In respiratory gating, the annual dose delivery accuracy tolerance is best described as within MU of the expected delivery?

• A. Within 50 MU of expected
• B. Within 100 MU of expected
• C. Within 150 MU of expected
• D. Within 200 MU of expected

Answer: (B)

In respiratory gating, the annual dose delivery accuracy tolerance is expressed in monitor units because it measures how close the actual delivered dose (in MU) over the year is to what was planned. Gating adds variability from when the beam turns on and off with the patient’s breathing, so using MU as the tolerance unit keeps the criterion tied directly to the delivered dose, regardless of how many MU a given plan uses.

Choosing within 100 MU of the expected delivery provides a practical balance. It is tight enough to catch gradual drift in machine output or gating synchronization over the year, yet lenient enough to accommodate day-to-day fluctuations that come with gated deliveries. For typical plans, this amounts to only a small, clinically acceptable deviation in total annual dose. A smaller tolerance like 50 MU would be overly sensitive to normal variations, while larger tolerances such as 150 or 200 MU could allow meaningful drift to go unnoticed.