OPTIMUM FIELD SIZE AND CHOICE OF ISODOSE LINES IN ELECTRON-BEAM TREATMENT

Purpose: A method is provided for the optimum field size and the choic e of isodose line for the dose prescription in electron beam therapy. Methods and Materials: Electron beam dose uniformity was defined in te rms of target coverage factor (TCF) which is an index of dose coverage of a given treatment volume. The TCF was studied with respect to the field size, the beam energy, and the isodose level for prescription fr om the measured data for various accelerators. The effect of the TCF o n air gap between electron applicator/cone and the surface was investi gated. Electron beams from scattering foil and scanned beam units were analyzed for the target coverage. Results: A mathematical method is p rovided to optimize a field size for target coverage by a given isodos e line in terms of TCF which is strongly dependent on the type of acce lerator and the design of the collimator. For a given type of collimat ing system, the TCF does not depend on the type of electron beam produ ction (scattering foil or swept scanned beam). Selection of isodose li ne for dose prescription is very critical for the value of the TCF and the dose coverage. The TCF is inversely proportional to the isodose v alue selected for the treatment and nearly linear with field size and beam energy. Air gap between applicator and the surface reduces the do se uniformity. Tertiary collimator moderately improves the lateral cov erage for high energy beams. Conclusions: To adequately cover the targ et volume in electron beam treatment, lateral and depth coverage shoul d be considered. The coverage at depth is strongly dependent on the ch oice of isodose line or beam normalization. If the dose prescription i s at d(max) (i.e., the 100% isodose line is selected), the choice of b eam energy is not critical for depth coverage since d(max) is nearly i ndependent of energy for smaller fields. The 100% isodose line should not be chosen for treatment because of the significant constriction of this isodose line and inadequate coverage at depth. For a higher TCF, a minimum air gap between the cone to the surface of the patient is d esired. If such is not possible, then a tertiary collimator at the ski n is required. Whenever, a tertiary collimator is used, it is advised to increase the collimator field size by a factor of 1.4.