PNNL

Abstract

Monte Carlo simulation of electrons stopping in liquid water was used to model the penetration and dose distribution of electron beams incident on the full-thickness EpiDermTM skin model (MatTek, Ashland, VA). This 3D tissue model has a fully developed basement membrane separating an epidermal layer of keratinocytes in various stages of differentiation from a dermal layer of fibroblast embedded in collagen. The simulations were motivated by a desire to selectively expose the epidermal layer to low linear-energy-transfer (LET) radiation in the presence of a non-irradiated dermal layer. Using the variable energy electron microbeam at the Pacific Northwest National Laboratory (PNNL) as a model of device characteristics and irradiation geometry, we find that at the highest beam energy available (90 keV), the estimated 90th percentile of penetration remains in the epidermal layer. To investigate the depth-dose distribution, we calculated lineal energy spectra for 10um thick layers near the 10th, 50th, and 90th percentile of penetration by the 90 keV electron beam. Biphasic spectra showed an increasing component of "stoppers" with increasing depth. Despite changes in the lineal energy spectra, the main effect on dose deposition with increasing depth is the screening effect of tissue above the layer of interest.