Measurements of polarized electron cyclotron emission from runaway electrons have been achieved for the first time in the TCV tokamak. The experimental configuration uses a vertical viewing chord to avoid ambiguities on the location of the emitting electrons [1]. The diagnostic thus establishes a one-to-one correspondence between the measured frequencies and the electrons kinetic energy provided there is no harmonic overlap and no radiation from multiple wall reflections. In this talk we discuss the reconstruction of the line-average distribution function of runaway electrons with energies up to ~200 keV, from their measured X3/O3 emission leaning on LUKE[2] and SOFT[3] for the modelling. [1] A. Tema Biwole, L. Porte, A. Fasoli, A. Simonetto, O. D’Arcangelo and the TCV team, "Performance of a high vacuum, high temperature compatible millimeter-range viewing dump for the vertical ECE experiment on TCV", Fusion Eng. Des. 2020 **112079**,[https://doi.org/10.1016/j.fusengdes.2020.112079]. [2] J. Decker, Y. Peysson, "DKE: a fast numerical solver for the 3D drift kinetic equation", Tech. Rep. EUR-CEA-FC-1736, Euratom-CEA (2004). [3] M. Hoppe, O. Embréus, R. A. Tinguely, R. S. Granetz, A. Stahl and T. Fülöp, "SOFT: a synthetic synchrotron diagnostic for runaway electrons", Nucl. Fusion **58** 026032 [https://doi.org/10.1088/1741-4326/aa9abb].