Poster - Thurs Eve-06: Maximizing eclipse IMRT dose accuracy by adjusting the dosimetric leaf gap parameter

The dosimetric leaf gap (DLG) is a parameter used by Eclipse to model the rounded leaf ends of Varian MLCs. The DLGs were determined for the Millennium (M120) and High-Definition (HD120) model MLCs and taken as the difference between measured (0.6mm diode, IBA) and nominal MLC-defined profile FWHM values. Configuring the Eclipse pencil beam algorithm with the measured DLG gave poor agreement between measured and calculated IMRT dose distributions for the HD120 but not the M120. Agreement was optimized by adjusting the DLG for the HD120; 0.3mm changes in DLG were enough to cause significant variations in field dose agreement. Optimal DLG values of 0.04cm and 0.05cm were found for the 6MV HD120 and 10MV HD120, respectively, and 0.135cm 0.175cm for the 6MV M120 and 18MV M120, respectively. Agreement between measured and calculated dose distributions worsened for the AAA algorithm indicating separate DLG values may be required. A leaf calibration software upgrade also reduced agreement by changing the physical leaf position for a given location value. The change was detected using film and the picket fence MLC-pattern which places the two banks of opposing leaves at the same position but at different times. The DLG value can be adjusted from its measured physical value to improve the dosimetric accuracy of Eclipse IMRT plans and compensate for the effects of treatment planning algorithm and varying leaf calibrations. Since leaf calibrations are variable it is important to define the dosimetric leaf gap for each accelerator and clinic.