comparison of measured and monte carlo calculated dose distributions from “circular collimators” for radiosurgical beams

background: stereotactic radiosurgery is an important clinical tool for the treatment of small lesions in the brain, including benign conditions, malignant and localized metastatic tumors. a dosimetry study was performed for elekta ‘synergy s ’ as a dedicated stereotactic radiosurgery unit, capable of generating circular radiation fields with diameters of 1-5 cm at isocentre using the beam/egs4 monte carlo code. materials and methods: the linear accelerator elekta “synergy s” equipped with a set of 5 circular collimators from 10 mm to 50 mm in diameter at isocentre distance was used. the cones were inserted in a base plate mounted on the collimator linac head . a pinpoint chamber and wellhofer water tank chamber were selected for clinical dosimetry of 6 mv photon beams. the results of simulations using the monte carlo system beam/egs4 to model the beam geometry were compared with dose measurements . results: an excellent agreement was found between monte carlo calculated and measured percentage depth dose and lateral dose profiles which were performed in water phantom for circular cones with 1, 2, 3, 4 and 5 cm in diameter. the comparison between calculation and measurements showed up to 0.5 % or 1mm difference for all field sizes. the penumbra (80-20%) results at 5 cm depth in water phantom and ssd=95 ranged from 1.5 to 2.1 mm for circular collimators with diameter 1 to 5 cm . conclusion: this study showed that beamnrc code has been accurate in modeling synergy s linear accelerator equipped with circular collimators.