For SRS arc rotation mode, what is the tolerance for gantry angle vs delivered?

• A. 0.5° or 1%
• B. 2° or 2%
• C. 1° or 2% (whichever is greater)
• D. 0.5° or 2%

Answer: (C)

In arc rotation mode, the allowable difference between the planned gantry angle and what is actually delivered is set by a dual criterion: use the larger of 1 degree or 2 percent of the planned gantry angle. This means for small angles the absolute limit (1°) governs, while for larger angles the relative limit (2% of the angle) becomes the controlling value. For example, at a 20° plan, 2% is 0.4°, so the tolerance is 1°. At 120°, 2% is 2.4°, so the tolerance is 2.4°. This approach keeps tight precision for small rotations while scaling with angle for larger arc deliveries, which is important for maintaining accurate dose delivery in SRS.