What is the monthly backup monitor chamber constancy tolerance?

• A. 1%
• B. 2% (at IMRT dose rate)
• C. 3%
• D. 4%

Answer: (B)

The monthly backup monitor chamber constancy tolerance checks how much the chamber’s response to radiation can drift from its established baseline. This chamber is what senses the beam to regulate monitor units, so its constancy is crucial to keep dose delivered per MU consistent over time. The standard tolerance is two percent when testing at the IMRT dose rate, because IMRT deliveries use higher and more variable dose rates, making this condition most representative of clinical use. If the chamber’s response shifts by more than about two percent at the IMRT dose rate, it indicates drift that should be addressed with recalibration or service before continuing treatments.