Comparison of beam characteristics of a gold x-ray target and a tungsten replacement target.

A new W-Cu target was designed to replace the existing Au target on a linear accelerator model in common use in radiotherapy. This work shows that targets of different material composition can be designed to produce beams with the same dosimetric character over a wide range of beam energies without adjusting the beam energy. The target design objective was to improve mechanical robustness, replacing water in the beam path with a Cu heat sink, without altering the beam properties for the nominal clinical energy range of 4-25 MV. The W-Cu could then be installed in place of the Au target without recommissioning. The effect of the target swap was measured in the test cells for 11 different beams ranging in nominal energy from 4 to 25 MV, with focus on open field dose distributions, including diagonal profiles taken for the largest (40x40 cm) field, measured at 4 different gantry angles. Depth dose curves agreed to 0.4% or better, profiles to 1.2% or better. Monte Carlo simulations of the treatment head were done for representative energies of 6 and 18 MV. Calculated and measured dose distributions generally matched within 1%, although dose measured in the build-up region of large fields was significantly more than in the simulations. Calculated spectral distributions on the central axis and angular distributions of energy fluence matched for the two targets, whereas angular distributions of fluence were significantly different. Matching energy fluence gave a more favorable match of dose profiles than matching fluence. The target was further tested on several machines operating in a radiotherapy clinic. Measurements were made for a wide range of open field sizes and with selected wedges and blocks. Dose distributions for the two targets agreed to 1.4% or better, including the dose in wedged fields. Wedge factors changed by no more than 0.5%, transmission through a 4.4 HVL block no more than 1.5%. The response of the monitor chamber was found to change, generally by 1%-2%. Therefore, when the W-Cu target was used to replace the Au target, the output of the machine was measured and adjusted appropriately, but there was no requirement for recommissioning.