Which tolerance describes the monthly phase/amplitude beam control?

• A. 1%
• B. ±1 mm (≤ ±1 mm for SRS/SBRT)
• C. Functional
• D. 2%

Answer: (B)

The key idea is how precisely the treatment beam’s position and shape must match the plan on a monthly basis. Phase and amplitude beam control relate to steering the beam and shaping its dose distribution, and small shifts or distortions can change where the dose lands. That kind of accuracy is measured as a physical distance at the isocenter, so the tolerance is expressed in millimeters. A tolerance of ±1 mm keeps the beam aligned within a very tight spatial bound, which is essential for accurate coverage of the target and protection of nearby structures. For highly precise treatments like SRS or SBRT, this is even more strict, hence the note that the tolerance should be within ±1 mm in those cases. The other options don’t fit because they describe dose-based tolerances (percent) or are qualitative rather than providing a specific spatial limit.