Unstable rectum.

It’s not as bad as it sounds, at least according to this study examining pitch and roll corrections during a course of prostate radiation using daily cone-beam CT alignment. You see, anybody who’s anybody these days is bouncing around a LINAC couch in 6-degrees of freedom (6DOF). So how useful is 6DOF in correcting prostate position? We know that rectal distension can seriously alter it relative to bony anatomy, but we’re not sure how much is altered in the x, y, and z planes. Until now. About half the patients had what the authors deemed “unstable rectums,” meaning there was increased daily variabilities in rectal cross-sectional area. Interestingly, rectal distension was not associated with pitch or roll, and pitch/roll corrections were similar and minimal (< 0.4 degrees all around) between the stable and unstable groups. TBL: Rectal distension doesn’t appear to cause significant pitching and rolling of the prostate, even for those with unreliable rectums. | Charret, Pract Radiat Oncol 2018