AbstractID: 7561 Title: An Approximation of the Rectum Based on Orthogonal Films for
Patients Receiving Intracavitary Brachytherapy
Low dose rate intracavitary brachytherapy (ICB) treatment planning often consists of obtaining orthogonal radiographs, reconstructing the source locations, and calculating dose to a limited number of points. For the rectum, these points consist of the ICRU reference point and may include points determined from markers in a rectal tube.
However, to fully optimize treatment planning, one needs to know the dose delivered to the entire rectal volume. The goal of this study is to develop an approximate method for determining the rectal volume from orthogonal radiographs and to calculate the subsequent dose delivered to this volume. Our approach is as follows. At the time of simulation, contrast is injected into the rectum. Anterior and lateral radiographs are obtained, and the rectal contrast is digitized on both views. On tranverse slices located 3 mm apart, the rectum is reconstructed by fitting the boundaries of the contrast (right-left and anterior-posterior) to an ellipse. In order to validate this approach, the rectum of 9 gynecologic patients who had a CT simulation was outlined on orthogonal radiographs.
The approximate rectal contours were then compared to those drawn on axial CT slices.
On average, the percent difference in volume obtained using these approaches was 11%.
Most of this difference occurred in the inferior part of the rectum away from the implant.
This method provides a simple means of calculating doses to the entire rectal volume and may improve the planning and delivery of ICB.
Supported by a grant from the Whitaker Foundation.