Quantification of local rectal wall displacements by virtual rectum unfolding

Background and purpose: To develop a method to project surface elements of a bent tubular organ, e.g. the rectum, in order to create a two-dimensional (2D) map and to use this method to quantify on a local scale shape and position variations of the rectum. Patients and methods: For this study we used data of 19 patients, who each received a planning CT scan and 9-13 repeat CT scans that were considered representative for the radiotherapy course. We combined maps from multiple CT scans of the same patient to quantify local rectal wall displacements. To make a map we first computed a central axis through the rectum and divided it into segments of equal length assuming that the length of these segments was invariant under rectum shape and position changes. Next, we constructed for each segment a planar cross section through the rectum, which was oriented orthogonally to that segment. The amount of rectal wall tissue was assumed to be constant in all orthogonal cross sections throughout the entire rectum. We unfolded the cross-sected rectal wall at the dorsal side and projected either the associated dose or the coordinates onto the map. Results: The largest variation in the position of the rectal wall during the treatment course occurred at the upper anterior, left and right side (1 SD=5-7 mm). Near the anus the variation was <3 mm (1 SD) and at the posterior side of the rectum <4 mm (1 SD). The anterior-posterior (AP) and left-right displacements between the rectum in the planning CT scan and the mean rectum shape during the treatment were localized between 40 and 80% of the central axis. At the upper anterior, left, and right side the displacements were 5-8 mm (1 SD). These rectal wall displacements correlated with the rectum volume in the planning CT scan. At the upper anterior side the correlation coefficient between the AP displacements and the planning rectum volume was 0.85. Conclusions: We quantified variations in rectum shape and in dose in the rectal wall. The systematic error in rectal wall position was found to be larger than the random shape and position variations. We successfully developed a method to virtually unfold a rectum and to project the dose onto a 2D map. The spatial information of the dose distribution can be used in the analysis of rectum complications.

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