Calculating SF₆ Leak Rate in Linear Accelerators From Digital Pressure Logs

Area of Science:

• Environmental Science
• Medical Physics
• Engineering

Background:

• Sulfur hexafluoride (SF₆) is a potent greenhouse gas used in radiation oncology.
• SF₆ is a dielectric in linear accelerator waveguides, requiring leak monitoring due to regulatory and environmental concerns.
• Truebeam Linac diagnostic logs capture SF₆ pressure data every 2 hours.

Purpose of the Study:

• To analyze SF₆ pressure data from a fleet of Varian Truebeam Linear Accelerators over one year.
• To assess the reliability of infrequent SF₆ pressure sampling for detecting small leaks.
• To develop a more accurate method for calculating SF₆ leak rates.

Main Methods:

• Parsing diagnostic log files from 15 Varian Truebeam Linear Accelerators.
• Analyzing high time-resolution SF₆ pressure data over a 1-year period.
• Investigating the impact of temperature fluctuations on SF₆ pressure readings.

Main Results:

• Average SF₆ leak rates were low (8.6 g/year/Linac) but highly variable (std dev 17.7 g).
• Temperature-induced pressure fluctuations mimicked SF₆ losses over monthly intervals.
• Calculated leak rates were lower than literature and manufacturer specifications.
• Infrequent pressure sampling proved unreliable for detecting small leaks due to temperature effects.

Conclusions:

• SF₆ pressure is an unreliable metric for detecting small leaks when sampled infrequently due to temperature variations.
• A novel method utilizing high-resolution log file data can mitigate temperature effects for more accurate SF₆ leak rate calculation.
• Proactive monitoring of SF₆ leaks is crucial given its environmental impact.