Real-time dynamic MLC tracking for inversely optimized arc radiotherapy.

BACKGROUND AND PURPOSE Motion compensation with MLC tracking was tested for inversely optimized arc radiotherapy with special attention to the impact of the size of the target displacements and the angle of the leaf trajectory. MATERIALS AND METHODS An MLC-tracking algorithm was used to adjust the MLC positions according to the target movements using information from an optical real-time positioning management system. Two plans with collimator angles of 45 degrees and 90 degrees , respectively, were delivered and measured using the Delta(4)(R) dosimetric device moving in the superior-inferior direction with peak-to-peak displacements of 5, 10, 15, 20 and 25 mm and a cycle time of 6s. RESULTS Gamma index evaluation for plan delivery with MLC tracking gave a pass rate higher than 98% for criteria 3% and 3 mm for both plans and for all sizes of the target displacement. With no motion compensation, the average pass rate was 75% for plan 1 and 70% for plan 2 for 25 mm peak-to-peak displacement. CONCLUSION MLC tracking improves the accuracy of inversely optimized arc delivery for the cases studied. With MLC tracking, the dosimetric accuracy was independent of the magnitude of the peak-to-peak displacement of the target and not significantly affected by the angle between the leaf trajectory and the target movements.