Room: Exhibit Hall | Forum 9
Purpose: To develop a sliding window approach for improved VMAT dose calculation accuracy
Methods: VMAT plans were generated for ten patients in pinnacle using 4Â° gantry spacing without MLC movement constraint. For each patient, the VMAT was converted to a sliding window (SW) IMRT plan where each two consecutive control points ni and ni+1 in the arc were replaced by a fixed-gantry SW beam placed in the mid angle between ni and ni+1 and having two control points N1 and N2. N1 was given the same leaf positions of ni and a dose weight of zero while N2 was given the same leaf positions and dose weight of ni+1. Another VMAT plan was created with gantry spacing of 1Â° by interpolating the original VMAT beam. This plan was considered as the gold standard. The quality of the original, the interpolated and the SW plans were compared. The original plans were delivered on an Elekta Versa HD and measured with Mapcheck2 using an in-house developed sub-arc method. The dose calculation of the three plans were compared to the continuous delivery.
Results: The isodose distribution and DVH between the SW and interpolated plans were indistinguishable. However, there were significant differences between the original VMAT plan and either the SW or the interpolated plan, indicating that 4o gantry spacing without MLC movement constraint is inadequate. The average passing rate between the original VMAT plan and measurements was 84%. For both the interpolated and SW plans, the average passing rate was 96%.
Conclusion: A SW approach for VMAT dose calculation has been developed. The proposed method showed plan quality that closely approaches the continuous VMAT delivery without increase in dose calculation time.