Portal imaging for evaluation of daily on-line setup errors and off-line organ motion during conformal irradiation of carcinoma of the prostate

Abstract

Purpose: To use portal imaging to measure daily on-line setup error and off-line prostatic motion in patients treated with conformal radiotherapy to determine an optimum planning target volume (PTV) margin incorporating both setup error and organ motion. Results: A total of 2549 portal images from 33 patients were acquired over the course of the study. Of these patients, 23 were analyzed for setup errors while the remaining 10 were analyzed for prostatic motion. Setup errors were characterized by standard deviations of 1.8 mm in the anterior-posterior (AP) direction and 1.4 mm in the superior-inferior (SI) direction. Displacements due to prostatic motion, with standard deviations of 5.8 mm AP and 3.3 mm SI, were found to be more significant than setup errors. Conclusions: Taking into account both setup errors and target organ motion, optimum PTV margins to ensure 95% coverage are 10.0 mm AP and 5.9 mm SI. The portal imaging protocol established in this study allows radiation therapists to accept or adjust a treatment setup based upon daily on-line image matching results. The successful localization of radiopaque fiducial markers on a significant number of portal images acquired in the study gives hope that more accurate on-line targeting verification may soon be possible through the visualization of the prostate itself as opposed to the surrounding bony structures of the pelvis.