Electron mass scattering powers: Monte Carlo and analytical calculations

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Abstract
Values of electron mass scattering power, T/ρ, for various materials have been calculated by using the EGS4 Monte Carlo system and by integration of the Molière multiple‐scattering distribution. The energy range covered is 0.5–100 MeV. Monte Carlo calculations test the concept of T/ρ “experimentally” and assess the contribution to electron mass scattering power from effects such as Moller scatter and energy‐loss straggling. The Monte Carlo results agree within 2% with the analytical results calculated from Molière multiple‐scattering theory at energies less than 20 MeV for high‐Z materials and for energies less than 50 MeV for low‐Z materials. At higher energies the Monte Carlo calculations include the effects of bremsstrahlung production which can significantly increase values of T/ρ. For low‐Z materials and electron energies less than 60 MeV, the Monte Carlo calculated T/ρ values are generally 22% higher than those given by ICRU Report 35, while those for high‐Z materials and energies less than 25 MeV are found to be consistent (within 1%) with ICRU Report 35. The effects of Moller scatter, which significantly affect T/ρ for low‐Z materials, as well as bremsstrahlung effects, are included in the present Monte Carlo calculations. If the tabulated T/ρ data of ICRU Report 35 are modified to include the Moller scatter effect, then for energies less than 60 MeV they are generally 6% less than the present Monte Carlo data for low‐Z materials as well as for copper. It is shown that T/ρ is a well‐defined constant over an appropriate range of slab thickness except when bremsstrahlung effects are significant. It is found that T/ρ is proportional to E−n, where n is in the range of 1.5–2.0 for the energies considered here. The Monte Carlo calculations are shown to agree well with various relevant experimental measurements. Accurate T/ρ data, which should include the effect of Moller scatter, are necessary in electron‐beam treatment planning, especially for a small field size. The choice of the depth step in the implementation of pencil‐beam codes should not violate the slab‐thickness limits for T/ρ data.

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