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

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

Answer: (B)

Understanding how gating affects delivered dose is key here. Even when the beam is turned on only during specific parts of the breathing cycle, the total dose delivered over the treatment session must stay consistent with the machine’s established baseline. For monthly quality checks, the standard tolerance for beam output constancy with respiratory gating is within about 2% of the reference value. In other words, the gated delivery should produce an output that is within ±2% of what the machine delivers under normal, non-gated conditions.

This 2% limit ensures dose accuracy across the different phases of respiration and the varying on/off times of the gate, so patient dose remains predictable and within plans. A tighter 1% would be stricter than normally required for monthly checks. A spatial tolerance like ±1 mm isn’t relevant to beam output, and a non-numeric option like “Functional” doesn’t specify a measurable standard.