  We calculate large mass diphoton exclusive photoproduction in the framework
of collinear QCD factorization at next to leading order in {\alpha}s and at
leading twist. Collinear divergences of the coefficient function are absorbed
by the evolution of the generalized parton distributions (GPDs). This result
enlarges the existing factorization proofs to 2 -> 3 processes, opening new
reactions to a trustable extraction of GPDs.
