Monte Carlo study on electron contamination and output factors of small field dosimetry in 6 MV photon beam

The purpose of this study was to investigate the characteristics of electron contamination and Output Factors (OFs) from Varian Trilogy Clinac iX 6 MV photon beam at small field sizes. EGSnrc Monte Carlo (MC) code system was used to model the photon beam for this Linear Accelerator (Linac) head and analyze the electron contamination and OFs from this treatment head. The electron contamination was analyzed for field sizes of 1x1, 2x2, 3x3, 4x4, and 5x5cm(2). The number of electron contamination increases with increasing field sizes, but the maximum energy of the electron contamination stays constant (at around 1.87MeV for each field size). The contaminants contribute to the dose at the surface of the water phantom (1-5cm from the surface) for field size 4x4 and 5x5cm(2) and this dose decreases with depth. The OFs are simulated by EGSnrc code system and have a good agreement with measurement (deviation 3.45, 1.76, and 0.86 for field of 2, 3 and 4, respectively). This study presented that MC methods have great potential to accurately predict the electron contamination and OFs for 6 MV photon beam.