Electron contamination scatter factor from different megavoltage therapy machines

Abstract

Electron contamination can be defmed as the undesirable electrons which accompany high energy photon beam during radiotherapy. It arises from the interaction between incident photon beam and the metallic constituents of gantry head (such as flattening filter & collimator jaws) and the air volume separating between the treatment head and the point of measurement. Electron contamination is known to increase with photon beam energy & field size, and decrease with Source Axis Distance (SAD). It is also affected by machine collimator system design. Contamination with secondary electrons would probably degrade the skin sparing advantage of high-energy x-rays resulting in a possible increase in the surface dose to patient and skin injury. Since electron contamination depends on many parameters that vary potentially between radiotherapy machines and measurement conditions, it should be calculated independently for each machine.

In this work collimator scatters factors in air (Sc(air)) and in mini-phantom (Sc(mph)) are measured using Perspex build-up cap & miniphantom, respectively. By dividing Sc(air) by Sc(mph) electron contamination scatter factor (Sce1) is calculated for three linear accelerators, Varian 600C (6 MV without MLC), Varian DBX (6 MV with MLC) and Elekta precise (6&15 MV with MLC). Measurements are done at 80, 100 & 120 em SAD for square fields and at 100 em SAD for asymmetric, conformal & IMRT fields.

Results show that the maximum recorded Scel value for square fields is 4.84% with respect to reference field for Elekta 15 MV at 80 em SAD. At I 00 em SAD, the value of Scel is 1.91% for the same machine. For Varian 600C, Varian DBX & Elekta 6 MV, the