H Hooshangnejad ¹*, D Han², C Chen³, K Ding⁴, (1) Johns Hopkins university , Baltimore , MD, (2) Johns Hopkins University , Baltimore, MD, (3) Johns Hopkins National Proton Therapy Center, Washington, DC, (4) Johns Hopkins University, Baltimore, MD

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  H Hooshangnejad

(Saturday, 4/4/2020)   [Mountain Time (GMT-6)]

Purpose: To demonstrate the dosimetric benefits of incorporating beam specific margin and reduced setup uncertainty from imaging guidance in stereotactic body proton therapy (SBPT) for locally advanced pancreatic cancer (LAPC).

Methods: We designed SBPT plans (33Gy in 5 fractions) for 5 patients from our institute, with LAPC, using two different gross tumor volume (GTV) expansion schemes. To obtain optimization target volume (OTV), depending on the imaging modality being used for IGRT, 3D-CBCT or 2D-KV planar imager, we expanded GTV with two methods, beam specific water equivalent thickness (BS-WET) and conventional 5mm, accounting for setup and range uncertainties. The BS-WET expansion is comprised of 2mm expansion covering CBCT setup uncertainties laterally, accompanied with the 3.5% proton beam penetration depth uncertainty in beam direction. All plans are completed on Raystation TPS 9A. Target coverage (V33Gy, V25Gy) and OAR Sparing (V33Gy, V20Gy, and V15Gy) are compared to evaluate the quality of the plans.

Results: Both optimization schemes achieved decent target coverage V33Gy>98% and V25Gy>100% for all patients. However, BS-WET expansion scheme shows a considerable increase in OARs sparing for Duodenum, V33Gy by 67% (p-value=0.05), V20Gy by 46% (p-value<0.001), and V15Gy by 36% (p-value<0.001), as opposed to 5mm expansion scheme. OAR sparing also improves for proximal Stomach and small bowel.

Conclusion: The results of this study demonstrate a considerable improvement in SBPT plan quality with BS-WET, OTV margin over 5mm margin. A systematic comparison of robustness evaluation is warranted to determine the deliverability of the plans

Funding Support, Disclosures, and Conflict of Interest: Research reported in this publication was supported by the National Cancer Institute of the National Institutes of Health under Award Number R37CA229417. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

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