For respiratory gating, what is the annual beam output constancy tolerance?

• A. 0.5%
• B. 1%
• C. 2%
• D. 5%

Answer: (C)

The question tests how much the machine’s delivered dose rate can drift over a year while using a gated beam. When treatment is gated, the beam is only on during certain phases of breathing, so even small changes in the overall output can lead to dose discrepancies over many fractions. To keep patient doses within safe, expected limits, a standard tolerance is set for annual beam output constancy.

Two percent is the typical allowance. It reflects the balance between practical measurement variability—instrument response, phantom setup, daily warm-up, and long-term drift—and the need to maintain accurate dose delivery during gated treatments. If the output drifts beyond this level, recalibration or service is warranted to restore consistency.

The other options are either unrealistically tight for routine QA or too lenient to protect patient safety. A half‑percent tolerance would be difficult to measure reliably in practice; one percent, while possible in very tight setups, is not the standard; and a five percent tolerance would risk meaningful dose errors in the gated regimen.