Which tolerance includes both a linear and an angular component?

Which tolerance includes both a linear and an angular component?

• A. Accessory tray position tolerance
• B. Wedge placement accuracy tolerance
• C. Treatment couch position indicator tolerance (2 mm / 1°)
• D. Cross-hair centering tolerance

Answer: (C)

The key idea is that some tolerances are defined to control both how far something can shift linearly and how much it can tilt or rotate. The couch position indicator is designed to track both translation and rotation of the treatment couch. Its tolerance is given as 2 mm in linear displacement and 1 degree in angular deviation, capturing the two ways the couch can affect patient setup and beam alignment. If either the linear shift exceeds 2 mm or the couch tilts beyond 1°, the setup could move the patient off the intended isocenter, altering dose delivery. The other tolerances typically address just a single aspect—either a linear distance or an angular measurement—without combining both, so they don’t include both components.

The key idea is that some tolerances are defined to control both how far something can shift linearly and how much it can tilt or rotate. The couch position indicator is designed to track both translation and rotation of the treatment couch. Its tolerance is given as 2 mm in linear displacement and 1 degree in angular deviation, capturing the two ways the couch can affect patient setup and beam alignment. If either the linear shift exceeds 2 mm or the couch tilts beyond 1°, the setup could move the patient off the intended isocenter, altering dose delivery. The other tolerances typically address just a single aspect—either a linear distance or an angular measurement—without combining both, so they don’t include both components.