Study the effect of source to diaphragm distance (SDD) on beam penumbra width of Gamma Knife machine model 4C using Monte Carlo simulation

Introduction: Stereotactic radiosurgery (SRS) is one of the conformal radiotherapy methods and Gamma Knife is a non-invasive SRS technique to treat intracranial lesions with 201 Co- 60 sources without need to surgery and bleeding. Beam penumbra phenomenon leads to irradiation outside the field and normal tissues. Therefore, considering it is momentous to attain the acceptable treatment plan.   Materials and Methods: In this study, Gamma Knife unit was modeled using EGSnrc/BEAMnrc Monte Carlo code. SDD (Source to Diaphragm Distance) parameter that is one of the affecting factors on physical penumbra width was studied. For this purpose, SDD was increased for 4, 8, 14 and 18 mm collimator sizes in five steps and each step as much as one centimeter. Single beam profiles were calculated using EGSnrc/DOSXYZnrc code at isocentre depth in a cubic Plexiglas phantom. MATLAB and EXCEL software were used to plot profiles and measure physical penumbra width . Results: Based on the results, physical penumbra width (90%-50%) does not show a regular trend with increasing SDD. This may be attributed to scatter radiation which is due to the low distance between the Gamma Knife collimator end and phantom surface. Best results were observed for 4 and 8 mm collimators in 1 cm increase in which penumbra width decreased from 0.785 and 0.790 mm to 0.749 and 0.736 mm for 4 and 8 mm collimator sizes, respectively. Conclusion: It is recommended to increase SDD of Gamma Knife 4C around 1 centimeter to reduce penumbra width for small tumor sizes located near the critical nerves.