Room: Exhibit Hall
Purpose: Modelling of electron beams is restricted to applicator based delivery, with its inherent limitations. The use of a photon MLC for applicator free electron (AFE) field shaping and eventual modulation has been reported by several authors over previous decade.
Methods: In this case, we conducted experiments to measure the depth doses, profiles, scatter kernels and modulation transfer functions (MTF) for a Varian True Beam with a high definition MLC (HDMLC), at SSDâ€™s ranging from 60cm to 100cm. Sufficient data was obtained to begin electron beam modeling and validation in Ray Station (RaySearch labs, Sweden). Measurements were made using the IC profiler and MapCheck 2 from SNC (Melbourne, FL) and various ionization chambers at SSDâ€™s varying from 60 to 100cm, for energies of 9, 12 and 16 MeV and field sizes from 10 to 20cm.
Results: Flatness and symmetry of 3% was obtained with the central 80% field size for electron blocks using apertures defined at the detector surface. PDDs measured with ionization chamber and were within 2%/2mm from applicator-aperture PDDâ€™s. Single leaf scatter kernels were derived from individual leaf profile measurements for the use in beam modelling. Wedge shaped profiles were created using leaf modulation. Profile and depth data was imported into Ray Station and compared with computed values. A contrast ratio, defined as 100% x (Max-Min)/(Max + Min) of a profile of 50% was obtained for 12MeV electrons with 0.5 line pairs/cm at 60cm SSD, and 0.25line pairs/cm at 100cm SSD, indicating that a modest amount of electron modulation for conformal electron treatments can be generated by HDMLC AFE.
Conclusion: We conclude that electron beam modeling can be extended to include AFE delivered beams.