We study solutions of Leontovich—Fock parabolic equation which describe the structure of a paraxial quasi-optical wave beam in a medium with a strong spatial inhomogeneity of the absorption coefficient across the direction of beam propagation. Using as an example the simplest reference problem of a wave beam propagating along an infinitely narrow plane dissipative layer, it is shown that in this case the energy dissipation and the beam diffraction affect each other, therefore they must be determined simultaneously while the application of a perturbation approach can lead to qualitatively incorrect results.