Random variation in rectal position during radiotherapy for prostate cancer is two to three times greater than that predicted from prostate motion

Objective: Radiotherapy for prostate cancer does not explicitly take into account daily variation in the position of the rectum. It is important to accurately assess accumulated dose (D-A) to the rectum in order to understand the relationship between dose and toxicity. The primary objective of this work was to quantify systematic (Sigma) and random (sigma) variation in the position of the rectum during a course of prostate radiotherapy. Methods: The rectum was manually outlined on the kilovoltage planning scan and 37 daily mega-voltage image guidance scans for 10 participants recruited to the VoxTox study. The femoral heads were used to produce a fixed point to which all rectal contours were referenced. Results: Sigma [standard deviation (SD) of means] between planning and treatment was 4.2mm in the anteroposterior (AP) direction and 1.3mm left-right (LR). sigma (root mean square of SDs) was 5.2mm AP and 2.7mm LR. Superior-inferior variation was less than one slice above and below the planning position. Conclusion: Our results for Sigma are in line with published data for prostate motion. sigma, however, was approximately twice as great as that seen for prostate motion. This suggests that D-A may differ from planned dose in some patients treated with radiotherapy for prostate cancer. Advances in knowledge: This work is the first to use daily imaging to quantify Sigma and sigma of the rectum in prostate cancer. sigma was found to be greater than published data, providing strong rationale for further investigation of individual D-A.