What is the annual collimator rotation isocenter tolerance?

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

Answer: (A)

Rotational isocenter tolerance defines how far the beam’s isocenter can shift when a rotating component, like the collimator, turns. The isocenter is the fixed point through which the beam axis should pass no matter how the machine is oriented. For annual QA, this rotational isocenter displacement is limited to about 1 millimeter from the baseline. This level of precision aligns mechanical accuracy with the dose-delivery accuracy needed for reliable treatments, ensuring that as the collimator rotates, the beam still targets the intended point.

A tolerance of ±1 mm reflects a standard balance between achievable mechanical stability and the geometric precision required for accurate dose distribution. Tighter limits, such as ±0.5 mm, would be more demanding than typical practice and could be impractical to maintain consistently across all units, while looser limits like ±2 mm or ±1.5 mm could allow misalignments that meaningfully affect treatment accuracy.