Title:
Impact of the number of discrete angles used during dose computation for TomoTherapy treatments.

Abstract:
To quantify systematically the effect on accuracy of discretizing gantry rotation during the dose calculation process of TomoTherapy treatments.
Up to version 4.0.x included, TomoTherapy treatment planning system (TPS) approximates gantry rotation by computing dose from 51 discrete angles corresponding to the center of the projections used to control the binary multileaf collimator. Potential effects on dose computation accuracy for off-axis targets and low modulation factors have been shown previously for a few treatment configurations. In versions 4.1.x and later, TomoTherapy oversamples the projections to better account for gantry rotation, but only during full scatter optimization and final calculation (i.e., not during optimization in "beamlet" mode). The effect on accuracy of changing the number of angles was quantified with the following framework: (1) predict the impact of the discretization of gantry rotation for various modulation factors, target sizes, and off-axis positions using a simplified analytical algorithm; (2) perform regular quality assurance using measurements with EDR2 radiographic films; (3) isolating the effect of changing the number of discretized angles only (51, 153, and 459) using a previously validated Monte Carlo model (TomoPen). The diameters of the targets were 2, 3, and 5 cm; off-axis central positions of target volumes were 5, 10 and 15, and 17 cm (when accepted by the treatment unit); planned modulation factors were 1.3 and 2.0.
For extreme configurations (3 cm tumor, 1.3 modulation factor, 15 cm off-axis position), effects on dose distributions were significant with 89.3% and 95.4% of the points passing gamma tests with 2%∕2 mm and 3%∕3 mm criteria, respectively, for TPS software version 4.0.x (51 gantry angles). The passing rate was 100% for both gamma criteria for the 4.1.x version (153 gantry angles). Those differences could be attributed almost completely to gantry motion discretization using TomoPen. Using 51 gantry angles for dose computation, TomoPen reproduced within statistical uncertainties (<1% standard deviation) dose distributions computed with version 4.0.x. Using 153 and 459 gantry angles, TomoPen reproduced within statistical uncertainties measurements and dose distributions computed with version 4.1.x.
When low modulation factors and significant off-axis positions are used, accounting for gantry rotation during dose computation using at least 153 gantry angles is required to ensure optimal accuracy.