Abstract

Image-guided radiation therapy (IGRT) improves the geometric accuracy of radiation treatment by using on-board imaging to localize the patient and target immediately before or during treatment. However, repeated kilovoltage cone-beam computed tomography (kV-CBCT) imaging introduces an additional radiation dose to healthy tissues, creating a need for accurate characterization of the imaging beam and ultimately for patient-specific imaging-dose estimation. This work developed a Monte Carlo model of the kV X-ray source and collimation system of the X-ray Volume Imaging (XVI) system integrated into an Elekta Synergy linear accelerator.

The X-ray tube, filtration system, and available collimator configurations were modeled using the EGSnrc/BEAMnrc Monte Carlo framework. Simulations were performed with 25 × 10⁶ electrons incident on a tungsten target, and phase-space data were generated at different stages of the beamline and at the treatment isocenter. Nine collimator configurations were successfully simulated. Radiation transport parameters appropriate for kilovoltage photon beams were implemented, including Rayleigh scattering, atomic relaxation, electron-impact ionization, and directional bremsstrahlung splitting to improve computational efficiency.

The simulations showed that approximately 95% of the bremsstrahlung photons produced in the tungsten target were absorbed by the filtration cone, while only about 1% of the generated photons exited the collimation system. Approximately 90% of the photons leaving the X-ray unit had their last interaction in the tungsten target. Beam spectra and energy-fluence profiles demonstrated progressive beam hardening as a consequence of filtration and the anode heel effect. Mean photon energy varied with field of view and collimator geometry, and characteristic tungsten K-shell X-ray peaks were observed at approximately 57–59 keV and 67–69 keV. The calculated beam characteristics were in good agreement with previously published Monte Carlo simulations of the Elekta XVI system.

The developed Monte Carlo model therefore provides a detailed physical characterization of the Elekta Synergy kV-CBCT imaging beam and establishes a foundation for subsequent calculations of absorbed dose from image-guidance procedures. Such modeling may support more accurate assessment of cumulative imaging dose and the development of methods for minimizing unnecessary radiation exposure during radiotherapy.

Keywords: Monte Carlo simulation; EGSnrc; BEAMnrc; cone-beam computed tomography; CBCT; image-guided radiation therapy; kilovoltage X-rays; radiation dosimetry; Elekta Synergy.