Duffin-Kemmer-Petiau Theory in the Causal Approach
  In this paper we consider the scalar sector of Duffin-Kemmer-Petiau theory in the framework of Epstein-Glaser causal method. We calculate the lowest order distributions for Compton scattering, vacuum polarization, self-energy and vertex corrections. By requiring gauge invariance of the theory we recover, in a natural way, the scalar propagator of the usual effective theory. 
J. T. Lunardi, L. A. Manzoni, B. M. Pimentel and J. S. Valverde