Apocrine Intraductal Carcinoma With A Frankly Invasive Salivary Duct Carcinoma Component: A Case Report Showing Novel Genetic Alterations

Intraductal carcinoma (IDC) and salivary duct carcinoma (SDC) are both rare tumors of the salivary glands that most frequently occur in the parotid glands. We present an example of apocrine IDC with a frankly invasive SDC component in the parotid gland. The clinicopathologic and molecular features were analyzed in this study. A 76-year-old female patient was admitted to hospital with a parotid mass. The biopsy result showed SDC, and radical resection surgery was performed. The tumor was characterized by an intraductal proliferation forming solid and cribriform structures with abundant eosinophilic cytoplasm, accompanied by comedo necrosis and apocrine secretion, some of which showed infiltrative growth without myoepithelial cells surrounding. Identifiable intravascular tumor thrombi and extensive lymph node (12 of 30) metastases were observed. The tumor cells were positive for androgen receptor and HER2, while negative for S100 and SOX10. The final diagnosis was apocrine IDC with an invasive SDC component. Next-generation sequencing revealed MET gene fusions and other gene mutations in the tumor cells. The rare MET gene fusions have not been reported in either IDC or SDC, which may enhance our understanding of the genetic alterations in a subset of SDCs that derive from IDC.