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A Systematic Analysis of Penumbra Characteristics in Conformal Arc Delivery for SBRT and Its Utilization for Negative Margin Technique (NMT), a Novel Planning Strategy for Improving Dose Conformation

S Kim

S Kim*, C Serago, A Smith, Mayo Clinic, Jacksonville, FL

SU-D-211-5 Sunday 2:15:00 PM - 3:00:00 PM Room: 211

Purpose: To systematically analyze penumbra characteristics in co-planar arc-beams and demonstrate how optimal aperture margin (OAM) can be systematically estimated.

Methods: 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 (2x2, 4x4, 6x6, and 8x8), and two materials (50x50x50 cmxcmxcm water and 50x50x50 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.

Results: Overall, larger HDG was observed in 6 MV (than 10 MV), smaller fields, LDD (than RDD), and in water (than lung). 6F showed slightly larger HDG than 6X while 10X did than 10F. Dose fall-off started significantly farther from the CAX in RDD than LDD, supporting the NM concept. With the systematic margin estimation applied, expected PILs matched closely with isodose points having tight target coverage, indicating the applicability of the method.

Conclusions: Among tested, the 6F beam provides the most optimal dose distribution for dose conformation. The range of optimal PIL decreases as field-size increases. It has been demonstrated optimal AMs can be systematically estimated as proposed in this study.

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