Monte Carlo estimation of electron contamination in a 18 MV clinical photon beam

Background: The electron contamination may reduce or even diminish the skin sparing property of the megavoltage beam. The detailed characteristics of contaminant electrons are presented for different field sizes and cases. Materials and Methods: The Monte Carlo code, MCNPX, has been used to simulate 18 MV photon beam from a Varian Linac-2300 accelerator. All dose measurements were carried out using a PTW-MP2 scanner with an ionization chamber (0.6 CC) at the water phantom. Results: The maximum electron contaminant dose at the surface ranged from 6.1 % for 5 × 5 cm2 to 38.8 % for 40 × 40 cm2 and at the depth of maximum dose was 0.9 % up to 5.77 % for the 5 × 5 cm2 to the 40 × 40 cm2 field sizes, respectively. The additional contaminant electron dose at the surface for the field with tray increased 2.3 % for 10 × 10 cm2, 7.3 % for 20 × 20 cm2, and 21.4 % for 40 × 40 cm2 field size comparing to the standard field without any accessories. This increase for field with tray and shaping block was 5.3 % and 13.3 % for 10 × 10 and 20 × 20 cm2, respectively, while, the electron contamination decreased for the fields with wedge, i.e. 2.2 % for the 10 × 10 cm2 field. Conclusion: The results have provided more comprehensive knowledge of the high-energy clinical beams and may be useful to develop the accurate treatment planning systems capable of taking the electron contamination in to account. Iran. J. Radiat. Res., 2011 9(1): 15-28