What is the monthly electron beam energy constancy tolerance?

• A. 1%
• B. 2% or 2 mm
• C. 3%
• D. 5%

Answer: (B)

Energy constancy for electron beams is about keeping the beam’s penetrating power stable over time. In TG-142, the monthly tolerance for electron energy is given as two percent, or equivalently a two-millimeter shift in the depth-dose characteristics (commonly tracked with R50 or practical range). This matters because even a small change in energy moves where the dose is deposited in tissue; a 2% change in the energy indicator directly reflects a potential shift in dose distribution, while a 2 mm shift in the depth parameter captures the same issue in a depth-dose sense. In practice, you measure the beam quality indicator in a water phantom and compare to baseline; if the change exceeds either 2% or 2 mm, you investigate and adjust the beam.

Numbers like 1%, 3%, or 5% are not the standard monthly tolerance for electron energy in TG-142—they either are stricter or looser than the defined criterion.