IMPROVING DOSE CALCULATION ACCURACY AND PLANNING EFFICIENCY OF VOLUMETRIC MODULATED ARC THERAPY

Abstract

Most modern treatment planning systems (TPS) approximate Volumetric Modulated Arc therapy (VMAT) with static beams which results in dosimetric errors. The first part of this work focused on developing a sliding-window (SW) methodology for more accurate VMAT dose calculation. In this method, for any two adjacent VMAT control points (CPs) n and n+1, dose distribution was approximated by a 2-CP SW IMRT beam with the gantry angle fixed in the middle of the two VMAT CPs. VMAT plans were generated in Pinnacle using 4° gantry spacing and then were converted to SW IMRT plans. The SW plans showed more accurate dose calculation when compared with the original plan. The average passing rate between the SW plan and measurements was 96%. The dose calculation times of the original VMAT plans and the SW plans were similar. The second part of this work focused on developing a planning strategy for head and neck VMAT plans using the commercial TPS. Head and neck plans with different MLC motion constraints (0.3, 0.46, 1 & 2 cm/⁰) and gantry spacing of 4⁰ were generated and compared with the proposed SW method calculation for dosimetric accuracy. It was concluded that for best compromise between planning efficiency and plan accuracy, optimization with 4⁰ gantry spacing followed by post-optimization interpolation to 2⁰ is necessary. The third part of this work investigated the sensitivity of dose accuracy on the gantry angles with SW method and the potential to further reduce dose calculation time by using larger gantry spacing. SW plans with 8⁰-gantry spacing were created and compared to VMAT plans with 1⁰ gantry spacing. It was found that using SW with 8⁰ gantry spacing did not sacrifice dose calculation accuracy. We conclude that the proposed SW approach improved VMAT dose calculation accuracy with the potential to improve planning efficiency.