PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s):For large-field static IMRT/VMAT treatment plans for pelvic & abdominal targets, multi-leaf collimator (MLC) leakage is a dominant factor that contributes to low-dose regions, and is related to maximum jaw opening and/or multiple isocenters, and field-size dependent MLC leakage. The goal of this study was to evaluate the impact of jawless flattening-filter-free (FFF) double-MLC machine recently introduced in the clinic on low-dose regions for large-field pelvic & abdominal targets and to compare its delivery efficiency with conventional linear accelerators using total monitor unit (MU) as a metric.
Materials/Methods:Ten challenging clinical pelvic & abdominal cases were retrospectively selected and treatment plans were generated using commercial treatment planning software for the jaw-less FFF double-MLC layer machine, and these research plans were comparable to clinical plans in terms of target coverage and Organ-at-Risk (OAR) doses. Prescription dose was 45Gy in 25 fx in all but 3 replanned cases. The cases were all large-field IMRT/VMAT cases with the median target size of 1713 cm3 (max=2269 cm3, SD=505 cm3). To minimize any subjective bias, static IMRT or VMAT techniques with the same arcs or gantry angles, which were identical to the clinical plans, were used for dose calculation. After achieving comparable target coverage as well as OAR doses, low-dose regions (10%, 20%, & 30% of prescription dose) and total MU were compared between two different plans.
Results:Target coverages were found to be over 95% with the median values of 96.3% for clinical plans and 97.3% for this study, and doses for various OARs were mostly comparable between clinical plans and this study. The median total MU was 684(SD=227) for the clinical plans and 878(SD=287) for this study. The higher MU for this study was related to the limitation of maximum field size for the jawless double-MLC layer machine; two isocenters had to be used for three cases due to the limited maximum field size of 28cm. Since the separation of two isocenters for the jawless machine should be within 8cm apart, total number of arcs or fields for the jawless machine were twice as that of clinical plans for three out of 10 clinical cases. The median value of low-dose region (10% of prescription dose) was 48.4% (SD=11.8%) for clinical plans vs. 48.6% (SD=11.7%) for this study, and the ratios of low-dose regions (10%, 20%, & 30%) between clinical plans and this study were 1.03(SD=0.04), 1.01(SD=0.03), and 0.97(SD=0.07), respectively. The comparable low-dose regions for this study resulted from the double-MLC layer features, which shows the extremely low measured MLC leakage rate of 0.1%
Conclusion:For large-field static IMRT/VMAT cases, low-dose regions (10%, 20%, & 30% of prescription dose) for the jawless FFF double-MLC layer machine were comparable to those of conventional machines.
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