Calculation of x-ray spectra for radiosurgical beams

As conformal radiosurgery using micromultileaf collimators gains feasibility, dose calculation algorithms based on Monte Carlo or convolution techniques may become necessary. These require radiosurgical x-ray spectra. The most accurate method currently available to estimate clinical radiosurgery spectra is the Monte Carlo method. In this study the EGS4 Monte Carlo system was used to simulate the thick target of a 6 MV linear accelerator used for radiosurgery in our center. The calculated spectrum was attenuated through any significant mass thickness of material downstream from the target. The attenuated thick-target spectral distributions calculated both with and without the flattening filter were compared to the attenuated, thin target spectrum based on the small angle Schiff analytical spectrum calculated for the same target and attenuator material, as well as with a published spectrum from a full Monte Carlo simulation of a treatment head with a flattener in place. The Schiff spectrum neglects contributions from lower-energy scattered electrons that significantly degrade the quality of the beam. The flattener is removed from our accelerator during radiosurgery to increase the dose rate to approximately 750 cGy/min for a 10 x 10 cm2 field at the depth of dose maximum. This leaves a substantial fluence of photons below 1 MeV that are not observed in published spectra calculated for accelerators with flattening filters. Removal of the flattening filter has a measurable effect on the central axis depth dose, reducing the percentage dose at 10 cm depth from 59.2% to 54.3% for a 10 mm diam field. Radiosurgical off-axis ratios and percentage depth dose distributions calculated from these spectra with the EGS4 Monte Carlo code were compared to measured data. Measured and calculated dose distributions both with and without flattener were in good agreement. The dose distributions were found to be insensitive to the differences in the various calculated spectral distributions. Thus, although the attenuated Schiff spectrum is significantly harder than the clinical beam, it is adequate for dose calculations of radiosurgical beams.