Room: Exhibit Hall | Forum 4
Purpose: The purpose of this study is to develop a method to estimate the robustness of a VMAT plan by calculating the geometry-oriented dose uncertainty (gDU) and a simple aperture-based MU calculation.
Methods: A head-and-neck (HN) VMAT plan with a D95%=70Gy prescription was created on RANDO Phantom using Eclipse treatment planning system (original plan). DICOM RT Structure and Plan were analyzed by in-house analysis software. Dose per MU (DPM) to each voxel was calculated by multiplying tissue-to-maximum ratio, Sc and Sp, followed by the inverse square correlation. The gDU was defined as the differential of DPM in the superior-inferior (SI) direction for each voxel within the planning target volume (PTV). The other plans with non-irradiating gantry angles (Avoidance Sector, AS) with 15 and 30 degrees covering local maxima of gDU around each shoulder were created. Estimated DU (eDU) was defined as the sum of segmental MU-weighted gDU within the multileaf collimator for all arcs. The eDU was then compared with the SI-shift-induced variation of the mean dose of PTV for original and AS plans.
Results: The eDU showed local maxima around G=0, 90 and 270 degrees in the original plan. By setting AS at local maxima of gDU, eDU around G=90 and 270 degrees were drastically reduced while eDU outside AS was increased. Among the original and AS plans, the SI-shift-induced variation of the mean dose was proportional to eDU. This result indicated that eDU gives a good estimation of the robustness of VMAT plans.
Conclusion: We developed a method to estimate the robustness of VMAT plans by analyzing structures and simple MU calculation taking the segmental apertures into account. The accuracy of eDU will be improved by taking the actual displacement of individual parts such as shoulders or head.