  Expansion of non-spherical relativistic blast waves is considered in the
Kompaneets (the thin shell) approximation. We find that the relativistic motion
effectively "freezes out" the lateral dynamics of the shock front: only
extremely strongly collimated shocks, with the opening angles $\Delta \theta
\leq 1/\Gamma^2$, show appreciable modification of profiles due to sideways
expansion. For less collimated profiles the propagation is nearly ballistic;
the sideways expansion of relativistic shock becomes important only when they
become mildly relativistic.
