What is the annual electron output constancy vs gantry angle tolerance?

• A. ±0.5% from baseline
• B. ±0.8% from baseline
• C. ±2% from baseline
• D. ±1% from baseline

Answer: (D)

The main idea is that annual checks of dose output across different gantry positions are meant to catch any slow drift in the beam as the machine rotates. For electron beams, the tolerance for this annual output constancy versus gantry angle is set at ±1% from the baseline. This keeps the delivered dose consistent no matter which gantry angle is used, which is important for maintaining accurate dose distributions in patients.

This is the best choice because it matches the standard allowance used in TG-142-style QA for this particular cross-angle constancy check. In practice you’d measure the electron output with a calibrated detector in a suitable phantom at several standard gantry angles, compare each reading to the baseline measurement obtained at the reference angle (with appropriate environmental corrections), and verify that all readings stay within ±1%.

Other values would be less consistent with the typical annual tolerance: being tighter (like ±0.5% or ±0.8%) would be unusually strict for an annual cross-angle check and could be impractical given measurement variability, while being looser (like ±2%) would risk overlooking small but clinically relevant drifts.