For SRS/SBRT procedures, what is the annual coincidence tolerance between radiation and mechanical isocenters?

• A. ±2 mm (Non-IMRT)
• B. ±1 mm (Non-IMRT)
• C. ±1 mm (SRS/SBRT)
• D. ±2 mm (SRS/SBRT)

Answer: (C)

The key idea is that SRS/SBRT demand extremely precise alignment between where the machine thinks the beams will converge (mechanical isocenter) and where the radiation actually converges (radiation isocenter). Because SRS/SBRT targets are very small and dose fall-off is steep, even a tiny mismatch can cause underdosing the target or overdosing adjacent normal tissue. That’s why the annual coincidence tolerance is kept very tight: plus or minus 1 millimeter. In contrast, non-SRS/SBRT treatments tolerate a looser alignment (typically around plus or minus 2 millimeters) since the targets are larger or less dose-constrained. So the best choice reflects the stricter requirement for SRS/SBRT: ±1 mm.