Which QA mechanical test is ALWAYS 2 mm regardless of technique?

• A. Gantry angle accuracy
• B. ODI
• C. MLC leaf position accuracy
• D. Isocenter coincidence

Answer: (B)

The main idea here is that some mechanical QA checks use a fixed tolerance that doesn’t depend on which treatment technique you’re using. The ODI test is designed to be a consistent, technique‑independent geometry check between imaging and the beam, so its acceptance threshold is set as a constant 2 mm. In other words, regardless of whether you’re doing conventional treatments or advanced techniques like IMRT or VMAT, you’re looking to keep the imaging-to-beam alignment within 2 mm. This makes ODI the best fit for a criterion that stays the same across techniques.

Other tests involve tolerances that can change with technique or are governed by different measurements. For example, gantry angle accuracy is an angular tolerance, and MLC leaf position accuracy can depend on field size and leaf design, so their acceptable deviations aren’t fixed at a single value across all techniques. Isocenter coincidence is also a geometry check, but the ODI phrasing in this context is the one specified as a fixed 2 mm standard, hence why it’s the correct choice.