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 wan. Methods and Materials: A mathematical model for the rectum wall is develope d, incorporating the stretching of the rectum wall due to variable rectal f illing and neighboring structures. The model is based on the assumption tha t the amount of intersected rectum wall tissue normal to the central axis o f the rectum is constant. The main objective of the model is to determine t he thickness of the rectum wall in each wall element. Two approaches are de scribed, each yielding a DVH of the rectum wall, based only on the delineat ed outer contours of the rectum. In the first approach, the model is used t o 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 sampl ing space, which is subsequently binned into a normalized dose-surface hist ogram (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 variati on between DWHs, NDSHs, and DVHs is investigated using normal tissue compli cation 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 ti ssue per unit length rectum varied considerably between patients (27%, 1 SD ). In all analyzed patients, the DWHs and NDSHs corresponded well to the DV Hs of the rectum wall. Much more discrepancies were observed between the DV Hs of the total rectum and the DVHs of the rectum wall. Conclusion: The applied methods yield accurate dose distributions of the re ctum wall, without delineating the inner surface of the rectum. This reduce s both the workload and variations due to inaccurate delineation of the rec tum wall. The DWH and NDSH are effective tools to evaluate 3D dose distribu tions of the rectum wall and to estimate the complication probability of th e rectum in high-dose conformal radiotherapy. (C) 1999 Elsevier Science Inc .