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A Novel Robust and Delivery-Efficient Spot Scanning Arc Therapy to Spare Hippocampus and Cochlea in Whole Brain Radiotherapy

X Ding

X Ding*, J Zhou , K Blas , X Li , Y Wang , A Qin , C Stevens , D Yan , I Grills , P Kabolizadeh , William Beaumont Hospital, Royal Oak, MI


SU-F-205-2 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 205

Purpose: Recently, there have been significant interests towards whole-brain radiotherapy (WBRT) with hippocampal and cochlea avoidance. Herein, we present a novel robust, continuous and delivery-efficient spot-scanning proton arc therapy technique (SPArc) to improve dosimetric outcome.

Methods: Eight whole brain patients were used to evaluate the feasibility of using SPArc for hippocampal and cochlea avoidance WBRT. Both SPArc and robust optimized Intensity Modulated Proton Therapy (ro-IMPT) plans were generated using the robust optimization with ±3.5% range and 3mm setup uncertainties compared to the Volumetric Modulated Arc Therapy (VMAT). Root-mean-square deviation doses (RMSDs) Volume Histogram, or RVH, was used for plan robustness evaluation. Total delivery time was compared based on a full gantry rotation with 1RPM, 2ms spot switching time, 0.01 minimum spot monitor unit, and energy-layer-switching-time (ELST) from 0.1 to 5 seconds.

Results: SPArc plans showed significant dosimetric improvements in terms of reduction of the mean dose to the hippocampus 6.20Gy[RBE] compared to VMAT 10.89Gy[RBE] (p<0.001) and ro-IMPT 9.38Gy[RBE] (p<0.001); D100% to the hippocampus is reduced to 4.50Gy[RBE] compared to VMAT 9.16Gy[RBE] (p=0.001) and ro-IMPT 7.02Gy[RBE] (p=0.002); cochlear mean dose is reduced to 7.75Gy[RBE] compared to VMAT 11.52Gy[RBE] (p=0.018) and ro-IMPT 10.15Gy[RBE], (p=0.037); Skin maximum dose is reduced to 33.84Gy[RBE] compared to ro-IMPT 36.37Gy[RBE]. RVH shows SPArc is more robust in Organ-at-risks(OARs) sparing e.g. hippocampus, cochlea, lens and eyes. The average total estimated delivery time was 412s, 627s, 1694s based on ELST of 0.2s, 1s, and 5s for SPArc plans, compared with the respective values of 547s (p<0.001), 626s (p=0.484), and 1025s (p=0.002) for ro-IMPT plans. Hence, SPArc plans could potentially achieve similar or faster delivery time in the modern proton machine with ELST less than 1s.

Conclusion: SPArc could significantly reduce the dose delivered to the hippocampus and cochlea in patients treated with WBRT.

Funding Support, Disclosures, and Conflict of Interest: A patent has been filed based on this study

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