[en] An empiric method for approximate restoration of the X radiation spectrum in any point of a heterogeneous medium on the basis of experimental measurement of ''effective'' energy with thermoluminescent detectors is suggested. Formulas for calculation of a mean geometrical factor of plane, spherical, and cylindrical cavities in cases when electrons formed in the bone surrounding the cavity do not reach the center of the cavity are presented. The results of calculation of the mean bone marrow doses in various regions of the skeleton of rats and mice under the most common conditions of irradiation are discussed