Dosimetry implications of VMAT delivery angles and intra-fraction motion for SBRT prostate treatments

For a cohort of retrospectively treated patients with prostate carcinoma using a Stereotactic body radiation therapy (SBRT) technique and with implanted localization fiducials, this study determined whether there were any correlations between theoretical fiducial visibility using intra-fraction megavoltage imaging and intra-fraction motion induced dosimetric effects. Treatment planning data for 20 retrospective patients that used a SBRT prostate technique were investigated in this study. An in-house script was created that split each of the 360-degree volumetric modulated arc therapy arcs into 12 sectors, with each sector being 30 degrees. The script created a total of 24 sectors for each SBRT plan ranging from 180 to 210 degrees to 180 to 150 degrees. Resulting data was assessed to determine whether there was dosimetric impact from intra-fractional prostate motion and if it correlated with the theoretical fiducial visibility. Forty percent of the 4 highest CTV D_98% mean dose differences occurred within the angles of 240 and 270 degrees and 25% occurred between the angles of 90 and 120 degrees. The 4 highest average percentage differences in PTV D_98% coverage during angular sectors of 270 to 240 degrees, 90 to 120 degrees, 240 to 270 degrees and 60 to 90 degrees were -1.19%, -1.14%, -1.10%, and 1.01% respectively. It was also the case for the PTV D_95%in sectors 90 to 120 degrees, 240 to 270 degrees, 270 to 240 degrees and 270 to 300 degrees, which reduced by -0.97%, -0.93%, -0.92%, and -0.82% respectively. From the 4 highest rectal dose differences in V_32Gy and V_18Gy, it was found that 50% of the highest rectum V_32Gyand V_18Gy dose increases occurred between the angles of 90 and 120 degrees and 37.5% occurred between the angles of 240 and 270 degrees. The MU for each sector showed that 240_270, 240_210, 270_240, and 120_90 had the highest average MU with 150.8, 134.6, 129.2, and 124.3 respectively. This study demonstrated that there is a high correlation between dosimetric impact of intra-fractional motion and the theoretical fiducial visibility. As a result, modification to treatment plans to enable fiducial visibility at all angular sectors throughout a treatment may not be required. Further sector analysis tests are required to develop patient specific megavoltage imaging gantry angles for SBRT prostate patients.