Comparison of three strategies to delineate the bowel for whole pelvis IMRT of prostate cancer

Purpose: To compare three different contouring approaches of the bowel before and during whole pelvis IMRT of localized prostate cancer. Materials: Nine patients were randomly selected among those treated for localized prostate cancer at UTMB from March 2004 to August 2006. On the planning CT, besides the usual organs at risk (OAR), for each patient we contoured the bowel according to three different definitions: each bowel segment ('BS'); 'BS+1', BS uniformly expanded by 1 cm; intestinal cavity ('IC') or the 'container' of the bowel loops up to the pelvic/abdominal walls. For each patient we generated three rival plans each considering a different bowel definition, otherwise identical. Provided that the same target coverage and other OAR spare had been achieved, plans were compared for their ability to minimize bowel dose at planning. Furthermore, after co-registering 6 weekly CT to the initial planning CT for each patient, we investigated which of the three definitions would allow the best bowel protection also during treatment. Results: All definitions provided a very similar average bowel DVH at planning. During treatment BS allowed an average ≈20 cc more of bowel to receive at least 45 Gy over BS+1 and IC (p = 0.008 and 0.029, respectively); on the contrary bowel V45 between IC and BS+1 were not significantly different (p = 0.65). Conclusion: A definition that takes into account internal organ motion is warranted to maximize bowel protection during treatment.