219. Daily MVCTs for assessing the bowel motion and probability distribution maps during pelvic tomoterapy

Purpose To assess inter-fraction bowel loops (BL) motion and quantify its impact on BL DVHs in prostate radiotherapy including pelvic nodes irradiation. Methods Ten patients treated by Helical Tomotherapy, with adjuvant (5) or radical (5) intent, delivering 52.5/51.8 Gy to pelvic-nodes in 30/28 fractions respectively, were enrolled. For each patient, BL contours were drawn on fifteen daily MVCTs, using a validated method for segmentation: BL were propagated from the planning CT to in-room MVCTs by elastic registration and then manually corrected. Based on BL daily contours overlayed on the planning CT, 50% and 80% coverage probability maps (CPM) of BL position were generated using an ad hoc module of the VODCA software (BMSInc.). Average BL DVH during radiotherapy (DVHm) and DVH of BL50%/BL80% were compared against planning DVH (DVHpl). Results The figure and Table 1 summarize the results: the differences between planning and BL50% values of V45-V50 were Conclusions Daily MVCTs permitted to quantify bowel CPM assessing the most likely location of stable BL during pelvic radiotherapy. BL next to external iliac lymphnodes were quite stable while small bowel was associated to large random motion. Bowel planning strategies should be implemented to take into account the anatomical segments more likely to remain stable during radiotherapy. Download high-res image (111KB) Download full-size image