Comparison of measured and Monte Carlo calculated dose distributions from “Circular Collimators “ for Radiosurgical beams

Stereotactic radiosurgery is an important clinical tool for the treatment of small lesions in the brain, including benign conditions, malignant and localized metastatic tumours .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 Well hofer water tank chamber have been selected for clinical dosimetry in 6 MV photon beams. Results of simulations using the Monte Carlo system BEAM/EGS4 to model the beam geometry have been 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 shows that BEAMnrc code is capable in accurate modelling Synergy S linear accelerator equipped with circular

Comparison of measured and Monte Carlo calculated dose distributions from “Circular Collimators “ for Radiosurgical beams Keywords:

Monte Carlo , dosimetry , radiosurgery , circular cones