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An Aperture Preserved Approach for Adaptive Radiotherapy of Prostate Cancer Using Volumetric-Modulated Arc Therapy

A Price

A Price1*, J Dooley1 , T Zhu1 , R Pappafotis2 , A Wang1 , R Chen1 , S Das1 , J Lian1 , (1) University of North Carolina, Chapel Hill, NC, (2) Duke University, Durham, North Carolina, NC


SU-K-605-15 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 605

Purpose: Adaptive radiotherapy (ART) can often achieve better dosimetry compared to directly calculated plans on daily patient anatomy. However, a major limitation of ART applications is its need for delivery quality assurance (DQA) before treatment. To address this challenge, we propose an innovative ART technique to maintain segment shapes in ART of VMAT plans while only re-optimizing segment MU, thus eliminating the step of ART DQA.

Methods: Five previously treated prostate cancer patients were included in a retrospective simulation study. Each patient was treated 180cGyx25 fractions with daily CT-on-rails image guidance. For each patient, 5 weekly treatments with noticeable anatomic change were selected to perform ART. Treatment CTs were rigidly registered to the planning CT and contours were adjusted on the treatment CT. For each selected treatment, dose distributions from the original VMAT arcs were calculated on the daily treatment CTs (DCP plan). Weekly ART plans were created by optimizing segment MU only while preserving segment shapes.

Results: The average PTV coverage for all 25 ART plans was 94.91±0.40%, compared to the DCP plans of 81.83±11.39%. The rectal V40Gy for the average ART plans was 15.24±11.04cc, similar to the average DCP plans at 15.68±12.97cc. The composite displacement of the seminal vesicle centroid in the superior only and either the anterior or posterior direction caused a decrease in PTV dose conformality index (R² = 0.89). Excluding the 4 extreme ART plans whose seminal vesicle displacement was greater than 0.7cm, the rectal V40Gy improved to 11.15±4.34cGy. 5 selected ART plans passed measurement QA (3%/3mm) similar to the original plan.

Conclusion: We found that maintaining segment shapes while optimizing segment MU only in VMAT prostate plans could be an effective method for adaptive radiotherapy. It makes it possible to implement an ART plan on some patient’s treatments without the need of DQA.

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