What is the monthly asymmetric light/radiation field coincidence tolerance on a side?

• A. 2 mm or 1% on a side
• B. 1 mm or 1% on a side
• C. 1 mm
• D. 2 mm

Answer: (B)

The main idea is to ensure the light field matches the radiation field, edge by edge, so the patient receives the intended dose in the correct location. When the field is asymmetric, misalignment can show up differently on each edge, so the tolerance is defined per side.

For monthly checks of asymmetric fields, the allowable coincidence error on each side is 1 mm or 1% of the field dimension on that side, whichever is larger. This means you use the greater of 1 mm or 1% of the side length to decide if the light and radiation fields are still aligned on that edge. For example, a 20 cm side has 1% equal to 2 mm, so up to 2 mm of misalignment is tolerated on that side. A smaller 5 cm side would have 1% equal to 0.5 mm, but the minimum tolerance is still 1 mm, so you’d tolerate up to 1 mm there.

This combination—per side, with the larger of 1 mm or 1%—is what the correct choice reflects.