Which monthly QA tolerance is listed for the electron beam output?

• A. 1%
• B. 2%
• C. 3%
• D. 5%

Answer: (B)

The key idea is ensuring the electron beam output stays stable from month to month. In monthly QA, the output is compared to a reference value, and the allowed variation is set to ±2%. This 2% tolerance reflects a practical balance: it is tight enough to catch meaningful drifts that could affect patient dose, yet acknowledges typical measurement uncertainties from detectors, phantom setup, and calibration processes. If the measured output deviates beyond ±2% from the baseline, the machine should be rechecked and recalibrated as needed. A tolerance of 1% would be too strict given routine measurement noise, while 3% or 5% would let larger, clinically significant drifts go unnoticed.