What is the monthly electron beam profile constancy tolerance?

• A. 1%
• B. 0.5%
• C. 1.5%
• D. 2%

Answer: (A)

Profile constancy tracks how the electron beam’s transverse dose distribution remains the same from month to month. Keeping this profile stable is crucial because any shift in the shape of the beam affects how dose is delivered across the treatment field, even if the total dose stays the same.

A 1% tolerance means the measured profile at the chosen depth and setup should not differ from the reference profile by more than 1% at any measured off-axis position. This tight but achievable limit helps catch small drifts in beam shaping components or energy that could alter dose uniformity, without being so strict that normal measurement variability triggers false alarms. When the profile drifts beyond this limit, it signals a need to inspect or recalibrate the accelerator’s beam delivery system to restore consistent patient doses.

Smaller tolerances, like 0.5%, would be too prone to false alarms due to routine detector noise and setup differences. Larger tolerances, such as 1.5% or 2%, would let meaningful changes in the beam profile go unchecked, potentially affecting dose uniformity and treatment accuracy.