title: Extension of CadPlan algorithm to model the dose distribution under a motorized wedge.
abstract: The CadPlan treatment planning system models the dose distribution in the non-wedge direction under a wedged field by converting the wedge thickness to an equivalent water thickness. The algorithm estimates the off-axis ratio (OAR) in the non-wedged direction using the open field OAR at a depth deeper by this equivalent water thickness. This model has been shown to work well for a Siemens Mevatron KD-2 Linac. However, the motorized wedge of the Elekta (formerly Philips) accelerators is tapered off-axis to give a flat dose profile in the non-wedged direction. The CadPlan model assumes that the wedge has a uniform thickness in the non-wedged direction and so cannot model the off-axis dose for the motorized wedge. For a 4 MV beam of a SL75/5 accelerator this leads to a 7% overestimate and a 9% underestimate of the OAR under the thin and thick edge of the wedge respectively. For 6 and 18 MV beams of a SL20 accelerator and a 6 MV beam of a SL75/5 accelerator, the model underestimates the OAR in the order of 10% under the thick end of the wedge. We have shown that by appropriate modification of the effective water thickness values at off-axis distances, the algorithm models the OAR in the non-wedged direction to within 2.5% of the measured values for the 4, 6 and 18 MV beams, for the Elekta motorized wedge.
authors.affiliation: BC Cancer Agency, Cancer Centre for the Southern Interior, Kelowna, Canada.
authors.forename: R
authors.initials: R
authors.lastname: Rajapakshe
authors.forename: C
authors.initials: C
authors.lastname: Araujo
mesh_terms: D000465:Algorithms; D004867:Equipment Design; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation; D014867:Water
chemical_list: D014867:Water