title: A 3D photon superposition/convolution algorithm and its foundation on results of Monte Carlo calculations.
abstract: Based on previous publications on a triple Gaussian analytical pencil beam model and on Monte Carlo calculations using Monte Carlo codes GEANT-Fluka, versions 95, 98, 2002, and BEAMnrc/EGSnrc, a three-dimensional (3D) superposition/convolution algorithm for photon beams (6 MV, 18 MV) is presented. Tissue heterogeneity is taken into account by electron density information of CT images. A clinical beam consists of a superposition of divergent pencil beams. A slab-geometry was used as a phantom model to test computed results by measurements. An essential result is the existence of further dose build-up and build-down effects in the domain of density discontinuities. These effects have increasing magnitude for field sizes < or =5.5 cm(2) and densities < or = 0.25 g cm(-3), in particular with regard to field sizes considered in stereotaxy. They could be confirmed by measurements (mean standard deviation 2%). A practical impact is the dose distribution at transitions from bone to soft tissue, lung or cavities.
authors.affiliation: VARIAN Medical Systems International AG, Taefernstr. 7, CH-5405, Baden-Daettwil, Switzerland.|Max-Planck-Institute of Physics, Goettingen, Germany.
authors.forename: W
authors.initials: W
authors.lastname: Ulmer
authors.affiliation: VARIAN Medical Systems, Helsinki, Finland.
authors.forename: J
authors.initials: J
authors.lastname: Pyyry
authors.affiliation: VARIAN Medical Systems International AG, Taefernstr. 7, CH-5405, Baden-Daettwil, Switzerland.
authors.forename: W
authors.initials: W
authors.lastname: Kaissl
mesh_terms: D000465:Algorithms; D001822:Body Burden; D003198:Computer Simulation; D006801:Humans; D008954:Models, Biological; D015233:Models, Statistical; D009010:Monte Carlo Method; D017785:Photons / Q000627:therapeutic use; D011786:Quality Control; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D012062:Relative Biological Effectiveness