What is the annual gantry rotation isocenter tolerance?

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

Answer: (B)

The key idea is keeping the beam’s isocenter fixed as the gantry rotates. The isocenter is the point where all rotational axes (gantry, couch, collimator) should converge, so as the gantry turns, the beam should still aim at the same spot in space. The annual tolerance for how much that isocenter can drift from the baseline established during commissioning is plus or minus one millimeter. This level of precision is achievable with routine QA measurements and is tight enough to ensure accurate dose delivery from every angle, including IMRT/VMAT and stereotactic cases, while allowing for normal mechanical wear and measurement variability. If the measured shift exceeds this limit, maintenance or re-baselining is typically required. Tolerances that are tighter (like half a millimeter) would be harder to maintain in practice, while looser tolerances (two millimeters or more) could compromise dose conformity across angles.