Purpose: This study shares our clinical experience of the lifetime of Varian Kapton sealed ion chambers as a retrospective review. The data provided shows the average lifetimes of the ion chambers in our institution for nine clinical linear accelerators, so other physicists can be informed of the average lifetime and prepare for replacement.
Methods: The lifetime of ion chambers (N=11) has been analyzed using daily quality assurance, monthly quality assurance, delivered treatment field data, and machine applied gain values pulled from configuration files saved after each output adjustment. We investigate the length of time before replacement, beam hours, and filament hours captured during routine preventative maintenance inspections. The estimated clinical monitor units (MU) delivered, cumulative MU after ion chamber replacement, and output adjustments are also analyzed. Data for all clinical linear accelerators was exported and analyzed using a combination of Microsoft Excel and custom Python programs.
Results: The average number of MU delivered per machine before ion chamber replacement was 13.9 Million (STDEV 4.1 Million). The average amount of time before the first ion chamber replacement was 1,731 days (STDEV 567) which is approximately 4.75 years. The average increase in the applied gain to the ion chamber before replacement was 8.7% (STDEV 3.4%) with a maximum increase over the life of the ion chamber of 13.1%.
Conclusion: The average lifetime of an ion chamber is approximately 14 million clinically delivered MU. We have found replaced chambers have greater initial changes in output than would be anticipated based on current literature for output changes, and we would recommend this be closely monitored after replacement. We did not observe a plateau in the output changes over time as suggested by other reports.
Ionization Chamber, Quality Assurance
TH- External Beam- Photons: Quality Assurance - Linear accelerator