: MK
authors.lastname: Leung
authors.forename: Patricia E
authors.initials: PE
authors.lastname: Lindsay
authors.forename: David A
authors.initials: DA
authors.lastname: Jaffray
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D055592:Biophysical Phenomena; D051379:Mice; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons / Q000627:therapeutic use*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D014057:Tomography, X-Ray Computed / Q000706:statistics & numerical data