Impact of rectal distension on prostate CBCT-based positioning assessed with 6 degrees-of-freedom couch

Introduction: Prostate requires a daily correction of its 3-dimensional position in relation with rectal distension. In this study, we sought to determine whether rectal distension with respect to the rectal behavior might have an impact on prostate translations and/or rotations during prostate image guided radiation therapy using a 6 degrees-of-freedom (DOF) couch.

Methods and materials: We reviewed the data from 39 patients with localized prostate cancer treated with protracted external radiation therapy using a 6 DOF couch. Before each fraction, a kilovoltage cone beam computed tomography (kV-CBCT) scan was performed. The automatic fusion algorithm was set to fuse on soft tissue and allowed correction for translations in 3 dimensions and rotations in the longitudinal axis ("roll") and lateral axis ("pitch"). After contouring the rectum on each kV-CBCT, we determined the cross-sectional area (CSA) and relative CSA (CSA_rel) by dividing with the CSA of planning CT. The standard deviation of CSA_rel per patient was used to classify the patients in 2 groups: patients with a stable rectum and patients with an unstable rectum. The CSA_rel was compared between these 2 groups with a linear mixed model with group as fixed effect and patient as random effect.

Results: A total of 616 kV-CBCT were analyzed, and 2 subgroups of patients could be defined a posteriori: 19 patients had a stable rectum, mean CSA_rel (1.06 ± 0.08); the other 20 patients had an unstable rectum, mean CSA_rel (1.43 ± 0.08). The average pitch in the group with a stable rectum was 0.36° (±0.21) versus 0.40° (±0.20) (P = .898). The pitch was not correlated with the CSA_rel (P = -.065, r = 0.119). The average roll in the group with a stable rectum was 0.27° (±0.16) versus 0.05° (±0.16) (P = .137). The roll was not correlated with the CSA (P = .094, r = 0.068). The average CSA_rel was higher (P = .0013) and more variable (P = .035) in the unstable group.

Conclusion: Rectal distension had no impact on the pitch or on the roll, which suggest that a 6 DOF couch has little interest in daily practice for prostate image guided radiation therapy.