Assessment of monitor unit limiting strategy using volumetric modulated arc therapy for cancer of hypopharynx.

PURPOSE: To quantify relative merit of MU deprived plans against freely optimized plans in terms of plan quality and report changes induced by progressive resolution optimizer algorithm (PRO3) to the dynamic parameters of RapidArc.

MATERIALS AND METHODS: Ten cases of carcinoma hypopharynx were retrospectively planned in three phases without using MU tool. Replicas of these baseline plans were reoptimized using "Intermediate dose" feature and "MU tool" to reduce MUs by 20%, 35%, and 50%. Overall quality indices for target and OAR, integral dose, dose-volume spread were assessed. All plans were appraised for changes induced in RapidArc dynamic parameters and pre-treatment quality assurance (QA).

RESULTS: With increasing MU reduction strength (MURS), MU/Gy values reduced, for all phases with an overall range of 8.6-34.7%; mean dose rate decreased among plans of each phase, phase3 plans recorded greater reductions. MURS20% showed good trade-off between MUs and plan quality. Dose-volume spread below 5Gy was higher for baseline plans while lower between 20 and 35Gy. Integral dose was lower for MURS0%, not exceeding 1.0%, compared against restrained plans. Mean leaf aperture and control point areas increased systematically, correlated negatively with increasing MURS. Absolute delta dose rate variations were least for MURS0%. MU deprived plans exhibited GAI (>93%), better than MURS0% plans.

CONCLUSION: Baseline plans are superior to MU restrained plans. However, MURS20% offers equivalent and acceptable plan quality with mileage of MUs, improved GAI for complex cases. MU tool may be adopted to tailor treatment plans using PRO3.