Which monthly QA tolerance applies to the photon beam profile constancy?

• A. 0.5%
• B. 0.9%
• C. 1%
• D. 1.5%

Answer: (C)

Profile constancy checks focus on the shape of the photon beam across the field rather than its overall dose level. The monthly tolerance is set tight enough to catch small drifts in the beam’s profile that could affect dose distribution, yet realistic given measurement noise. In practice, the beam’s cross-field profile is measured at a reference depth with a detector array or film and compared to a reference profile; any point in the profile should agree with the reference to within about 1 percent. If deviations exceed this threshold, it signals a change in the beam shape—perhaps due to optics, alignment, or hardware aging—that warrants investigation before treatment continues.

Choosing a tighter tolerance (for example, 0.5% or 0.9%) would be overly stringent for routine monthly measurements, given typical detector noise and setup variability, while a looser tolerance (like 1.5%) could miss meaningful drifts in the beam profile. The 1% standard strikes a balance by protecting dose distribution accuracy without being unrealistically strict.