For respiratory gating, the annual dose delivery accuracy tolerance is within how many MU of the expected delivery?

• A. Within 50 MU of expected
• B. Within 75 MU of expected
• C. Within 100 MU of expected
• D. Within 125 MU of expected

Answer: (C)

Dose delivery accuracy in respiratory gating is evaluated by how closely the actual delivered monitor units (MU) match what was planned. Because gating relies on timing with the patient’s breathing, the beam is not on for the entire treatment, which introduces variability in the total MU that can be delivered in practice. The tolerance of 100 MU provides a practical balance: it is tight enough to catch meaningful drift in the system over the year, yet broad enough to accommodate normal timing jitter, calibration drift, and measurement noise that occur in routine operation. Tolerances that are much smaller would be overly stringent and prone to false alarms, while a larger tolerance could miss gradual drifts affecting dose accuracy. Therefore, the accepted annual dose delivery accuracy is within 100 MU of the expected delivery.