What is the acceptance criterion for Light & X-ray Field Alignment distance accuracy?

• A. +/- 5% of source-to-distance
• B. +/- 1% of source-to-distance
• C. +/- 2% of source-to-distance
• D. No tolerance

Answer: (C)

Light and X-ray field alignment distance accuracy checks that the light field, which guides patient positioning, matches the radiation field at the target distance. The distance accuracy part measures how far the light-field edge and the radiation-field edge are from each other and expresses that error as a percentage of the distance from the X-ray source to the plane where the measurement is made (the source-to-distance). The reason for using a ±2% tolerance is that it scales with distance, keeping the allowable misalignment proportional as geometry changes, and it maintains a safety and accuracy balance—tight enough to prevent exposing the wrong area but achievable with routine calibration. For example, at a source-to-distance of 100 cm, a 2% tolerance corresponds to a 2 cm allowable difference between the light and radiation field edges. A larger tolerance would permit more misalignment and could compromise targeting, while a much smaller tolerance would be impractical in daily practice.