title: Semi-analytical model for output factor calculations in proton beam therapy with consideration for the collimator aperture edge.
abstract: In the development of an external radiotherapy treatment planning system, the output factor (OPF) is an important value for the monitor unit calculations. We developed a proton OPF calculation model with consideration for the collimator aperture edge to account for the dependence of the OPF on the collimator aperture and distance in proton beam therapy. Five parameters in the model were obtained by fitting with OPFs measured by a pinpoint chamber with the circular radiation fields of various field radii and collimator distances. The OPF model calculation using the fitted model parameters could explain the measurement results to within 1.6% error in typical proton treatment beams with 6- and 12 cm SOBP widths through a range shifter and a circular aperture more than 10.6 mm in radius. The calibration depth dependences of the model parameters were approximated by linear or quadratic functions. The semi-analytical OPF model calculation was tested with various MLC aperture shapes that included circles of various sizes as well as a rectangle, parallelogram, and L-shape for an intermediate proton treatment beam condition. The pre-calculated OPFs agreed well with the measured values, to within 2.7% error up to 620 mm in the collimator distance, though the maximum difference was 5.1% in the case of the largest collimator distance of 740 mm. The OPF calculation model would allow more accurate monitor unit calculations for therapeutic proton beams within the expected range of collimator conditions in clinical use.
authors.affiliation: Proton Therapy Division, Shizuoka Cancer Center Hospital, 1007, Shimonagakubo, Nagaizumi-cho, Shizuoka 411-8777, Japan. Proton Therapy Division, Shizuoka Cancer Center Research Institute, 1007, Shimonagakubo, Nagaizumi-cho, Shizuoka 411-8777, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Kase
authors.forename: Haruo
authors.initials: H
authors.lastname: Yamashita
authors.forename: Makoto
authors.initials: M
authors.lastname: Sakama
authors.forename: Manabu
authors.initials: M
authors.lastname: Mizota
authors.forename: Yoshikazu
authors.initials: Y
authors.lastname: Maeda
authors.forename: Yuji
authors.initials: Y
authors.lastname: Tameshige
authors.forename: Shigeyuki
authors.initials: S
authors.lastname: Murayama
mesh_terms: D000465:Algorithms; D002138:Calibration; D003198:Computer Simulation; D006801:Humans; D008954:Models, Biological; D008962:Models, Theoretical; D009010:Monte Carlo Method; D061766:Proton Therapy; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012062:Relative Biological Effectiveness; D012542:Scattering, Radiation