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

• A. 1%
• B. 0.5%
• C. 2%
• D. 5%

Answer: (C)

Beam output constancy over time is about keeping the delivered dose per MU stable. When using respiratory gating, the beam is only on during parts of the breathing cycle, which can introduce extra variability in how we measure and monitor output. Despite that, the long-term (annual) tolerance is set at 2% to reflect a practical but still clinically meaningful limit on drift or changes in the gated beam.

A tighter tolerance like 1% or 0.5% would be hard to achieve consistently given measurement uncertainties and routine calibration processes. A looser tolerance like 5% could allow dose delivery to drift into clinically significant levels, especially important in gated treatments where timing and dose precision matter. Therefore, the appropriate annual tolerance for respiratory-gated beam output is 2%.