Idiopathic calcinosis cutis with unusual histomorphology and negative von Kossa stain: A diagnostic pitfall

Abstract

Introduction: Calcinosis cutis is characterized by basophilic crystalline or amorphous calcium within the dermis/subcutis. Calcinosis cutis has many different clinical subtypes and associated etiologies that may lead clinicians to consider this diagnosis. Von Kossa and/or Alizarin red special stains may be used by pathologists to highlight deposition of insoluble calcium salts, especially in histologically ambiguous or subtle cases. Case Report: We report the case of a 27-year-old male who presented with an inferior right buttock mass clinically diagnosed as epidermal inclusion cyst(s). Gross pathology revealed a gray-white pasty substance within a cyst-like structure. Histopathologic examination demonstrated pools of amorphous blue-gray material with rare deposits of admixed coarse calcifications surrounded by foreign-body giant cells. No cyst wall was seen in the specimen. Special stains, including von Kossa, were initially negative. Following additional review, it was discovered that surface decalcifying solution had been applied to the paraffin block in the histology lab prior to microtome sectioning. Hypothesizing that this could be the cause of the unusual morphology, the paraffin block was reprocessed and subsequent H and E stained sections displayed characteristic basophilic calcium deposits, which were correspondingly positive by von Kossa stain. Conclusion: The histopathologic diagnosis of calcinosis cutis is apparent by HandE in most cases, though von Kossa and/or Alizarin red special stains can be used to aid the pathologist. Given the unusual histomorphology following surface decalcification and initial lack of von Kossa stain prior to reprocessing, this report serves to make pathologists aware of this potential diagnostic pitfall.