What is the monthly localizing laser tolerance?

• A. 1 mm
• B. 2%
• C. ±1 mm (≤ ±1 mm for SRS/SBRT)
• D. Functional

Answer: (C)

The key idea is how closely the room’s localizing lasers must point to the treatment isocenter during setup. Monthly QA checks verify that the laser crosshair stays within a small distance of the true isocenter so patient alignment won’t introduce setup errors into the dose delivery. The standard tolerance is plus or minus 1 millimeter, with a note that for high-precision treatments like SRS/SBRT the limit remains within that same tight range. This small allowance matters because even tiny setup shifts can move the target away from the intended dose or increase dose to surrounding tissue. A single value like 1 mm doesn’t express the directionality of a tolerance (positive and negative), a percentage isn’t used for a spatial offset, and “Functional” isn’t a numeric tolerance.