The dosimetric effect of photon target degradation on C-series linear accelerators using Monte Carlo simulation

The impact of beam dosimetry changes brought on by target degradation has not been well investigated.

To determine the optimal dosimetry metrics for detecting target degradation and to investigate the impact of beam dosimetry changes brought on by target degradation.

The EGSnrc Monte Carlo programs were used to model the Varian Novalis Tx linear accelerator. By altering the central portion of the photon target with a cylindrical vacuum zone, few target degradation scenarios were simulated and compared with the nominal condition. For square photon beams defined by the jaws, the effects of target degradation on beam output, beam quality (percentage depth dose, PDD), beam profile (off-axis factor), relative output factor (ROF), and wedge factor (WF) were examined. Few degradation scenarios were used to retroactively model the effects on patient-specific quality assurance (PSQA).

Under the realistic degradation conditions (0.6-0.7 mm depth, 1.0-2.0 mm widths), the 6MV beam output decreased by 5.7%-24.1% and the 30 cm field diagonal profile sag in the horns by a max deviation of 3.8%-5.9%, however, the PDD_20,10 varied within 1%, and the flatness deviated by 0.74%-1.35%, respectively. As the photon target got more degraded, the beam output, beam quality and profile showed a downgrade trend from the baseline. The PDD_20,10 varied from 0.7% to -2.8% in the 10 cm field. Before the target degraded with a 2 mm width and 0.889 mm depth hole, ROF and WF were stable (within 2% deviation). The pelvic case gamma result showed obvious decrease (90.9%) under degradation depth 0.7 mm and width 2.0 mm.

A technical approach was created and verified for precise Monte Carlo simulation of linear accelerator target degradation scenarios. The parameters with the highest sensitivity for detecting target degradation were beam output and large field diagonal profile deviation in the 0.95 field size range.