What is the annual X-ray beam quality (PDD or TMR) tolerance?

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

Answer: (A)

Beam energy stability over time is what this item is checking, using PDD or TMR as the indicator of beam quality. Small shifts in energy change the shape of the depth-dose curve, so keeping the annual PDD or TMR within a tight range of the baseline ensures the dose distribution stays accurate.

The standard tolerance is ±1% from the baseline. This choice reflects a balance between the precision of measurements and the need to catch clinically meaningful energy drift without overreacting to routine setup variability. If a measurement strays beyond 1%, it prompts investigation of the beam spectrum and related components to prevent dose inaccuracies.

Smaller tolerances, like ±0.5%, would be difficult to achieve consistently given typical measurement variability. Larger tolerances, such as ±2% or ±3%, would allow energy changes that could noticeably affect patient dose distributions, undermining treatment accuracy.