SU-D-211-05: A Systematic Analysis of Penumbra Characteristics in Conformai Arc Delivery for SBRT and Its Utilization for Negative Margin Technique (NMT), a Novel Planning Strategy for Improving Dose Conformation

Medical Physics
S KimAshleya Smith

Abstract

To systematically analyze penumbra characteristics in co-planar arc-beams and demonstrate how optimal aperture margin (OAM) can be systematically estimated. Full arc-beam delivery was simulated with a Pinnacle RTP for a total of 32 cases. Considered factors were energy (6X, 6F, 10X, and 10F, where 'F' indicates flattening-filter-free), field-size (2×2, 4×4, 6×6, and 8×8), and two materials (50×50×50 cm×cm×cm water and 50×50×50 cmxcmxcm water with a 20 cm diameter sphere of 0.25 g/cc lung density at the center). The highest-dose-gradient (HDG) and its range in both radial-dose-distribution (RDD) and longitudinal-dose-distribution (LDD) were evaluated. For OAM estimation, start with simulated dose distributions in interest. Measure coverage size (CS) at the chosen PIL in the RDD. The difference between the target size (TS) and the CS is the margin needed. In radial-direction, choose a PIL where the CS is larger than the TS [resulting in 'negative margin (NM)'] to enhance conformation capability.Next, measure the CS at the chosen PIL in the LDD. Longitudinally, positive margin is expected in general. This process was applied to 6X and 10F cases in lung to illustrate its applicability. Overall, larger HDG was observed in 6 MV (than...Continue Reading

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