In respiratory gating, the annual beam output constancy tolerance is expressed as what percentage?

• A. 0.5%
• B. 2%
• C. 3%
• D. 4%

Answer: (B)

In respiratory gating, the annual beam output constancy tolerance expresses how much the linac’s delivered beam power is allowed to drift from year to year while delivering during gated treatments. The tolerated variation is two percent, meaning the measured output at annual QA should stay within ±2% of the nominal value. This level balances detecting meaningful drift with avoiding unnecessary recalibration, since the gating system only affects when the beam is on, not the overall stability of the beam output itself. Values like 0.5% are unrealistically tight for annual checks, while 3% or 4% would permit too much variation and could impact dose accuracy over the treatment course.