Which tolerance describes the arc mode allowance in MU per degree?

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

Answer: (D)

In arc therapy, the beam is delivered as the gantry rotates, so the output per degree must stay tied to a steady baseline to keep the dose consistent around the arc. The tolerance for MU per degree in arc mode is ±1% of that baseline, meaning the MU per degree should stay within one percent above or below the baseline. For example, if the baseline is 5 MU per degree, the allowed range is 4.95 to 5.05 MU per degree. This keeps dose uniform despite the continuous rotation. The other options don’t match the standard arc-mode allowance, and a non-numeric term isn’t a tolerance.