What is the annual X-ray output constancy vs gantry angle tolerance?

• A. ±0.5% from baseline
• B. ±2% from baseline
• C. ±1% from baseline
• D. ±3% from baseline

Answer: (C)

In X-ray QA, you’re checking that the beam output stays consistent as the gantry angle changes. You measure a baseline output at a reference angle and then compare outputs at other angles to that baseline. The goal is to keep any variation small so patient dose stays predictable during rotational imaging or treatment.

The accepted tolerance for annual X-ray output constancy across gantry angles is within plus or minus 1% of the baseline. This level balances practical measurement precision with the need to detect clinically meaningful drift. With good setup and a calibrated dosimeter, variations due to angle-dependent factors and measurement noise typically stay within about 1%, so this threshold is stringent enough to catch real changes without being unrealistically tight.

Larger tolerances, like plus or minus 2% or 3%, could mask meaningful differences in dose across angles, while a much tighter limit (such as plus or minus 0.5%) might be impractical given normal measurement variability in routine QA.