What is the annual SRS arc rotation mode tolerance (range 0.5-10 MU/deg)?

• A. Monitor units set vs delivered: 1.0 MU or 2% (whichever is greater); Gantry angle vs delivered: 1° or 2% (whichever is greater)
• B. 0.5 MU or 1%; 0.5° or 1%
• C. 2.0 MU or 4%; 2° or 4%
• D. 1.5 MU or 3%; 2° or 3%

Answer: (A)

SRS arc rotation mode tolerances are set by two checks against the treatment plan: the difference between the monitor units delivered and the monitor units planned, and the difference between the gantry angle delivered and the gantry angle planned. Each check uses a rule of “the greater of a fixed value or a percentage” to keep the tolerance sensible across different dose levels and rotation speeds. For annual QA in SRS arc rotation, a common and appropriate pairing is 1.0 MU (or 2%, whichever is greater) for the MU comparison, and 1° (or 2%, whichever is greater) for the gantry angle comparison. This combination ensures that both small absolute MU discrepancies and proportional percentage discrepancies are controlled, and that angular accuracy remains acceptable even at low doses or steep dose gradients.

The other options propose different fixed values or omit the “whichever is greater” approach, which would not align with the standard practice of applying a dual-criterion tolerance that scales with dose and rotation. This is why the pairing with 1.0 MU or 2% and 1° or 2% is the best match for the stated range.