What is the annual X-ray beam quality tolerance measured as PDD or TMR from baseline?

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

Answer: (A)

Beam quality constancy is checked yearly because the photon energy shapes the depth-dose curve. PDD and TMR measure how dose falls with depth for a given field, so comparing the current PDD or TMR to the baseline reveals whether the beam energy has shifted. The tolerance is set to within ±1% of the baseline to keep the energy in a tight range, since even small changes can alter dose at treatment depths and affect plan accuracy. Larger tolerances would let meaningful energy drift go unnoticed, while an even tighter tolerance would be impractical due to natural measurement variability and normal machine drift. Therefore, the annual X-ray beam quality tolerance is ±1% from baseline.