In this paper, we report a formulation for the exposure buildup factor by solving the one-dimensional photon transport equation in a heterogeneous slab by the LTS N method, assuming the Klein-Nishina scattering kernel as the scattering differential cross-section as well as the multigroup model in the wavelength variable. We present numerical simulations and comparisons with available results in the literature for a multilayered slab composed of water, iron and lead. We also report an analytical solution to the exposure buildup factor by solving the photon transport equation in a rectangle applying the LTS N nodal method. The exposure buildup factor formulation in a slab and rectangle geometry 103