Monte Carlo simulations of bremsstrahlung production in a carbon target for imaging in radiotherapy

Imaging in radiotherapy is gaining importance since the accuracy of radiotherapy has improved over the last decade. In this work, the X-ray beam is produced by a medical linac and a low MV X-ray spectrum is obtained. This spectrum provides a lot more contrast than conventionally used MV therapy spectra [1] and the method offers a clear relation between imaging and treatment position. The setup of a commercially available medical linac is implemented in detail in a Geant4 Monte Carlo simulation. The production of bremsstrahlung takes place in a carbon target followed by an aluminum absorber. A location map of the bremsstrahlung processes and the stopping of electrons is shown as well as the resulting low MV X-ray spectrum with a mean energy value of 468 keV. The primary and secondary electrons are stopped mostly in the vicinity of the bremsstrahlung processes but a slight contamination of the X-ray beam with electrons is detected after the dose chamber. The influence of the primary beam angle on the angular distribution of the resulting photon spectrum is high concerning φ and small concerning θ. The results give a deeper understanding of the setup which can be helpful for new designs.