137. Monitor Units constraints for SBRT plans and prostate cancer

Abstract

PurposeIn this work the correlation between total monitor units (MUs) constraints, dosimetric findings and pre-treatment verifications has been studied for prostate Stereotactic Body Radiation Therapy (SBRT) VMAT plans. MethodsA study on 5 SBRT prostate treatments has been performed.Plans were been accomplished with Varian Eclipse TPS on TrueBeam™ 2.5. The so called MU Objective tool implemented in VMAT optimization engine was used. This tool allows setting the range and the relative priority to the minimum and maximum number of MU employed for plan optimization.Dose prescription was 35 Gy in 5 fractions, using 10MV-FFF photon beams and plans were calculated using Acuros XB algorithm. All plans were calculated with and without constraints on MUs.For each case the 2 plans were compared in terms of DVH, MUs, modulation complexity coefficient (MU/Gy) and pre-treatment verifications performed with EPID and Epiqa software. ResultsAverage MUs were 973 with a modulation coefficient of 277.5 MU/Gy and 1120 with 320 MU/Gy, respectively for plans with and without forced MUs.Differences in DVHs have been found. In fact, for 3 of the 5 cases, use of “MU objective” involve a better PTV coverage and a reduced dose to OARs. Pre-treatments verifications showed a better agreement between calculated and delivered dose distribution for all SBRT plans obtained forcing MUs (see Table 1).All plans were acceptable with a GAI > 95%. ConclusionsImportant discrepancies have been found in terms of total MUs and modulation complexity coefficient, that are considerably reduced for plans obtained forcing the MUs.SBRT prostate plans obtained using forced MUs reveals an increasing of GAI value associated with MUs reduction.In our opinion, for SBRT treatments further studies are necessaries on a wider sample of cases to understand if minimizing modulation (i.e. decreasing total MUs) could improve SBRT plans quality.