190. Correlation between modulation degree and dosimetric accuracy of prostate VMAT treatment planning

Purpose To evaluate the impact of modulation degree (MD) on patient specific pre-treatment quality assurance for prostate VMAT treatment planning. Methods Ten patients with similar PTV and rectum size were included in the study. The prescribed fractional dose to PTV was 2,5 Gy daily. VMAT plans were calculated using a MonteCarlo-based TPS. For each patient we optimized 4 different plans using the same constraint cost-functions for PTV and organ-at-risk (OARs), the same plan parameters (arc number, maximum number of control points for arc) but changing the minimum distance between opposing MLC leafs (MDW = 0.5, 0.8, 1, 1.5 cm). Such parameter influences the MD and the plan quality. MD was calculated from the DICOM-RT files using the mean segment area value and total Monitor Unit. To evaluate the plan quality we took into account the PTV coverage and rectum dose. Gamma index analysis was performed to compare measured and calculated dose. Plans were accepted if 95% of the points satisfied the 2% dose-difference and distance-to-agreement 2 mm (γ2%,2 mm). All plans were delivered by a 6 MV Linac and the patient specific pre-treatment quality assurance verification was performed using a cylindrical diode array-phantom. Descriptive statistic was performed to evaluate the correlation between MD and gamma passing rate measured. Results We observed a negative linear correlation (Fig. 1) between MD values and γ2%,2 mm (correlation coefficient r = 0.7, p = 0.0004). The plans with MD   4 yielded a γ2%,2 mm inferior than 95% in 100% of cases (Fisher-exact-test p = 0.002). 93% of plans with MDW > 0.8 cm had a MD  Conclusions The gamma index value was significantly related to MD   4. Rival plans with a MD   0.8 cm allows to obtain a desirable MD value. The calculation of MD for prostate VMAT treatment planning is important to predict the patient-QA in term of gamma index analysis.