  The description of the electromagnetic interaction of charged spinless
particles is usually formulated by the Scalar Quantum Electrodynamics. However,
there is an alternative formulation given by the Duffin-Kemmer-Petiau theory:
the Scalar DKP gauge theory. The proof of the equivalence between these two
formulations has been discussed in many researches, but there is not yet a
conclusive proof. In this paper, we initiate a complete proof in the framework
of the Causal Perturbation theory, showing that both scalar formulations
provide the same results for the differential cross section at tree level.
