What is the annual coincidence of radiation and mechanical isocenter tolerance?

• A. ±2 mm from baseline (Non-IMRT), ±1 mm (SRS/SBRT)
• B. ±1 mm from baseline
• C. ±0.5 mm from baseline
• D. ±3 mm from baseline

Answer: (A)

The main idea is that the point where the radiation beams converge (radiation isocenter) must stay tightly aligned with the machine’s rotation point (mechanical isocenter) over time. The annual tolerance sets how much drift between these two centers is acceptable when checked once a year. For typical non-IMRT treatments, that allowance is two millimeters. For highly precise stereotactic procedures like SRS and SBRT, the tolerance is tighter—one millimeter—because these plans use very small targets and steep dose gradients, so even a tiny misalignment can affect the dose to the target and surrounding organs.

This alignment is evaluated with annual QA tests, often using a Winston-Lutz style setup with a phantom to measure the center of the radiation field against the mechanical rotation axes across angles. The baseline is established during commissioning, and annual checks ensure the system hasn’t drifted over time.

So the correct understanding is that the annual coincidence tolerance is two millimeters for non-IMRT and one millimeter for SRS/SBRT, reflecting the differing precision demands of these treatment types.