What is the annual arc mode (expected MU/deg) tolerance?

• A. ±2% from baseline
• B. ±0.5% from baseline
• C. ±1.5% from baseline
• D. ±1% from baseline

Answer: (D)

In arc therapy, the amount of dose delivered per degree of gantry rotation (MU per degree) should stay essentially constant over time. The yearly tolerance for this arc-mode delivery is set at plus or minus 1 percent from the baseline value. This reflects a balance: modern accelerators are stable enough that a 1% drift over a year is noticeable but not common, and it helps catch drifts in beam output, gantry speed, or MLC timing that could affect the dose distribution without being so strict that normal, small fluctuations trigger false alarms.

Choosing a tighter tolerance like 0.5% would be very challenging to maintain in routine practice given measurement and machine variation, while looser tolerances (such as 1.5% or 2%) could allow drift that meaningfully changes dose delivery, especially in highly modulated VMAT plans. If the arc MU/deg value deviates by more than 1% from the baseline, it typically prompts a re-baseline or investigation into calibration to restore accuracy.