Dosimetric coverage of anal sphincter by 3D conformal fields in patients undergoing neoadjuvant chemoradiation for rectal cancer: An implication in the concept of sphincter-preserving radiation therapy.

Abstract

558 Background: Functional impact related to dosimetric coverage of the anal sphincter in rectal cancer patients treated with neoadjuvant chemoradiation (NCRT) remains unknown. We evaluated the anatomic location of the anal sphincter to pelvic bony landmarks related to 3D conformal radiotherapy (3DRT) and studied the dosimetric coverage of the anal sphincter in patients undergoing NCRT for rectal cancer. Methods: Between June 2006 and March 2012, 64 patients with rectal cancer treated with NCRT were included. All patients were treated in prone position on a bellyboard by 3DRT with 3 or 4-field setup up to 5,040 (86%) or 5,400 (14%) cGy. The inferior borders of the RT fields used were the lower of either at 3-5 cm inferior to the lowest gross tumorous volume (GTV) or at the inferior border of the obturator foramina (IBOF). The anal sphincters were contoured retrospectively and dose distributions were determined using dose volume histograms. Results: The mean anal sphincter volume was 5.1 cm3 (range, 1.3 – 11.2 cm3). On average, the center of anal sphincter was 18.6 mm inferior to the IBOF. In 53 cases (82.8%), the anal sphincter was completely inferior to the IBOF. In the remaining 11 cases (17.2%), the anal sphincter was either overlapping the IBOF (10 cases; 15.6%), or completely superior to the IBOF (1 case; 1.7%). On average, the center of anal sphincter was 39 mm inferior to the lowest edge of GTV. In 19 cases (30%), the anal sphincter was completely outside of the inferior border of the RT fields. In another 10 cases (16%), the sphincter was more than 4 cm away from the GTV inferior border but was partially treated. The mean dose delivered to anal sphincter was 2795 cGy (range, 245–5441 cGy). Lower mean doses delivered to the anal sphincters were noted for larger distances from the center of the anal sphincter to the GTV. Conclusions: Minimizing dose delivered to anal sphincter is possible, especially for upper or mid rectal cancer. However, meticulous planning to define the inferior border of RT fields is necessary. Future studies will focus on the correlation between RT dose distribution to the anal sphincter and anal-rectal dysfunction.