SU-F-T-228: Analysis of Multi-Leaf Collimator Leaf Position Errors in Volumetric Modulated Arc Therapy for Nasopharyngeal Carcinoma.
PURPOSE
The purpose of this study was evaluate the position error of multileaf collimator in volumetric modulated arc therapy of nasopharyngeal carcinoma via MLC logs from Varian machines and to provide a method for detecting the leaf position accuracy.


METHODS
Thirty one patients with nasopharyngeal carcinoma were randomly selected. Each patient was created with two 358° arcs. The MLC errors for all arcs, arc1 and arc2, left-bank and right-bank leafs of each arc were evaluated.


RESULTS
99.99% of the leaf position errors of all plans were within ± 2mm. The proportion of the leaf position errors within ± 1.5mm,± 1mm and ± 0.5mm were 99.90%, 98.98% and 94.17%,respectively. The leaf position errors range of ARC1 and ARC2 were -2.95 ∼ 2.99mm and -0.22 ∼ 0.23mm (t = 2.35, P = 0.00), The proportion of the leaf position errors within ± 1mm were 98.08% and 100% for ARC1 and ARC2. The leaf position errors range of Bank A and Bank B were -2.95 ∼ 2.68mm and -2.92 ∼ 2.99mm (t = 1.64, P = 0.31), The proportion of the leaf position errors within ± 1mm were 98.97% and 99.01%.


CONCLUSION
During the VMAT treatment for NPC, it was found that the leaf positions were in control, the Dynalog files can be used as an effective way to analysis the leaf position errors.