Calculation of rectal dose surface histograms in the presence of time varying deformations

Purpose: Dose volume (DVH) and dose surface histograms (DSH) of the bladder and rectum are usually calculated from a single treatment planning scan. These DVHs and DSHs will eventually be correlated with complications to determine parameters for normal tissue complication probabilities (NTCP). However, from day to day, the size and shape of the rectum and bladder may vary. The purpose of this study is to compare a more accurate estimate of the time integrated DVHs and DSHs of the rectum (in the presence of daily variations in rectal shape) to initial DVHs/DSHs. Methods: 10 patients were scanned once per week during the course of fractionated radiotherapy, typically accumulating a total of six scans. The rectum and bladder were contoured on each of the studies. The model used to assess effects of rectal contour deformation is as follows: the contour on a given axial slice (see figure) is boxed within a rectangle. A line drawn parallel to the AP axis through the rectangle equally partitions the box. Starting at the intersection of the vertical line and the rectal contour, points on the contour are marked off representing the same rectal dose point, even in the presence of distortion. Corresponding numbered points are used to sample the dose matrix and create a composite DSH. The model assumes uniform stretching of the rectal contour for any given axial cut, and no twist of the structure or vertical displacement. A similar model is developed for the bladder with spherical symmetry. Results: Normalized DSHs (nDSH) for each CT scan were calculated as well as the time averaged nDSH over all scans. These were compared with the nDSH from the initial planning scan. Individual nDSHs differed by 8% surface area irradiated at the 80% dose level, to as much as 20% surface area in the 70-100% dose range. DSH variations are due to position and shape changes in the rectum during different CT scans. The spatial distribution of dose is highly variable, and depends on the field arrangement and weights used for each field. Two dose gradients exist, a slowly varying gradient (e.g. within the high dose volume) and rapidly varying gradient (e.g. dose gradient from the field edges). Superimposed on this variation are the sampling points from a deformable hollow organ, which can translate and deform in both slow and rapidly varying regions of dose. The individual scan nDSHs and average nDSH summarize the effects of these variations. Conclusion: The initial DSH for the rectum in a given plan can vary by 8% for moderate to high dose in comparison to the time averaged nDSH. The effect on NTCP is expected to be relatively small, but it is important to note that at any given instant of time, uncertainty in the absolute surface area of a structure at a given dose can vary by as much as 20% due to distortion and movement