Assessment of rectal distention in radiotherapy of prostate cancer using daily megavoltage CT image guidance

Abstract

Purpose Assessment of rectal distention in a group of patients who are not receiving daily rectum emptying procedures during a course of prostate cancer radiotherapy to investigate which patients could benefit from daily rectum emptying. Methods and materials Eighteen patients underwent daily megavoltage CT (MVCT) scanning with positioning based on bony anatomy. Emptying the rectum was only performed before planning CT and not during the actual treatment. The rectal average cross-sectional area (CSA) was determined on the MVCTs. The relative CSA (CSA rel) was defined as CSA on MVCT / CSA on planning CT. Additional prostate soft tissue matching was performed to verify the influence of rectal distention on prostate motion. Results Two distinct subgroups could be defined a posteriori. One group had a limited and stable rectal distention with a CSA (mean ± SD) of 6.6 ± 2.1 cm 2, in contrast with a second group with large and variable rectal filling with a CSA of 9.5 ± 3.7 cm 2 ( p < 0.01). Mean anterior-posterior prostate displacement was 0.4 ± 2.4 mm in the stable group versus -2.4 ± 6.1 mm in the unstable group ( p < 0.01). A mean CSA rel of 1.35 of the first 3 days as cut-off value allowed for a correct a priori classification of 90% and 85% of the patients from groups 1 and 2, respectively. Conclusion Based on a few measurements of the CSA by daily MVCT imaging at the first days of treatment, rectum emptying may be omitted in part of the patients.