OMEGA Monte Carlo test of the Gamma Knife dosimetry

The dosimetry of narrow Co-60 beams from Gamma Knife is based on the knowledge of (i) central axis depth-dose data (ii) beam profiles and (iii) collimator output factors. All three elements present a challenge to the current data acquisition and verification methods. In this work, most recent experimental and analytical data pertinent to the Gamma Knife dosimetry were verified using similar data generated via OMEGA Monte Carlo Codes. We have simulated one of the 201 source positions and thereby generated single beam data for PDD, beam profiles and output factors for all four Gamma Knife collimators currently in clinical use. Simulation geometry was an exact replica of dimensions utilized by the Gamma Knife manufacturer. The average percent deviation of the OMEGA derived PDD values are 1.5%, 1.8%, 2.7% and 3.2% respectively for the 18 mm, 14 mm, 8 mm and 4 mm diameter collimators at isocenter. Experimentally measured beam profiles for all collimators appear to have penumbra sharper than those generated by Monte Carlo. As for the output factors, a 5% difference from the corresponding published value is observed for the 4 mm diameter collimator and about 2% for the rest of the apertures. Thus, the agreement between the clinically used and Monte Carlo simulated sets of data is found to be very good