Study of scattered photons from the collimator system of Leksell Gamma Knife using the EGS4 Monte Carlo Code

• Published in: Medical physics (Lancaster) 2006-01, Vol.33 (1), p.41-45
• Main Authors: Cheung, Joel Y. C., Yu, K. N.
• Format: Article
• Language: English
• Subjects: ALGORITHMS; backscatter; Backscattering; cobalt; COBALT 60; Collimation; COLLIMATORS; Compton scattering; COMPUTER CODES; Computer Simulation; DOSIMETRY; Dosimetry/exposure assessment; EFFICIENCY; Electron scattering; Equipment Design; Equipment Failure Analysis; GEOMETRY; Leksell Gamma Knife; medical computing; Models, Biological; Models, Statistical; Monte Carlo; MONTE CARLO METHOD; Monte Carlo methods; PHANTOMS; Photon scattering; PHOTONS; Photons - therapeutic use; PLANNING; RADIATION DOSES; RADIATION PROTECTION AND DOSIMETRY; radiation therapy; RADIOLOGY AND NUCLEAR MEDICINE; Radiometry - methods; Radiosurgery - instrumentation; RADIOTHERAPY; Radiotherapy Dosage; Radiotherapy Planning, Computer-Assisted - methods; scattered photons; Scattering, Radiation; Secondary emission; SIMULATION; Software; SPECTRA; Treatment strategy
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.2143138

In the algorithm of Leksell GAMMAPLAN (the treatment planning software of Leksell Gamma Knife), scattered photons from the collimator system are presumed to have negligible effects on the Gamma Knife dosimetry. In this study, we used the EGS4 Monte Carlo (MC) technique to study the scattered photons coming out of the single beam channel of Leksell Gamma Knife. The PRESTA (Parameter Reduced Electron-Step Transport Algorithm) version of the EGS4 (Electron Gamma Shower version 4) MC computer code was employed. We simulated the single beam channel of Leksell Gamma Knife with the full geometry. Primary photons were sampled from within the Co 60 source and radiated isotropically in a solid angle of 4 π . The percentages of scattered photons within all photons reaching the phantom space using different collimators were calculated with an average value of 15%. However, this significant amount of scattered photons contributes negligible effects to single beam dose profiles for different collimators. Output spectra were calculated for the four different collimators. To increase the efficiency of simulation by decreasing the semiaperture angle of the beam channel or the solid angle of the initial directions of primary photons will underestimate the scattered component of the photon fluence. The generated backscattered photons from within the Co 60 source and the beam channel also contribute to the output spectra.