A strategy to determine off-axis dosimetric leaf gap using OSLD and EPID

Background: The aim of the study was to investigate the dosimetric feasibility of using optically stimulated luminescence dosimeters (OSLD) and an electronic portal imaging device (EPID) for central axis (CA X) and off-axis (OAX) dosimetric leaf gap (DLG) measurement. Materials and methods: The Clinac 2100C/D linear accelerator equipped with Millennium-120 multileaf collimator (MLC) and EPID was utilized for this study. The DLG values at CA X and +/- 1 cm OAX (1 cm superior and inferior to the CA X position, respectively along the plane perpendicular to MLC motion) were measured using OSLD (DLG(OSLD)) and validated using ionization chamber dosimetry (DLG(ICD)). The two-dimensional DLG map (2D DLG(EPID)) was derived from the portal images of the DLG plan using a custom-developed software application that incorporated sliding aperture-specific correction factors. Results: DLG(OSLD) and DLG(ICD), though measured with diverse setup in different media, showed similar variation both at CAX and +/- 1 cm OAX positions. The corresponding DLG(EPID) values derived using aperture specific corrections were found to be in agreement with DLG(OSLD) and DLG(ICD). The 2D DLG(EPID) map provides insight into the varying patterns of the DLG with respect to each leaf pair at any position across the exposed field. Conclusions: Commensurate results of DLG(OSLD) with DLG(ICD) values have proven the efficacy of OSLD as an appropriate dosimeter for DLG measurement. The 2D DLGEP map opens a potential pathway to accurately model the rounded-leaf end transmission with discrete leaf-specific DLG values for commissioning of a modern treatment planning system.