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Dosimetric Effect of Liver Motion for Scattering Proton Beam Therapy: A Proton Liver 4D Treatment Planning Strategy Study

Q Huang

Q Huang*, M Zhang , W Zou , N Yue , Rutgers Cancer Institute of New Jersey, New Brunswick, NJ


PO-BPC-Exhibit Hall-15 (Saturday, March 5, 2016)  Room: Exhibit Hall

Intrafraction respiratory motion in liver leads to variation between delivered and planned tumor coverage in proton therapy. In this study, we investigated strategies for designing liver proton plans with double scattering technique to minimize this variation.
Liver cancer patients with different tumor center motion treated with proton were retrospectively analyzed for the study. Free breathing scans (FB), maximum intensity projection scans (MIP), time averaged scan (Ave) and exhale phase scans (CT50) were used to create proton plans. Real-time dose distribution was simulated by calculating dose in each phase of 4DCT with same beam setting for according strategy. The delivered dose was determined by accumulating dose deposition of all phases using deformable image registration implemented by VelocityAl software. Delivered tumor coverage was defined as percentage volume of GTV receiving at least prescribed dose in delivered dose distribution while planned tumor coverage was defined as percentage volume of PTV receiving at least 95% prescribed dose in planned dose distribution. Tumor coverage was calculated to evaluate delivered and planned dose distribution for four planning strategies.
Dosimetric variation between delivered and planned dose for each strategy were observed. For the current patient we analyzed, delivered tumor coverage were 99.92%, 95.45%, 99.92% and 95.08% for FB, MIP, AIP and CT50 plans respectively. Corresponding planned tumor coverage were 97.54%, 98.9%, 99.66% and 98.51%. Maximum cord dose, maximum esophagus dose and small bowel doses were comparable between the planned and delivered dose. The result indicates proton plans designed based on time averaged scans is a good indicator for delivered dose distribution.
A systemic approach has been developed for comparing delivered and planned dose distribution for proton plans created on different static CT scans. The result indicates proton plans based on time averaged scan predicts tumor coverage better.

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