Which tolerance is identical across all modalities at isocenter?

• A. Daily laser localization tolerance
• B. Daily collimator size indicator tolerance
• C. ODI tolerance
• D. X-ray output constancy tolerance

Answer: (C)

The distance to isocenter is the single, shared reference point for every beam modality, so the measurement that guarantees this distance is accurate must be the same no matter which modality is used. The Optical Distance Indicator (ODI) directly checks how far the treatment head sits from isocenter, and its tolerance is defined to apply universally across all modalities. This keeps spatial accuracy consistent for imaging, planning, and treatment.

Other tolerances depend on specific devices or measurements: lasers assess room-laser alignment, which has its own limits; the collimator size indicator relates to how accurately the field size is read from the collimator, with different acceptable deviations; and X-ray output constancy concerns dose output, not distance to isocenter. That’s why ODI tolerance is the universal, identical standard at isocenter.