Dose planning of high energy electron beam in regions of inhomogeneous tissue

Choi, B C; Lee, D H; Chu, S S

Abstract

The dose distributions of high energy electron are perturbated strongly in regions of tissue inhomogeneity.The situation is quite complicated and, alghough the absorption of electrons is primarily determined by the numberof electrons per gram which is almost constant for all materials, scattering depends strongly upon atomic number.Since the dose at any point is determined by the dose from the primary beam plus the dose from the scatteredelectrons, the absorbed dose will be different in materials of different density. The dose distribution is alsodependent upon the range of the electrons which is inversely proportional to the density of the irradiated bodywhen Z/A is virtually constant and the atomic numbers do not radically differ. In treatment planning, it isserious problems that the perturbations of dose distribution about the regions of inhomogeneous tissue, aircavities, lung, bone in body. Using the equivalent materials of inhomogeneous tissues, we measured the dosedistribution from these and calculated the correction factor " mean coefficient of equivalent thickness" andanalyzed the perturbation of dose distribution. 1. For the inhomogeneities, appropriate correction of the dosedistribution could be made by considering the "mean coefficient of equivalent thickness, " of the materials asfollowing. = 0.5 for lung =1.1 for spongy bone =1.8 for compact bone = 0 for air cavity The isodoses were shiftedtowards the entry surface if >1 and towards greater depths if < 1. 2. Within and beyond tissues of lower density,such as lung, the decrease in attenuation resulted in an increase of the absorbed dose, and the decrease inscattering partially offset the elevation in dose. 3. The magnitude of the actual dose in and beyond low densityregions, relatived to that on a unit density basis, depended in a nonlinear manner on the location, extent, anddensity of the inhomogeneous region. 4. Within high atomic number material, such as bone, and immediately beyondin the interface region, there was an clevated dose, due to enhanced scattering. 5. The perturbation of dosedistribution adjacent to cavities had shown that the percentage of dose increased varied markedly as a function ofthe buile-up layer, the length and thickness of the cavities, and the electron energy.