  We calculate the spin-averaged amplitude for doubly virtual forward Compton
scattering off nucleons in the framework of manifestly Lorentz invariant baryon
chiral perturbation theory at complete one-loop order $O(p^4)$. The
calculations are carried out both in the infinite and in a finite volume. The
obtained results allow for a detailed estimation the finite-volume corrections
to the amplitude which can be extracted on the lattice using the background
field technique.
