Which monthly tolerance applies to the electron beam energy constancy?

• A. 1%
• B. 2%
• C. 2% or 2 mm
• D. 3%

Answer: (C)

Electron beam energy constancy is about keeping the beam energy stable so the depth dose stays predictable. For monthly QA, the tolerance for this check follows a combined rule: 2% or 2 mm, whichever is greater. This means the measured energy indicator must agree with the baseline within 2%, or the depth-related measure must stay within 2 mm of the reference—whichever limit is harder to meet. The reason for combining these two limits is that changes in energy shift the depth-dose distribution, so a percent difference captures energy changes, while there’s also inherent depth-measurement uncertainty of a couple millimeters. Using both criteria ensures you catch meaningful energy drift without overreacting to small measurement noise. The other options don’t match this standard combination.