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Sensitivity of MatriXX 2D Array in Detection of MLC Leaf Positioning Errors

Q Shang

Q Shang*, L Huang, P Qi, P Xia, Cleveland Clinic Foundation, Cleveland, OH

SU-E-T-90 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: MLC leaf positioning errors significantly impact the accuracy of IMRT delivery. This study was to investigate the detection sensitivity of a commercial MatriXX 2D ion chamber array for MLC leaf positioning errors.

Methods: Systematic MLC leaf positioning errors (±1, ±2, ±3 mm) were purposely introduced on one side of the leaf bank in a square field (10 X 10 cm2) and a clinical prostate IMRT plan with 5 coplanar fields (0°, 50°, 100°, 260°, and 310°) within a clinical treatment planning system (TPS). To test the effect of the ion chamber resolution (7.62 mm), the center of the square field was shifted to different locations at 1 mm intervals in relationship to the chamber positions. Both treatment plans, with and without leaf positioning errors, were delivered and measured with a commercial MatriXX device. To eliminate potential beam modeling errors from the TPS, the Gamma index of each field was directly calculated between measured planar doses with and without errors.

Results: For the square field with 1 mm and 2 mm leaf positioning errors, respectively, the Gamma indices of 3%/3mm were 99.8% and 98.8%; the Gamma indices of 2%/2mm were 98.8% and 97.5%. The Gamma index was independent of the error positions relative to the chamber positions. For the clinical prostate IMRT plan with 1 mm, 2 mm, and 3 mm leaf positioning errors, respectively, the average Gamma indices of 3%/3mm for 5 fields were (100 ± 0.0)%, (99.8 ± 0.1)%, and (98.6 ± 1.2)%; the average Gamma indices of 2%/2mm were (99.8 ± 0.2)%, (98.3 ± 0.8)%, and (96.6 ± 1.8)%.

Conclusion: With the Gamma index of 3%/3mm having a passing rate of 99%, the MatriXX 2D array can detect 2 mm leaf positioning errors for a single field, and 3 mm errors for IMRT plans with multiple fields.

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