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An Efficient Method to Quantitatively Detect Sub-Millimeter MLC Bank Alignment Errors Using a Well-Known QA Device

A Nguyen

A Nguyen1*, K Patton2 , T Paul1 , W Duggar3 , C Yang3 , (1) Ironwood Cancer and Research Center, Phoenix, AZ, (2) Elekta, Atlanta, GA, (3) University of Mississippi Med. Center, Jackson, MS


SU-E-T-102 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To introduce an ultra-sensitive procedure that can detect and quantify systematic MLC bank alignment error down to 0.2 mm or less. The procedure takes only 5 minutes to run and employs the familiar MapCheck2 device.

Methods: Elekta iCOM v13.0 was used to create and deliver a modified 4-Quadrant field on the Elekta Infinity Linac. The 2D dose distribution was collected on the MapCheck2, at 100 Source-Detector-Distance with no external buildup. Using SNC Patient v6.2.3 this planar dose was compared against a reference planar dose established preferably right after commissioning or the most recent Annual QA. To obtain baseline results and tolerance levels leaf bank alignment shifts ranging from -1.0 mm (in-field) to +1 mm (out-of-field) in 0.1 mm increment were intentionally induced. Each shifted field is matched against a reference field with zero shift. A Y-profile through the CAX, perpendicular to MLC traveling direction, was selected in the software. Percentage dose differences at selected points on this profile were recorded. The mean dose differences were then plotted against the corresponding shift distances.

Results: The test is extremely sensitive in picking up MLC alignment error. Shift down to 0.2 mm can be easily detected. Furthermore, the relationship between dose deviations and MLC bank alignment errors is shown to be quite linear. For our setup, each 0.1 mm shift in MLC bank would result in approximately 2.7% dose deviation from baseline. Based on this result, monthly QA tolerance level at was set at +/-7%, which translates roughly to 0.25 mm in alignment error.

Conclusion: A novel method to quantify MLC alignment error was introduced. The test is short, simple yet very effective. It’s been implemented clinically as part of our institution monthly machine QA.

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