Which tolerance is the monthly beam output constancy tolerance for respiratory gating?

• A. 1%
• B. Functional
• C. 2%
• D. ±1 mm

Answer: (C)

The main concept here is how much the beam output is allowed to vary from month to month when respiratory gating is used. For gated treatments, the beam output constancy is typically permitted to deviate by about 2% to account for normal fluctuations in linac performance, detector response, and the gating synchronization, while still keeping the delivered dose within clinically acceptable limits. A tighter limit, like 1%, would be more restrictive than standard practice and can be unnecessarily burdensome. A non-numeric term isn’t a tolerance, and a dimensional value like ±1 mm is a spatial rather than a dose-output tolerance. So, 2% is the appropriate monthly beam output constancy tolerance for respiratory gating.