Assesing multileaf collimator effect on the build-up region using Monte Carlo method

Previous Monte Carlo studies have investigated the multileaf collimator ( MLC) contribution to the build-up region for fields in which the MLC leaves were fully blocking the openings defined by the collimation jaws. In the present work, we investigate the same effect but for symmetric and asymmetric MLC defined field sizes ( 2x2, 4x4, 10x10 and 3x7 cm(2)). A Varian 2100C/D accelerator with 120-leaf MLC is accurately modeled for a 6 MV photon beam using the BEAMnrc/EGSnrc code. Our results indicate that particles scattered from accelerator head and MLC are responsible for the increase of about 7% on the surface dose when comparing 2x2 and 10x10 cm(2) fields. We found that the MLC contribution to the total build-up dose is about 2% for the 2x2 cm(2) field and less than 1% for the largest fields.