Limitations of the bowel bag contouring technique in the definitive treatment of cervical cancer.

Abstract

Incidence of acute grade 3 and 4 small bowel toxicity in the definitive treatment of cervical cancer is approximately 15%. Given uncertainties in position of the bowel at time of treatment, techniques including the contouring of a bowel bag have been suggested. The purpose of this study is to describe interfraction variability in bowel location for the female pelvis with intact reproductive organs and to characterize the ability of the bowel bag technique, as described in the Radiation Therapy Oncology Group pelvic normal tissue contouring guidelines, to account for organ motion in this specific clinical setting. Bowel position was assessed for 45 computed tomographic scans used in treatment planning for 9 consecutive cervical cancer patients. After a single operator contoured bowel loops, most superior, anterior, posterior, and inferior positions of bowel were recorded. Mixed effects models were used to assess significance of interfraction variability. Frequency of bowel loop migration outside of the bowel bag was then considered for each patient given all potential bowel bag volumes. Standardized scoring was used to determine additional margins that would be required to account for 95%, 90%, and 85% of significant bowel motion. Interfraction variability in the inferior-most bowel position was significant (P = .002). Median maximum variation in the inferior bowel position was 2.1 cm (range, 0.9 cm-4.8 cm). When applying the bowel bag technique, 100% of bowel motion was accounted for as the bowel translated laterally, anteriorly, posteriorly, and superiorly, though accounted for just 70.3% of motion in the inferior direction. A 4-cm inferior margin was required to account for 90% of motion in the inferior direction. In the intact female pelvis, the bowel bag technique is successful in accounting for most interfraction variability in bowel position but underestimates inferior motion. Until an improved approach to predicting small bowel motion can be routinely implemented, a focus on decreasing dose to potential bowel space should be emphasized.