Mucocyte-Rich/Extracellular Mucin-Poor Salivary Duct Carcinoma: An Unusual Morphological Variant Mimicking Mucoepidermoid Carcinoma

Salivary duct carcinoma (SDC) is an aggressive malignancy of the salivary gland. The rare mucin-rich variant is characterized by the presence of nests of malignant cells floating in pools of extracellular mucin. We present an unusual case of SDC with abundant mucocytes but complete absence of extracellular mucin, initially misdiagnosed as mucoepidermoid carcinoma (MEC) on fine needle aspiration (FNA). The patient, an 85-year-old woman, presented with a 3.2 cm heterogeneous, partially cystic, right parotid mass causing facial nerve paralysis. FNA revealed mixed pleomorphic and hyperchromatic epithelial cells with abundant mucocytes. The tumor cells were focally positive for p63 and androgen receptor (AR). The patient was diagnosed with intermediate-grade MEC. Subsequent parotidectomy revealed high-grade duct-forming carcinoma with extensive perineural invasion, frequent mitosis, and comedonecrosis arising from a pleomorphic adenoma. This was remarkable for numerous mucocytes and the absence of extracellular mucin. Immunohistochemically, the tumor cells expressed AR, CK7, GATA3, and GCDFP-15, confirming the diagnosis of SDC. Whole exome sequencing revealed RAD50 p.Lys973Arg and POLE p.Arg52Trp mutations, suggesting that the DNA damage repair pathway was affected. Given the potential for variability in individual marker expression, including aberrant phenotypes, such as p63-positive SDC or AR-positive MEC, employing a comprehensive panel of markers for accurate diagnosis is crucial. Markers such as GATA3 and GCDFP-15 can be useful in detecting highly aggressive SDC.