A Monte Carlo simulation study on the effectiveness of electron filters designed for telecobalt radiation therapy treatment

The aim of present study was to analyze the effectiveness of electron filters in the Telecobalt radiotherapy treatment by simulation technique.  The BEAMnrc Monte Carlo code was used to simulate the electron filters of thickness of 0.5 gm/cm2 below the trimmer bar for 35 × 35 cm2 field size in Theratron Equinox-80 telecobalt unit. The electron filters were made of an aluminum, copper, nickel, tin, PMMA, and lead with single or composite materials. The radiation beams at treatment distance were analyzed by generating profiles for photon and electron along the X-axis of radiation field.  The electron energy fluence for unfiltered beam was 0.32% of the photon energy. The photon energy fluence intensity reduction due to filter was 3.7%. The filters with low atomic number have shown poor electron contamination removal efficiency. The tin, copper and nickel were found effective filters, removing nearly 38% of contaminant electron energy. The lead filter is equally effective as tin, however the high energy electrons emitted from filter due to “photo peaks” adds significant dose at 3.0 to 4.0 mm depth. The tin filter dominates over to other filters on the subject of surface dose reduction and depth of dose maximum (dmax). It reduces the surface dose by 9.6% and 13.9 % of unfiltered beam for 15 × 15 and 20 × 20 cm2 field sizes respectively.