Assessing the impact of uncertainty in a VMAT plan for H&N cancer considering patient setup error (PSE).

In radiotherapy, accurate radiation dose delivery is critical, despite the uncertainties that can cause discrepancies between the prescribed and administered doses. This study examines the influence of uncertainties caused by patient positioning errors on dose accuracy and evaluates the potential consequences if these errors remain uncorrected. The investigation involves 670 treatment plans, including 10 clinically implemented plans and 660 with artificially induced uncertainties. These plans target patients undergoing volumetric arc therapy (VMAT) for various head & neck cancers using the Halcyon™ medical linear accelerator. Two scenarios are analyzed: one reflecting actual patient positioning shifts and another involving deliberate displacements ranging from 1 to 10 mm to assess the impact of uncertainty in a VMAT plan. Dosimetric validation is performed using EPID and the Octavius 4D phantom to introduce and assess the impact of these uncertainties. Results indicate that target coverage falls below the acceptable V95% threshold along the (X+, X-), (Y+, Y-), and (Z+, Z-) axes, when positioning errors exceed 5 mm. Additionally, both parotid glands are significantly affected by positioning uncertainties. Patient-specific quality assurance (PSQA) assessments confirm that positioning errors substantially affect the accuracy of dose delivery. To minimize uncertainties, strict adherence to recommended QA, clinical guidelines, imaging protocols, and other SOPs is crucial in clinical practice. Independent dosimetric audits are essential for evaluating the accuracy of dose delivery in treatment.