On twisted microdifferential modules I. Non-existence of twisted wave   equations
  Using the notion of subprincipal symbol, we give a necessary condition for the existence of twisted D-modules simple along a smooth involutive submanifold of the cotangent bundle to a complex manifold. As an application, we prove that there are no generalized massless field equations with non trivial twist on grassmannians, and in particular that the Penrose transform does not extend to the twisted case. 
Andrea D'Agnolo and Pierre Schapira