Po‐Poster ‐ 18: Investigation of tilted dose kernels for portal dose prediction in a‐Si electronic portal imagers

Abstract

The effect of beam divergence on dose calculation in an amorphous silicon electronic portal imaging device (EPID) was investigated with Monte Carlo generated dose kernels. The flat‐panel detector was simulated in EGSnrc (user code DOSXYZnrc) with a 3.0 cm water buildup; the model included details of the detector's imaging cassette and the front cover upstream of it. To approximate the effect of the EPID's rear housing, a 21 mm air gap and 10 mm water slab were introduced into the simulation as equivalent backscatter material. Kernels were generated with monoenergetic 2, 6, and 18 MeV photons with the orientation of the pencil beam varying from 0 to 14 degrees in 2 degree increments. Dose was scored in the phosphor layer of the detector. To reduce statistical fluctuations at large radial distances from the incident pencil beam, the kernels were first averaged bilaterally and then combined into square half rings. Profiles of the kernels were observed to demonstrate increasing asymmetry with increasing angle and energy, while the total energy deposited in the phosphor by the 2 MeV pencil beam decreased by greater than 2% at larger angles. Further investigation via comparison of superposition to convolution dose calculation methods is required to determine the effect these angled kernels have on calculation accuracy in clinical beam geometry.