  In this paper we describe the implementation of the QED process
$\gamma\gamma\to\gamma\gamma$ through a fermion loop into the framework of SANC
system. The computations of this process takes into account non-zero mass of
loop-fermion. We briefly describe additional precomputation modules used for
calculation of massive fermion-box diagrams. We present the covariant and
helicity amplitudes for this process and also some particular cases of $D_0$
and $C_0$ Passarino-Veltman functions. Whenever possible, we compare the
results with those existing in the literature.
