Calculating SF₆ Leak Rate in Linear Accelerators From Digital Pressure Logs

Sulfur Hexafluoride, one of the most potent greenhouse gases, is used in radiation oncology as a dielectric in linear accelerator waveguides. Given the high global warming potential of SF₆, its use is heavily regulated, and possible leaks should be proactively monitored. Truebeam Linac diagnostic log files record SF₆ pressure metrics every 2 h. These log files were parsed, and the high time resolution pressure data were studied across a fleet of 15 Varian Truebeam Linear Accelerators for a 1 year period. Average SF₆ leak rates were low, but highly variable (8.6 g/year/Linac, std dev 17.7 g). Pressure fluctuation due to temperature change was found to be a similar magnitude to real SF₆ losses over months. Leak rates calculated from pressure loss were significantly below those reported in the literature and manufacturer specifications. Pressure fluctuation proportional to temperature change caused by Linac use makes SF₆ pressure an unreliable metric to sample infrequently if the aim is to detect small leaks. A novel method for calculating leak rate using high sample rate log files mitigates this effect, giving a more accurate measurement.