Dose-wall histograms and normalized dose-surface histograms for the rectum: a new method to analyze the dose distribution over the rectum in conformal radiotherapy.

Purpose: To develop an accurate method to generate a dose-volume histogram (DVH) of the rectum wall, solely based on the outer contours of the rectum wall.
Methods and Materials: A mathematical model for the rectum wall is developed, incorporating the stretching of the rectum wall due to variable rectal filling and neighboring structures. The model is based on the assumption that the amount of intersected rectum wall tissue normal to the central axis of the rectum is constant. The main objective of the model is to determine the thickness of the rectum wall in each wall element. Two approaches are described, each yielding a DVH of the rectum wall, based only on the delineated outer contours of the rectum. In the first approach, the model is used to create a set of inner contours out of the axial outer contours. Both sets of contours are used to derive a dose-wall histogram (DWH) of the rectum. In the second approach, the model is used to generate a normalized 2D sampling space, which is subsequently binned into a normalized dose-surface histogram (NDSH). The model is verified using 20 sets of CT data (5 patients x 4 scans) in which both outer and inner contours of the rectum are carefully delineated. The DWHs and NDSHs are compared with DVHs of the rectum wall, which require contouring of the outer and inner surfaces of the rectum wall, and with DVHs of the total rectum (including rectal filling). The variation between DWHs, NDSHs, and DVHs is investigated using normal tissue complication probability (NTCP) calculations.
Results: The local wall thickness of the rectum as outlined on CT data was in conformity with the described rectum model. The amount of rectum wall tissue per unit length rectum varied considerably between patients (27%, 1 SD). In all analyzed patients, the DWHs and NDSHs corresponded well to the DVHs of the rectum wall. Much more discrepancies were observed between the DVHs of the total rectum and the DVHs of the rectum wall.
Conclusion: The applied methods yield accurate dose distributions of the rectum wall, without delineating the inner surface of the rectum. This reduces both the workload and variations due to inaccurate delineation of the rectum wall. The DWH and NDSH are effective tools to evaluate 3D dose distributions of the rectum wall and to estimate the complication probability of the rectum in high-dose conformal radiotherapy.