Validity of the Semiclassical Approximation and Back=reaction
  Studying two-dimensional evaporating dilatonic black holes, we show that the semiclassical approximation, based on the background field approach, is valid everywhere in regions of weak curvature (including the horizon), as long as one takes into account the effects of back-reaction of the Hawking radiation on the background geometry. 
Sukanta Bose, Leonard Parker and Yoav Peleg (UWM)