Which monthly QA tolerance is specified for the backup monitor chamber constancy?

• A. 1%
• B. 0.5%
• C. 1.5%
• D. 2% (at IMRT dose rate)

Answer: (D)

The main idea is to make sure the backup monitor chamber stays in step with the reference monitor so it can safely take over if the primary fails. Because the backup is a safety-only channel, it doesn’t have to be identical, but it does need to respond within a tight enough range to be trustworthy during IMRT deliveries.

Monthly constancy checks compare the backup chamber’s response to the reference chamber under IMRT dose-rate conditions. The specified tolerance is within 2% at the IMRT dose rate, meaning the backup chamber’s reading for a given MU and beam setup should be within ±2% of the reference chamber’s reading. This balance accounts for measurement uncertainties and detector differences while keeping patient safety intact. If the result exceeds 2%, calibration or cross-calibration of the chambers should be reviewed and corrected.