Evaluation of Patient-Specific Quality Assurance Results to Optimise Automatic Flash Margin in Monte Carlo Algorithm for VMAT of the Breast and Chest Wall Irradiation

Background/Objectives: To evaluate the impact of automatic flash (AF) margin thickness on plan complexity and patient-specific quality assurance (PSQA) outcomes in 4-partial-arc VMAT (4pVMAT) for breast and chest wall irradiation using the Monaco Monte Carlo treatment planning system. Methods: Twenty patients (11 left-sided, 9 right-sided) previously treated with 4pVMAT were retrospectively replanned with five AF thicknesses (AF0, AF3, AF5, AF10, AF15), yielding 100 plans on Elekta Versa HD/Agility MLC. Plan complexity was assessed via modulation factor (MF), total monitor units (MU), segment number, MU efficiency, and normalisation ratio. PSQA was performed using the IBA Matrixx Resolution detector, with gamma passing rates (GPRs) evaluated under 3%/3 mm, 3%/2 mm, and 2%/2 mm criteria. Friedman and Wilcoxon signed-rank tests were used with Bonferroni correction. Results: All five complexity metrics were statistically invariant across AF0–AF15 (Friedman p > 0.05; no pairwise comparison against AF10 reached significance). Mean GPR increased monotonically up to AF10—92.28 ± 4.83% (AF0) to 93.73 ± 4.55% (AF10) at 3%/3 mm—and plateaued at AF15. After Bonferroni correction, AF0 vs. AF10 (p = 0.0003) and AF3 vs. AF10 (p = 0.005) remained significant under the 3%/3 mm criterion. The proportion of plans meeting GPR ≥ 95% at 3%/3 mm rose from 25% (AF0) to 45% (AF10). Conclusions: AF thickness has no clinically meaningful effect on plan complexity but significantly affects delivery accuracy in 4pVMAT breast and chest wall irradiation. AF10 emerges as the optimal margin—maximising delivery accuracy at a plateau without increasing plan complexity—with AF5 as the practical lower bound and AF15 as the upper bound.