SU‐E‐T‐773: Dosimetric and Geometric Evaluation of a Hybrid Strategy of Online Image Guidance and Offline Adaptive Planning for Prostate Cancer Radiotherapy
Purpose: In fractionated radiotherapy of prostate cancer, both online and offline image‐guided strategies have been used to manage uncertainties such as setup errors and organ motions. In general, online image guidance is more effective than offline image guidance and is currently widely implemented. The purpose of this study is to evaluate the dosimetric and geometric benefit of a hybrid strategy of online image guidance and offline adaptive replanning in terms of margin reduction. Methods: A hypofractionated treatment protocol was simulated with 28 patients each having at least 16 helical CT scans. Both low‐risk patient (LRP, CTV=prostate) and intermediate‐risk patient (IRP, CTV=prostate+seminal vesicles) were studied. The hybrid strategy consists of online image guidance and offline adaptive planning. The online positioning correction was simulated by the shift of CTV center‐of‐mass (COM). Two registration methods were performed for IRP: COM shift of entire CTV (Type A) and prostate only (Type B). In offline adaptive planning, the internal target volume was constructed from the CTVs in the first five fractions. Both 3D‐CRT and IMRT were investigated. The dosimetry was evaluated for CTV, bladder and rectum. Results: Compared with online image guidance only, the hybrid strategy improves the target dose coverage significantly for both 3D‐CRT and IMRT at small margin. As margin increase, the magnitude of improvement decreases. Type A registration is better than Type B registration for IRP. The dosimtric margin benefit depended on the criteria and varied from 0 to 1.3 mm. Doses to rectum and bladder were significantly lower in IMRT than in 3D‐CRT, also lower in hybrid than online image guidance. Conclusions: The hybrid strategy can effectively account for the residual and patient‐specific interfractional organ deformation. It can further reduce the margins safely from the online image guidance alone. However, the dosimetric margin is smaller than its geometric counterpart. © 2011, American Association of Physicists in Medicine. All rights reserved.
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