What is the annual electron flatness change tolerance from baseline?

• A. 0.5%
• B. 3%
• C. 1%
• D. 2%

Answer: (C)

Long-term stability of the electron beam’s flatness is what this question targets. The baseline flatness is set during commissioning, and over time the beam profile can drift due to aging components or small changes in the beam path. The annual tolerance defines how much the flatness is allowed to change from that baseline in one year. A limit of 1% strikes a balance: it’s small enough to catch clinically meaningful drift while still being attainable given typical measurement repeatability and normal day-to-day variations. If the annual measurement shows more than a 1% change from baseline, the beam should be retuned and the baseline refreshed to maintain consistent dose distribution. Larger tolerances would risk dose differences in treatment, while a stricter limit would be harder to maintain routinely.