Dose distribution characteristic study of the Rotating Gamma Knife (RGK) by using the geometry analytic method

The purpose of this study was to realize the processing of dose distribution of RGK at the treatment isocenter at any gantry rotational angle by using an analytic geometry method to avoid inadequate arc therapy angles when implementing the treatment plan. Gaf chromic film was used for dose evaluation. A calibration curve was first obtained using linear accelerator irradiation. The 50% dose relative to the central axis at fixed gantry angles at the x-, y- and z-axes was obtained using Gaf chromic film and was compared to the analytic geometry method. The full width half maximum (FWHM) on the x-, y- and z-axes was predicted for the RGK dose distribution characteristic analysis. The FWHM on the x-, y-, and z-axes varies with different gantry and rotational plate angles. The most dramatic intersection variation appeared at a static gantry angle of 25°. The ratio of the FWHM of the y- and z-axes to that of the x-axis was up to 9 and 10. The geometric analytic method can be used for an accurate analysis of dose distribution in RGK replacing the actual film exposure experiment. It is essential to select the best arc irradiation angle to prescribe the dose to avoid excess irradiation of normal tissue. The geometric method used in this study can also be applied for rotational arc therapy dose analysis such as tomotherapy, linear-based stereotactic radiotherapy, or volume matrices arc therapy.