Room: Karl Dean Ballroom C
Purpose: A main limitation for practical application of adaptive radiotherapy (ART) in IMRT/VMAT is the need for delivery quality assurance (DQA) before treatment. To address this challenge, we previously proposed an innovative technique to maintain segment shapes for the re-optimization of ART plans. Herein, we studied the effectiveness of this method on 100 treatment sessions with variety of patient anatomy changes.
Methods: 20 previously treated prostate cancer patients with daily CT-on-rails guidance were retrospectively planned with VMAT and IMRT. For each patient, 5 weekly treatment CTs were selected to perform ART. Contours were transferred from planning CT to treatment CTs and further corrected manually. Weekly ART plans were created by optimizing segment monitor units (MU) only, while preserving segment shapes of original plan and observing minimum MU constraints. Dose directly calculated on the treatment CT from the original VMAT arcs or IMRT fields were used as the reference dosimetry of treatment day (DCP plan). The seminal vesicle (SV) centroid displacement from planning CT to treatment CTs was measured for all treatments.
Results: Among 100 adaptive sessions each, 79% of VMAT ART plans and 77% of IMRT ART plans showed improvement from DCP to ART, either >5% more target prescription dose coverage, >10% volume decrease of rectal V40Gy (cc), or both. 12% VMAT and 10% IMRT ART plans resulted in a â‰¥115% hot spot and were considered failures. These failed plans correlated strongly with large (>0.75cm) composite displacements of SV from the planning CT. 5 VMAT and 5 IMRT ART plans with large anatomic changes passed measurement based QA with similar passing rates to the original plans.
Conclusion: We found that preserving segment shapes during ART optimization in VMAT/IMRT prostate plans could benefit most treatment fractions. This makes it feasible to implement an ART strategy without the need of DQA.
Image-guided Therapy, Intensity Modulation, Quality Assurance