Linac Quality Assurance

, Foster D. Lasley², Indra J. Das², Marc S. Mendonca² and Joseph R. Dynlacht²

(1)

Department of Radiation Oncology, CHRISTUS St. Patrick Regional Cancer Center, Lake Charles, LA, USA

(2)

Department of Radiation Oncology, Indiana University School of Medicine, Indianapolis, IN, USA

What Is Quality Assurance?

AAPM definition of QA: “To assure that machine characteristics do not deviate significantly from their baseline values acquired at the time of testing and commissioning.”
- Linac measurements are taken during acceptance and commissioning.
- The AAPM requires daily, monthly, and annual QA tests to ensure that linacs are able to deliver treatment as planned.
- TG–40 set standards for basic linacs without MLCs, on-board imaging, or respiratory gating.
- TG–142 adds standards for MLCs, IMRT, SBRT, on-board imaging and respiratory gating.
QA intervals and instrumentation:
- Daily QA may be done using relatively imprecise “spot check” instruments such as a daily QA device.
- Monthly QA should use more precise instrumentation than daily QA, as determined by the physicist.
- Annual QA should use the most precise commissioned instruments, including TG–51 calibration with National Institute for Standards and Technology (NIST)-traceable ion chambers.
Tolerance level and action level:
- The tolerance levels are considered “acceptable” for clinical treatment.
  
  Tolerances are typically 3 % for daily checks, 2 % for monthly/annual checks, or 1 % for stereotactic.
  
  If the equipment consistently violates tolerance levels, treatment should be stopped until the equipment is brought into compliance.
- However, daily output checks may be more variable due to their rapid and imprecise nature.
  
  An action level of 5 % is recommended for stopping treatment after a daily output check deviation.
  
  If the deviation is between 3 and 5 %, treatment may continue “for the short term” until the discrepancy can be addressed.