Which tolerance is commonly described as walkout?

• A. Gantry/collimator angle indicator tolerance
• B. Accessory tray position tolerance
• C. Asymmetric light/radiation field coincidence tolerance
• D. Cross-hair centering (walkout) tolerance

Answer: (D)

The concept being tested is how well the light field crosshair aligns with the actual radiation field, i.e., the center of the beam, across the field. “Walkout” refers to the crosshair drifting away from the true beam center as you move away from the isocenter or when the beam is adjusted. The tolerance that specifically describes this behavior is the crosshair centering tolerance, often called the walkout tolerance, because it measures how much the light crosshair can “walk” from the radiation field center and still be considered acceptable. This is distinct from tolerances for gantry or collimator angle indicators, tray position, or light/radiation field edge coincidence in asymmetric setups. In practice, you test this by comparing the light crosshair to the actual beam center at a set distance, using a detector or film, and ensuring any offset stays within the specified small limit.