What is the annual couch rotation isocenter tolerance?

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

Answer: (C)

The main idea is how precisely the couch’s rotation axis must align with the planned isocenter over time. The couch rotation isocenter tolerance sets how much this intersection point can drift from baseline across a year, accounting for wear, cooling, and small mechanical changes. For TG-142 QA, the annual tolerance for the couch rotation isocenter is ±1 mm. This means during yearly checks, any shift in the couch’s isocenter position relative to the baseline must stay within 1 millimeter; if it drifts more, the couch system should be recalibrated or serviced to restore accuracy. The other options are either more stringent or looser than the standard, but the accepted guideline is 1 mm.