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Multileaf Collimator Performance and Validation of Quality Control Tolerances


D Letourneau

D Letourneau*, K Wang, B Norrlinger, A Nurul, P Homer, P Lee, D Jaffray, Princess Margaret Hospital, Toronto, ON

SU-E-T-94 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
The automated quality assurance system (AQUA) is a centralized quality control (QC) software designed to automate QC tests. Statistical analysis of AQUA results was performed to assess the geometric accuracy and long-term reproducibility of a commercially available multileaf collimator (MLC) and examine the applicability of the American Association of Physicists in Medicine (AAPM) tolerances for MLC QC.

Methods:
The MLC was first calibrated with AQUA by minimizing leaf-positioning errors on megavoltage images for 5 different leaf-bank positions (-60 to 100 mm from radiation isocenter). Leaf-positioning accuracy and reproducibility was assessed by repeating the AQUA test 5 times/week. The range of leaf-positioning error over leaf-bank positions and time was reported. Measured leaf-positioning errors were then separated into systematic and random error components. The systematic error corresponds to the variation (standard deviation) in mean positioning errors between leaves over leaf-bank positions and time. The random error quantifies the leaf position variations around its mean and is calculated as the root-mean-square of the individual leaf position standard deviations.

Results:
To date, 2 different MLCs have been calibrated using AQUA and 9-18 datasets have been acquired to assess performance. For the unit with the longest follow up, the range of leaf-positioning errors was -0.62 to 0.85 mm and 98% of the measured leaf positions (n=7200) were within ±0.5 mm of the nominal position. The systematic error was the main error component (±0.15 to ±0.2 mm) and was attributed to the residual errors after calibration. The random error was ±0.07 mm for both units and demonstrated good leaf-positioning reproducibility and limited uncertainty of the AQUA measurements.

Conclusions:
Preliminary results show that after MLC calibration with AQUA, leaf-positioning errors on two different units are well within the AAPM-recommended ±1 mm tolerances. Additional MLC performance improvement is possible if residual errors after calibration can be reduced further as the MLC demonstrated high reproducibility.

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