What is the annual X-ray/electron output calibration tolerance (TG-51)?

• A. ±0.5% absolute
• B. ±2% absolute
• C. ±3% absolute
• D. ±1% absolute

Answer: (D)

The requirement is about how closely the accelerator’s stated output must match the true dose delivered under reference conditions. TG-51 sets an acceptable tolerance for the annual X-ray or electron output calibration at about ±1% in absorbed dose to water. In practice, when you perform the calibration with a calibrated chamber and apply the appropriate beam-quality and chamber factors, the measured dose should agree with the true dose to within 1%. This level accounts for measurement uncertainties in the chamber, electrometer, depth and field corrections, and beam quality factors, while keeping dose delivery accurate across clinics. If the result strays outside that 1% window, corrective action is needed before use.