The effect of depth and control point number for MLC transmission and dosimetric leaf gap

Purpose/Objective: MLC transmision (MLCT) and dosimetric leaf gap (DLG) in Treatment Planning System (TPS) are important parameters for dose calculation and they provide accuracy of delivering IMRT plans. MLCT and DLG account the leakage of MLC and the effect of rounded leaf ends, respectively. The aim of this study is to examine control point (CP) and depth dependance for MLCT and DLG. Materials and Methods: MLCT and DLG parameters were measured with PTW 0.125 cc semi-flex ionization chember in a solid water phantom for Varian Trilogy machine with Millenium MLC. MLCT measurements were obtained 15x15cm2 field size at different depths (Dmax,5,10 and 15cm) for 6 MV photon beam. To derive the DLG measurement , a series of Sliding Window (SW) fields of 2,4,7,10,15 and 20 mm gap widths were used. These SW fields were prepared with different number of CP (2,3,5,7,9,11,13,16 and 21) and DLG measuremets were performed at same depths and field size. DLG values were calculated for each depth using MLCT parameters which were measured at same depth.