False-negative tumor-free margins following Mohs surgery

The inability to identify a neoplasm upon examination of an initial Mohs histologic section is not uncommon. Although often the neoplasm has been enclosed within the layer or extirpated by a prior biopsy, there are instances when the negative marginal reading is incorrect. This report describes circumstances when the initial Mohs layer failed to show positive tumor margins in the face of residual in situ neoplasm.