Abstract
Hypofractionated treatments generally increase the complexity of a treatment plan due to the more stringent constraints of normal tissues and target coverage. As a result, treatment plans contain more modulated MLC motions that may require extra efforts for accurate dose calculation. This study explores methods to minimize the differences between in‐house dose calculation and actual delivery of hypofractionated volumetric‐modulated arc therapy (VMAT), by focusing on arc approximation and tongue‐and‐groove (TG) modeling. For dose calculation, the continuous delivery arc is typically approximated by a series of static beams with an angular spacing of 2°. This causes significant error when there is large MLC movement from one beam to the next. While increasing the number of beams will minimize the dose error, calculation time will increase significantly. We propose a solution by inserting two additional apertures at each of the beam angle for dose calculation. These additional apertures were interpolated at two‐thirds’ degree before and after each beam. Effectively, there were a total of three MLC apertures at each beam angle, and the weighted average fluence from the three apertures was used for calculation. Because the number of beams was kept the same, calculation time was only increased by about 6%‐8%. For a lung plan, areas of high local dose differences (>4%) between film measurement and calculation with one aperture were significantly reduced in calculation with three apertures. Ion chamber measurement also showed similar results, where improvements were seen with calculations using additional apertures. Dose calculation accuracy was further improved for TG modeling by developing a sampling method for beam fluence matrix. Single element point sampling for fluence transmitted through MLC was used for our fluence matrix with 1 mm resolution. For Varian HDMLC, grid alignment can cause fluence sampling error. To correct this, transmission volume averaging was applied. For three paraspinal HDMLC cases, the average dose difference was greatly reduced in film and calculation comparisons with our new approach. The gamma (3%, 3 mm) pass rates have improved significantly from 74.1%, 90.0%, and 90.4% to 99.2%, 97.9%, and 97.3% for three cases, for calculation without volume averaging and calculation with volume averaging, respectively. Our results indicate that more accurate MLC leaf position and transmission sampling can improve accuracy and agreement between calculation and measurement, and are particularly important for hypofractionated VMAT that consists of large MLC movement.PACS number(s): 87.55.kd
Highlights
Volumetric-modulated arc therapy (VMAT) delivers dose continuously, while gantry and multiple leaf collimator (MLC) leaves move dynamically with varying dose rate
It was previously shown that dose calculation accuracy does not depend on the number of beams used in arc approximation, but it depends on the MLC aperture and monitor units (MU) variation between the neighboring beams.[3]. While most commercial treatment planning systems impose a maximum limit on MLC displacement, there is no constraint for MLC motion for VMAT planning using our in-house algorithm as an attempt to increase the degree of freedom in optimization
Our approach is simpler by using only two interpolated apertures between beams while maintaining our beam resolution at 2°. Another difference in our approach is that, for each static beam angle, MLC apertures are interpolated from the current beam, and its preceding and following nearest neighbors, such that the gantry angle used for ray-path calculation is centered in this small arc, whereas Bedford’s method uses only two beams for interpolation and the gantry angle used for ray-path calculation was the ending angle of the small arc
Summary
Volumetric-modulated arc therapy (VMAT) delivers dose continuously, while gantry and multiple leaf collimator (MLC) leaves move dynamically with varying dose rate. From the viewpoint of the target, Webb and McQuaid[4] found that ray paths from a rotating gantry can be approximated as coming from fixed orientations with small angular spacing of 5°. Based on this finding, we have developed a method to improve dose calculation accuracy by modifying the fluence at each CP by taking into account the additional fluence from two interpolated MLC apertures between adjacent CPs
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