SU‐E‐T‐14: A Feasibility Study of Using Modified AP Proton Beam for Post‐Operative Pancreatic Cancer Therapy

Abstract

Purpose:Due to the unpredictability of bowel gas movement, the PA beam direction is always favored for robust proton therapy in post‐operative pancreatic cancer treatment. We investigate the feasibility of replacing PA beam with a modified AP beam to take the bowel gas uncertainty into account.Methods:Nine post‐operative pancreatic cancer patients treated with proton therapy (5040cGy, 28 fractions) in our institution were randomly selected. The original plan uses PA and lateral direction passive‐scattering proton beams. Beam weighting is about 1:1. All patients received weekly verification CTs to assess the daily variations(total 17 verification CTs). The PA direction beam was replaced by two other groups of AP direction beam. Group AP: takes 3.5% range uncertainty into account. Group APmod: compensates the bowel gas uncertainty by expanding the proximal margin to 2cm more. The 2cm margin was acquired from the average bowel diameter in from 100 adult abdominal CT scans near pancreatic region (+/‐ 5cm superiorly and inferiorly). Dose Volume Histograms(DVHs) of the verification CTs were acquired for robustness study.Results:Without the lateral beam, Group APmod is as robust as Group PA. In Group AP, more than 10% of iCTV D98/D95 were reduced by 4–8%. LT kidney and Liver dose robustness are not affected by the AP/PA beam direction. There is 10% of chance that RT kidney and cord will be hit by AP proton beam due to the bowel gas. Compared to Group PA, APmod plan reduced the dose to kidneys and cord max significantly, while there is no statistical significant increase in bowel mean dose.Conclusion:APmod proton beam for the target coverage could be as robust as the PA direction without sacrificing too much of bowel dose. When the AP direction beam has to be selected, a 2cm proximal margin should be considered.