SU‐FF‐T‐80: Hybrid Image Guided Radiation Therapy for Hypofractionated Prostate IMRT: Feasibility Study

Purpose: In prostate IGRT employing hypofractionation, two methods are commonly used: patient re‐positioning and re‐planning. We propose a hybrid IGRT technique in this study. The dosimetric benefits and efficiency improvement of the technique is quantitatively evaluated by comparing with the two conventional techniques. Method and Materials: Five prostate patients were studied, each had 1 planning CT and 11 daily CBCTs. The structure‐of‐interest (SOIs) including CTV (prostate and seminal vesicles), bladder, and rectum were contoured on CT and CBCTs. Initial IMRT plans were generated on Eclipse. Three IGRT techniques (“re‐positioning”, “re‐plan”, and “hybrid”) were applied on each CBCT. “Re‐positioning” utilizes target soft‐tissue matching for patient position correction. “Re‐plan” re‐aligns the patient then reoptimizes the original plan using daily SOIs. “Hybrid” first finds the best‐fit plan from plan‐library for the daily CBCT. If no suitable plan found, “re‐plan” is performed and new plan is added to library. The daily and cumulative doses were calculated and compared for all three techniques. Results: (1) Cumulative dose: For CTV and bladder, the difference among the three IGRT techniques are <1% and <5% at all dose levels. “Re‐positioning” slightly underdoses the CTV. For rectum, the differences between “re‐plan” and “hybrid” are <2% at all dose levels. The Dmean/D30 doses to the rectum of the “re‐positioning” are 5%/10% higher than those of “re‐plan”. (2) Daily dose: The average(min‐max) D30 dose for “re‐positioning” and “hybrid” are [rectum: 84%(67%–94%) and 70%(51%–78%)] and [bladder: 57%(42%–75%) and 59%(46%–70%)]. (3) “Hybrid” required the re‐optimization to be performed 6, 4, 1, 2, and 3 times for patient 1 to 5, respectively. Conclusion: Hybrid‐IGRT retains the target coverage and critical structure sparing of the “re‐plan”, yet it reduces the workload of the re‐optimization by >70% and therefore is a viable option for hypofractionated prostate IGRT. Conflict of Interest: Research sponsored by Varian Medical Systems. © 2009, American Association of Physicists in Medicine. All rights reserved.