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

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

Answer: (B)

In gated radiotherapy, the beam is delivered only during a specific part of the breathing cycle, so the total amount of radiation delivered over a course can drift due to breathing variability, gating window calibration, machine output drift, and patient setup differences. To ensure safety and treatment effectiveness, there is a fixed annual tolerance that compares the delivered monitor units (MU) to what was planned. Being within 100 MU of the expected delivery means the total MU actually delivered over the year stays close enough to the plan to be clinically acceptable, despite the inherent variability of gating.

Choosing a smaller tolerance like 50 MU would be harder to achieve given routine daily variations, while a larger tolerance like 200 MU would be looser than typical QA standards. Stating that it must be exactly 100 MU would ignore the practical allowance for minor fluctuations.