The higher-order perturbations in the application of perturbation theory to the γ-transport equation are discussed. An analytic expression is obtained for the perturbation flux of arbitrary order arising from a variation in the electron density of the medium, regardless of its geometry. This expression can be used in the constant-cross-section approximation to find the perturbation due to a change in the chemical composition of a plane layer. The numerical calculation is illustrated with an example.