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

Abstract

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 <5 cc/10 cc for adjuvant/radical patients. V45-V50 of BL80% were much smaller, (3–5 times less than DVHpl), with higher values for radical pts. BL50% contour roughly follows the planning BL contour, excepting a more caudal position of descending and sigmoid colon in 2 adjuvant patients due to a different average bladder filling during Tomotherapy. BL80% volumes were much smaller, particularly for adjuvant patients, mostly located next to the right (cecum) and left external iliac lymphnodes. Small bowel was always associated with random motion, resulting in an almost absent BL80% in the central part of the intestinal cavity. Conclusions To assess inter-fraction bowel loops (BL) motion and quantify its impact on BL DVHs in prostate radiotherapy including pelvic nodes irradiation. 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). The figure and Table 1 summarize the results: the differences between planning and BL50% values of V45-V50 were <5 cc/10 cc for adjuvant/radical patients. V45-V50 of BL80% were much smaller, (3–5 times less than DVHpl), with higher values for radical pts. BL50% contour roughly follows the planning BL contour, excepting a more caudal position of descending and sigmoid colon in 2 adjuvant patients due to a different average bladder filling during Tomotherapy. BL80% volumes were much smaller, particularly for adjuvant patients, mostly located next to the right (cecum) and left external iliac lymphnodes. Small bowel was always associated with random motion, resulting in an almost absent BL80% in the central part of the intestinal cavity.