In vivo dosimetry for prostate cancer patients using an electronic portal imaging device (EPID); demonstration of internal organ motion

Purpose: To investigate the use of a commercially available video-based EPI D for in vivo dosimetry during treatment of prostate cancer patients. Methods: For 10 prostate cancer patients, the inter-fraction variation with in measured portal dose images (PDIs) was assessed and measured PDIs were c ompared with corresponding predicted PDIs based on the planning CT scan of the patient. Results: For the lateral fields, the average standard deviation in the meas ured on-axis portal doses during the course of a treatment was 0.9%; for th e anterior fields this standard deviation was 2.2%. The difference between the average on-axis measured portal dose and the predicted portal dose was 0.3 +/- 2.1% (1 SD) for the lateral fields and 0.7 +/- 3.4% (1 SD) for the anterior fields. Off-axis differences between measured and predicted portal doses were regularly much larger (up to 15%) and were caused by frequently occurring gas pockets inside the rectum of the patients during treatment o r during acquisition of the planning CT scan. The detected gas pockets did sometimes extend into the gross tumour volume (GTV) area as outlined in the planning CT scans, implying a shift of the anterior rectum wall and prosta te in the anterior direction (internal organ motion). Conclusions: The developed procedures for measurement and prediction of PDI s allow accurate dosimetric quality control of the treatment of prostate ca ncer patients. Comparing measured PDIs with predicted PDIs can reveal inter nal organ motion. (C) 1998 Elsevier Science ireland Ltd. All rights reserve d.