What is the X-ray monitor unit linearity tolerance for MU values ≤5?

• A. ±2%
• B. ±5%
• C. ±1%
• D. ±3%

Answer: (A)

The main idea is how accurately the beam output scales with the number of monitor units, especially at low MU. For MU values up to 5, the allowed variation from perfect linearity is ±2%. That means if you expect 5 MU to deliver a dose of 5 times the calibration dose per MU, the actual measured dose should be within 2% of that value (for example, between 4.9 and 5.1 when expressed in the same units). This tighter tolerance at low MU helps ensure small-dose deliveries are still reliable, which is important for precise treatments or when using small fractions. The other percentages are looser or tighter than this standard, so ±2% is the established tolerance.