Frequent apocrine changes in pleomorphic adenoma with malignant transformation: a possible pre-malignant step in ductal carcinoma ex pleomorphic adenoma

BACKGROUND: The most common type of carcinoma ex pleomorphic adenoma (CPA) is histologically equivalent to salivary duct carcinoma, which has an apocrine phenotype. Invasive CPA is often accompanied by non-invasive or in situ carcinoma, an observation that suggests the presence of precursor lesions. The aim of this study was to identify candidate precursor lesions of CPA within pleomorphic adenoma (PA). METHODS: Eleven resected cases of CPA with residual PA and 17 cases of PA with atypical changes were subjected to immunohistochemistry (IHC) for p53, human epidermal growth factor receptor 2 (HER2), androgen receptor (AR), pleomorphic adenoma gene 1, gross cystic disease fluid protein-15 (GCDFP-15), and anti-mitochondrial antibody. RESULTS: Invasive or in situ carcinoma cells in all CPAs were positive for AR, GCDFP-15, and HER2. Atypical foci in PAs corresponded to either apocrine or oncocytic changes on the basis of their reactivity to AR, GCDFP-15, and anti-mitochondrial antibody. Atypical cells in PAs surrounding CPAs had an apocrine phenotype without HER2 expression. CONCLUSIONS: Our study identified frequent apocrine changes in residual PAs in CPA cases, suggesting a possible precursor role of apocrine changes. We recommend the use of HER2 IHC in atypical PAs, and that clinicians take HER2 positivity into serious consideration.