What is the daily X-ray output constancy tolerance?

• A. 2% (all energies)
• B. 3% (all energies)
• C. 3% (for some energies)
• D. 4% (all energies)

Answer: (B)

Daily output constancy checks ensure the dose delivered is consistent from day to day. The standard tolerance used is three percent for all energies, meaning the measured daily output should stay within ±3% of the established baseline regardless of the photon or electron energy. This uniform limit is chosen because day-to-day drift in machine output tends to affect all energies similarly, and a single criterion keeps QA practical while remaining sensitive enough to catch meaningful changes in performance. In practice, you measure with a calibrated detector under a fixed setup (same field size and geometry), compare to the baseline, and if the result is within 3%, you’re good. If it exceeds 3%, you investigate and correct components such as the monitor chamber calibration, MU calibration, and beam output. The other options would be either overly strict, too lenient, or imply energy-specific limits, which isn’t how daily constancy is specified.