What is the monthly wedge placement accuracy tolerance?

• A. 1 mm
• B. 2 mm
• C. 2.5 mm
• D. 3 mm

Answer: (B)

Wedge placement accuracy tolerance measures how far the wedge can be off from its intended position in the beam during monthly QA. Because the wedge shapes the dose distribution, even a small misalignment shifts the dose gradient across the field, potentially affecting target coverage and normal-tissue sparing. The practical, clinically acceptable limit that aligns with typical mechanical repeatability and QA measurement capabilities is two millimeters. Staying within this tolerance helps ensure the delivered dose closely matches the planned distribution from month to month. If the wedge is off by more than about 2 mm, the resulting dose pattern could become clinically significant, prompting recalibration or service. To check this, a phantom-based measurement compares the wedge edge position against a baseline; deviations beyond 2 mm indicate maintenance is needed. The other values would either be too strict for routine maintenance (1 mm) or would permit larger misplacements that could noticeably impact the dose (2.5 mm or 3 mm).