Beyond IGRT: Daily real time planning (RTP)-Treatment of prostate cancer, clinical implementation, and technique.

191 Background: Image guided radiation therapy (IGRT) corrects for the interfractional movements of the target/CTV. However, the anatomic changes of CTV, and movements of the adjacent organs, are not accounted for. By modifying an online adaptive radiation technique proposed by Li, we have clinically deployed a daily "real-time treatment planning" (RTP) technique for treatment of primary prostate cancers. The technique, rationale and experience of our first 60 RTP treatments are presented. To our knowledge, this is the first clinical implementation of daily treatment planning. METHODS The RTP process is as follows: a) daily CT images are acquired via an in-room diagnostic CT-on-Rails; b) while IGRT evaluation is performed, new target and tissue contours (prostate, rectum and bladder) are drawn, c) new IMRT plan is computed (RTP plan); d) IGRT plan is compared to the RTP plan; e) if RTP plan is dosimetric superior, it will be delivered after f) necessary QA process. This entire process takes less than 20 minutes (excluding IMRT delivery). 60 RTP's were delivered (10 daily RTP/patient) in 6 consecutive patients. RESULTS Auto-contouring via auto-segmentation was inadequate when compared to manual anatomic re-contouring. The RTP plan is always superior to or equal to the original IGRT plan. In 20% of the cases, the CTV-DVH by RTP improved by >10%. Slight changes in manual contouring variation can lead to significant change in volume variation. For example, with initial prostate volumes of 15, 20, 30, 40 and 50 cc, a 1 mm increase in contour variation results in an increase volume of 26%, 24%, 21%,19% and 17% respectively, and absolute volume increase of 3.9cc, 4.8cc, 6.2cc, 7.4cc and 8.6cc respectively. Rectal dose improved significantly with RTP when compared to IGRT. In one extreme case, the rectal dose improvement is shown (Table). CONCLUSIONS Daily RTP is feasible for prostate cancer treatments and is superior to IGRT. RTP is especially necessary with extreme anatomic changes. [Table: see text].