What is the monthly photon beam profile constancy tolerance?

• A. 0.5%
• B. 0.8%
• C. 1.2%
• D. 1%

Answer: (D)

Monthly checks verify that the photon beam’s shape across the field stays consistent over time. The tolerance used for this constancy is 1%, meaning when you compare the current profile to the baseline, the relative dose at any point in the profile should not differ by more than ±1% after normalizing (typically to the central axis or peak value). This value strikes a balance between catching meaningful changes in the beam’s flatness or symmetry and not overreacting to normal measurement noise or minor fluctuations. Values tighter than 1% would often flag normal variations as problems, while looser values would allow drift that could affect patient dose distributions.