Is High Definition MLC Dosimetrically Superior to Standard Definition MLC for SIB-SBRT for Carcinoma Prostate

OBJECTIVE: The study was conducted to quantitatively evaluate the dosimetric effects of high definition (2.5 mm) and standard definition (5.0 mm) MLC on the quality of SBRT plans using SIB-IMRT and SIB-VMAT technique for carcinoma prostate and also to evaluate the dosimetric advantage of one technique over the other. MATERIALS AND METHODS: Seventeen annonymized planning CT data sets were used to generate plans for both VMAT and IMRT techniques using 2.5 mm and 5.0 mm MLC.The prescription to the nodule was 45Gy in 5 fractions and to the prostate was 35Gy in 5 fractions.CI, GI, D(2%), D(98%), D(50%) and V(95%) for target; D(2%), Dmean, V(80%), V(20%) for OAR’s; V(5%) of the irradiated volume, and delivered MU’s were analyzed.An independent t-test was used to compare the plans. Patient specific QA for all plans were also performed and analyzed. RESULTS: Minor difference in dosimetric indices was observed between 2.5mm and 5mm MLC VMAT plans, except D(2%) (PTV35) and D(98%) (GTV45) were better in 2.5mm MLC plans (p<0.05).D(2%) and D(50%) to GTV45 with p<0.009 and <0.03 respectively, and D(2%) of bladder (p<0.03) were significantly lesser for VMAT plans compared to IMRT plans. GI was significantly better for 2.5mm MLC plans in both techniques (p<0.05). 2.5mm MLC plans on an average had 9.9% and 7% more MU for VMAT and IMRT plans respectively. CONCLUSION: All the plans provided adequate dose coverage to target. Highly gradient plans with better conformity were achieved with 2.5mm MLC and VMAT combination with an increase in MU delivered, compared to 5mm MLC plans.