Organ Motion During Neoadjuvant Chemoradiotherapy for Rectal Cancer – A Call to Update the RTOG Contouring Guidelines

Abstract

Motion of the rectum and bladder during neoadjuvant radiation for rectal cancer has been studied mostly in the prone position and was the basis for the RTOG contouring recommendations for rectal cancer. Radiation planning and delivery at our center are conducted in the supine position. The aim of this study was to evaluate the deformation and displacement of the mesorectum, rectum, and bladder during a typical 5-6 week course of neoadjuvant chemoradiotherapy (CRT) for rectal cancer. After obtaining the required regulatory approvals, 23 patients (14 male, 9 female) were prospectively recruited to this study. Cone Beam CT (CBCT) images were acquired in supine position on the first two days of treatment and once weekly thereafter. CBCTs were fused with treatment planning CT scans based on bony anatomy. Bladder, rectum, and mesorectum were contoured on each CBCT and each planning CT by a single Radiation Oncologist. The overlapping volume (OV) between the planning CT bladder and mesorectum in each CBCT was also contoured. Bladder and rectum volume, as well as the OV were evaluated for each CBCT. The extreme anterior position in the lower, middle and upper rectum as well as mesorectum were evaluated in each CBCT. Linear-mixed models were used for statistical analysis. A total of 148 CBCT data sets were analyzed. Anterior rectal wall and mesorectal fascia showed significant change in position over time. There was also a significant directional movement of bladder with time. The interfractional change in the overlap volume of the planning bladder contour and CBCT mesorectal contours was significant and correlated with gender, bladder, and rectal volumes. This study demonstrated that the change in bladder, rectal, and mesorectal volumes and movement during the course of neoadjuvant CRT for rectal cancer are significant and may affect the chosen margins. The change in position of the bladder-mesorectal interface threatens the coverage of clinical target volume (CTV). An Internal Target Volume (ITV) based volume expansion for rectal cancer contouring is recommended to control for these uncertainties.