The Stability of Intracavitary Applicator Placement during Patient Transfer for Cervical HDR Brachytherapy

Abstract

limiting factor in fractionated radiotherapy for pancreatic cancer. Interand intra-fraction motion due to residual setup errors, respiratory motion and deformation results in deviations between planned and delivered doses. This motion and the dose deviations they cause prevent the widespread adoption of intensity modulated radiation therapy (IMRT) and smaller margins in pancreatic radiotherapy which could be used to reduce the planned OAR doses and toxicity risks. Daily cone beam CT (CBCT) image-guidance is currently used to minimize setup errors, which can be corrected using a simple 3D shift of the treatment couch. However, other types of 3D and 4D motion cannot be corrected in this manner. Deformable image registration (DIR) is a novel technique that allows residual motion in internal anatomy to be incorporated into the dose distribution, thus improving estimates of the normal tissue doses. This study aims to evaluate the accuracy of DIR algorithms to model organ deformation and evaluate the clinical impact of this technique by comparing deviations between planning doses and delivered doses estimated using DIR of CBCT. Method/Process: Weekly CBCT images of 20 patients treated with IMRT for locally advanced pancreatic cancer will be retrospectively evaluated in the treatment planning system (RayStation). OARs will be delineated manually on CBCT. DIR of the planning CT to each CBCT will be performed to auto-propagate planning OAR contours to each CBCT. Accuracy of the DIR will be quantified by calculating the discrepancy of the two deformed contours (manual vs. DIR generated). The cumulative dose will be calculated by summing the DIR doses of the weekly CBCT. Differences in clinically relevant dose metrics will be evaluated using paired t-tests (where P<0.05 is considered significant). Results/Benefits/Challenges: Pending. Conclusion/Impact/Outcomes: DIR and dose accumulation will improve our estimate of the delivered dose to patients. This allows us to apply margin reduction in treatment planning and enhance normal tissue sparing which consequently can decrease toxicity.