What is the annual X-ray monitor unit linearity tolerance?

• A. ±2%
• B. ±1%
• C. ±3%
• D. ±5% (≥5 MU), ±2% (≤5 MU)

Answer: (D)

The idea being tested is how consistent the beam output is with respect to the monitor unit (MU) setting across the range of exposures used in practice. To ensure dose accuracy, QA standards set a two-tier tolerance: when the delivery is five MU or less, the output must stay within tight bounds (about two percent) to guard against nonlinear behavior where the signal is small and measurements can be more sensitive to noise; for higher deliveries (above five MU), a looser bound (about five percent) is allowed because the dose signal is larger and small absolute fluctuations are less impactful. This combination helps catch meaningful nonlinearities at low-dose levels while recognizing practical stability at higher dose levels.