High-energy electron-beam absorbed-dose computation for shaped fields

The high-energy electron beams produced by the medical linear accelerator or betatron are widely used for treatment of cancers. The determination of the electron-beam absorbed dose distribution in tissue, with sufficient accuracy, is a complex problem in computational methods that need to be confirmed with measured doses. The electron treatment fields are often shaped in various forms from a standard beam applicator using secondary blocking to fit the treatment area or volume. Shaping a field within the limit of practical range of the electron beam energy (a) changes the absorbed dose distribution within the irradiated volume, (b) shifts the d/sub max/ (the point of maximum absorbed dose), and also (c) alters the output factor (ratio of the output for a shaped field to the output for a reference field). In this study, an investigation of the validity of electron-beam absorbed dose computational methods using (a) a strip-beam summation method and (b) a narrow-beam summation method for shaped fields was performed. The results indicate that the finite length strip-beam model is the most accurate to predict the d/sub max/-shift, output factor variation and also the depth dose as well as isodose distributions for shaped fields. The important finding in this study was that the d/sub max/-shift is strongly dependent upon the smallest dimension of the shaped field as long as the other dimension is larger than the practical range. This study also shows that the output factor variation can be predicted within a 5% accuracy compared to the measured value. A complete isodose distribution can be produced without a direct measurement