What is the monthly electron output constancy tolerance?

• A. 2%
• B. 1%
• C. 3%
• D. 5%

Answer: (A)

The key idea is monitoring how stable the electron beam output is from month to month. A typical monthly constancy tolerance is 2%, meaning the measured electron output should stay within ±2% of the established reference value. This threshold strikes a balance between accounting for normal measurement variability and catching meaningful drift in the machine’s output.

If the monthly measurement stays within 2%, the beam is considered stable. If it drifts beyond 2%, you would investigate potential causes such as calibration of the monitor chamber, changes in beam energy or flatness, changes in setup (like SSD or field size), or environmental and instrumental factors, and perform any necessary recalibration or service.

Choosing a smaller tolerance like 1% would be harder to achieve reliably given measurement uncertainties, while larger tolerances (3% or 5%) could allow clinically significant drift to go unchecked.