An evolutionary algorithm for the optimization of parameters in radiation beam profile modeling for the CyberKnife X-band linear accelerator

The calculation of the radiation absorbed dose to tissue requires the modeling of several parameters to describe the collimated beam. One of these parameters, the off-center ratio (OCR), describes the beam profile, i.e. the shape of the beam along a particular axis orthogonal to the direction in which the beam is aimed. The OCR is defined as the ratio of the dose at a point a specified distance from the center of the beam to the dose at the center of the beam. In the present work, the OCR of the CyberKnife X-band linear accelerator was modeled using error functions, with each individual error function representing a scatter kernel, and the summation describing the shape of the circular beam, with the assumption of azimuthal symmetry about the central axis of the beam. An evolutionary algorithm was used to optimize the amplitudes and standard deviations associated with the error functions, as well as the radius of the profile. Calculated OCR values were plotted against measured OCR values acquired during the commissioning of the CyberKnife. The model correctly predicts the beam profile.