What is the annual electron beam quality tolerance?

• A. ±0.5 mm
• B. ±1 mm
• C. ±2 mm
• D. ±1 cm

Answer: (B)

Electron beam quality is about the beam’s energy and penetrative power, which directly shapes where the dose is deposited in tissue. Because energy can drift as the accelerator ages or after maintenance, the QA program uses a tight annual tolerance to catch any drift before it affects patient dose distributions. In practice, this means the energy-related depth metric (such as practical range or the depth of a reference dose point) must stay within about one millimeter of its reference value. A shift any larger could produce a clinically meaningful change in where the dose falls, prompting recalibration or service.

Smaller tolerances would demand unrealistically precise measurements given routine QA uncertainties, while larger tolerances would permit drift that could impact treatment accuracy. A centimeter-scale tolerance would be far too loose for the precision required in electron therapy. Hence, the standard annual tolerance is set at about one millimeter.