Determination of the relative linear collision stopping power and linear scattering power of electron bolus material

Abstract

The linear collision stopping power and linear scattering power for machineable wax relative to water have been determined for electron energies between 2 and 20 MeV. Knowledge of these quantities is necessary for the use of this wax as bolus in electron pencil-beam dose algorithms. The atomic composition of the wax ( rho =0.920+or-0.001 g cm-3) was obtained by having the wax assayed. The formalisms expressed in the ICRU Report 35 were used to calculate the relative linear collision stopping and linear scattering powers of the wax. The calculated relative linear collision stopping powers of 2 to 20 MeV electrons in the wax ranged from 0.949+or-0.005 to 0.952+or-0.005, and the calculated relative linear scattering powers ranged from 0.734+or-0.004 to 0.729+or-0.004. As a check of the calculation method, the relative linear collision stopping power was measured by determining the shift in electron central-axis depth-ionization curves when varying thicknesses of water were replaced by wax. These measurements, made using 10, 12, 15 and 18 MeV electron beams with wax thicknesses from 1.0-4.0 cm, resulted in a mean value of 0.931+or-0.008. Determination of the relative linear stopping power and the linear scattering power by using the measured CT number to extract values from patient data tables resulted in values of 0.933+or-0.009 and 0.746+or-0.016, respectively, indicating that it should be acceptable to use the Hounsfield values obtained with CT scans for treatment planning dose calculations.