Measurement of the Mean Energy of the Off-axis Incident Electron without the Beam Guide : Using a Scintillating Fiber Beam-Energy Monitor for Electron Beam Therapy

Abstract

In the case of total skin electron therapy without the beam guide, the electron beam is scattered just outside the gantry exit, dose uniformity in the field is broken, and dose is spread outside the light field. The aims of this study were to measure the mean energy of the off-axis incident electron beam without the beam guide and to establish a reference for the clinical situation. For the measurement, a 4 MeV electron beam was selected among several energies from the linear accelerator. A scintillating fiber beam energy monitor measured the mean energy of the incident electron beam. This energy monitor is a small, light-weight piece of equipment composed of a wedge absorber, scintillation fiber, and photodiode. We found the relationship between electron energy and the indicated value of the energy monitor by means of the estimation of correction factors for five different kinds of electron energy. The preferable linear correlation of 0.997 of the coefficient of determination (R^2) was obtained. From the results of measurement at each point, those variations due to the off-axial distance were about 5% within the measured area. It was assumed that the energy did not change rapidly beyond the light field. Clinically, this amount of variation in energy may not cause any problem.