What is the annual spot check of field size dependent output factors tolerance?

• A. ±1% from baseline
• B. ±2% from baseline
• C. ±3% from baseline
• D. ±5% from baseline

Answer: (B)

Field size dependent output factors show how the machine’s output changes as you change the field size. Over time, the beam can drift a little due to aging, energy changes, or calibration shifts, so you perform a spot check to verify those factors remain consistent with the baseline. The accepted annual tolerance is ±2% from that baseline, which is a practical balance between detecting meaningful drift and accommodating normal measurement variability with standard dosimetry equipment. If a measurement falls outside this window, recalibration or service is needed. For example, if a reference field size has an output factor of 1.00, the current value should be within 0.98 to 1.02. Tolerances tighter than this could be overly sensitive to noise, while looser tolerances could allow clinically significant dose differences to go unnoticed.