Nosological Reappraisal of Primary Cutaneous Apocrine Carcinoma Based on a Clinicopathological and Genetic Analysis of 30 Cases

AIMS:The nosology of primary cutaneous apocrine carcinoma remains controversial, as the term has often been used as a heterogeneous diagnostic category. The aim of this study was to clarify the clinicopathological and molecular features of strictly defined apocrine carcinoma using modified diagnostic criteria. METHODS AND RESULTS:Thirty cases were collected from three institutions. Male predominance (28/30, 93%), predominance in elderly patients (median age, 72 years; range, 33-89 years) and the favourite site of the axilla (29/30, 97%) were observed. Lymph node and distant metastases occurred in 64% (16/25) and 28% (7/25) of cases, respectively. Two patients died of the disease (2/25, 8%), with a median follow-up period of 2 years and 1 month. Histopathologically, all tumours harboured cytoplasmic zymogen-like granules; however, some (8/30, 27%) lacked apocrine secretion features. Nucleoli were either moderately prominent (n = 17) or prominent (n = 13). Apocrine gland hyperplasia was identified in 5 (17%) of the 30 cases. Immunoexpression of androgen receptor, BerEP4, cytokeratin 7, GATA3 and GCDFP15 was observed in all tested cases. In contrast, BCL2, CEA, oestrogen receptor and progesterone receptors were negative in all tested cases. Only 13 cases underwent molecular studies. PIK3CA hotspot mutations were detected in 8 of the 13 cases. Panel sequencing revealed KMT2D (MLL2/4) mutations in three cases, all of which have no PIK3CA mutations. CONCLUSIONS:Primary cutaneous apocrine carcinoma is a distinct entity characterized by remarkably consistent clinical, cytopathological, immunohistochemical and molecular features.