Evaluating the influence of plan complexity, mechanical accuracy, and dose calculation algorithms on PSQA performance in IMRT and VMAT

To assess how treatment plan complexity affects patient-specific quality assurance (PSQA) performance in intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT), and differentiate mechanical, algorithm-related, and detector-related sources of QA discrepancies. A total of 110 clinical IMRT and VMAT plans were grouped by modulation complexity score (MCS) tertiles. Multileaf collimator (MLC) positional errors were quantified from trajectory-log. Dosimetric agreement was evaluated using log-based dose reconstruction, portal dosimetry with an electronic portal imaging device (EPID), ion-chamber array measurements, and dose-calculations with the Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB). Plan complexity was associated with increased multileaf collimator positional variability (p < 0.001). However, log-based dose reconstruction demonstrated minimal dosimetric impact from these submillimeter deviations, with gamma passing rates exceeding 99.5%. In contrast, measurement-based PSQA performance decreased with increasing plan complexity. Algorithm-dependent differences were observed in both planar gamma analysis and DVH metrics, with larger discrepancies for AAA than for AXB in highly complex plans (p < 0.05). Mechanical MLC deviations increased with plan complexity but produced minimal dosimetric impact in log-based reconstruction. Reduced PSQA agreement in highly modulated plans was driven predominantly by algorithm- and measurement-related factors rather than mechanical delivery errors.