What is the daily ODI tolerance at isocenter for Non-IMRT, IMRT, and SRS-SBRT?

• A. 1 mm (all modalities)
• B. 2 mm (Non-IMRT), 3 mm (IMRT), 2 mm (SRS-SBRT)
• C. 2.5 mm (all modalities)
• D. 2 mm (all modalities: Non-IMRT, IMRT, SRS-SBRT)

Answer: (D)

ODI checks are a quick mechanical verification that the beam’s isocenter aligns with the patient setup by comparing the gantry-to-isocenter distance shown on the Optical Distance Indicator to the true isocenter position. The tolerance is 2 mm for all modalities because this value provides a practical balance: it catches setup or machine-geometry errors without causing excessive treatment delays, and it establishes a consistent baseline across Non-IMRT, IMRT, and SRS-SBRT. For higher-precision deliveries, other imaging and QA steps are used, but the daily ODI tolerance remains 2 mm to ensure basic alignment before every treatment. If the reading exceeds 2 mm, recalibration or repositioning is needed before proceeding.