What is the annual electron monitor unit linearity tolerance?

• A. ±2%
• B. ±1%
• C. ±3%
• D. 0%

Answer: (A)

The thing this question is checking is how much the machine’s dose output can deviate while the number of monitor units (MU) is varied, over the course of a year, for electron beams. Monitor-unit linearity means the delivered dose should scale proportionally with MU, so doubling MU should double the dose (within a small, acceptable tolerance). Over a year, small changes in electronics, ionization chamber response, and setup can cause slight nonlinearity, so a tolerance is defined.

For electron beams, the annual MU linearity tolerance is ±2%. That means when you test several MU settings across the clinically used range, the measured dose should stay within 2% of the expected linear relationship. This keeps dose delivery predictable and accurate without demanding perfection impossible to achieve in practice.

The other options aren’t used because zero tolerance isn’t realistic due to measurement and calibration uncertainties, and tolerances of ±1% or ±3% aren’t the standard TG-142 value for this parameter.