In Silico Study of Proton Adaptive Prostate SBRT Radiation Therapy
B Maraghechi, Y Hao*, A Price, T Zhao, T Zhang, B Sun, A Darafsheh, E Laugeman, N Knutson, G Hugo, L Henke, B Baumann, S Perkins, T Mazur, Washington University School of Medicine, St. Louis, MO
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PO-GePV-T-170 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: To study the feasibility of adaptive proton prostate SBRT treatment and evaluate the differences between non-adapted (NADD) and adapted (ADD) delivered doses to target and organs-at-risk (OARs).
Methods: Imaging and treatment plans for five patients who received online adaptive prostate SBRT treatments (3625 cGy in 5 fractions) using a photon-based adaptive treatment platform (Varian Ethos) were transferred to a treatment planning system (RayStation 11A, RaySearch Laboratories) with a clinical beam model for a pencil beam scanning proton system (Mevion S250i Hyperscan). Data included 1) simulation CT images and initial treatment plans and 2) per session synthetic CT images derived from CBCTs along with session-specific and physician-approved contours for target volumes, bladders and rectums. Initial IMPT plans were created on simulation CT images using opposed-lateral fields with layer-specific apertures via multi-field optimization techniques. Optimization was designed to robustly achieve PTV D98%>3625 cGy assuming 2% range uncertainty while constraining bladder and rectum D0.03cc to less than 39 and 38 Gy respectively. These plans were evaluated on synthetic CBCTs to simulate NADD per fraction, and then identical beam geometries were re-optimized on each session based on session-specific contours to produce ADD. Coverage and OAR metrics were then compared per fraction for both NADD and ADD.
Results: Median (inter-quartile range, IQR) values for 1) bladder and 2) rectum D0.03cc across all fractions for NADD and ADD were 1) 38.8 and 38.9 Gy (p=0.41) and 2) 39.4 and 37.6 Gy (p=1.2E-5), respectively. Median (IQR) values for CTV D98% for NADD and ADD were 36.3 and 36.6 Gy (p=9.5E-5).
Conclusion: In this study, we demonstrated feasibility and dosimetric benefits of adaptive prostate SBRT delivered via IMPT. For the sampled cases maximum dose to the rectum can be reduced by approximately 2 Gy over 5 fractions via online adaptation while preserving CTV coverage.
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