A Hybrid Strategy of Offline Adaptive Planning and Online Image Guidance for Prostate Cancer Radiotherapy

摘要

The offline adaptive radiotherapy (ART) has been used to effectively correct and compensate the prostate motion and reduce the required margin. The efficacy depends on the characteristics of the patient setup error and interfraction motion through the whole treatment, specifically, the systematic errors are corrected and random errors are compensated through the margins. In online image-guided radiation therapy (IGRT) of prostate cancer, the translational setup error and interfractional prostate motion are corrected through pre-treatment imaging and couch correction at each fraction. However, the rotation and deformation of target are not corrected and only accounted for with margins in treatment planning. The purpose of this study was to investigate whether the offline ART strategy is necessary for an online IGRT protocol and to evaluate the benefit of the hybrid strategy. First, to investigate the rationale of the hybrid strategy, 592 cone-beam computed tomography (CBCT) images taken before and after each fraction for an online IGRT protocol from 16 patients were analyzed. Specifically the characteristics of prostate rotation were analyzed. It was found that there exist systematic inter-fractional prostate rotations, and they are patient-specific. These rotations, if not corrected, are persistent through the treatment fraction, and rotations detected in early fractions are representative of those in later fractions. These findings suggest that the offline adaptive re-planning strategy is beneficial to the online IGRT protocol with further margin reductions. Second, to quantitatively evaluate the benefit of the hybrid strategy, 412 repeated helical CT scans from 25 patients during treatment course were included in the replanning study. Both low risk patients (LRP, clinical target volume, CTV = prostate) and intermediate risk patients (IRP, CTV = prostate + seminal vesicles) were included in simulation. Contours of prostate and seminal vesicles were delineated on each CT. The benefit of margin reduction to compensate for both rotation and deformation in hybrid strategy was evaluated geometrically. With the hybrid strategy, the planning margins can be reduced by 1.4 mm for LRP, and 2.0 mm for IRP, compared with the standard online IGRT only, to maintain the same 99% target volume coverage. The average relative reduction in planning target volume (PTV) based on internal target volume (ITV) from PTV based on CTV is 19% for LRP, and 27% for IRP, respectively.