Intraductal carcinoma of the parotid gland, mixed intercalated duct and oncocytic subtype with mucinous and serous acinar differentiation: cytologic and histologic features of a novel morphology.

Intraductal carcinoma (IDC) of the salivary gland is rare. Histologic subtypes include intercalated duct, oncocytic, apocrine, and hybrid/mixed. Molecular correlates have been described, with intercalated duct IDC typically harboring NCOA4::RET, while TRIM33::RET, or BRAF pathogenic variants predominating in oncocytic IDC. We describe the case of a 77-year-old female with a parotid mass. Fine needle aspiration showed a population of low-grade epithelial cells with frequent cytoplasmic vacuolization, which were positive for S-100, SOX-10, and mammaglobin, and was interpreted as compatible with secretory carcinoma. A dual cell population was recognized on surgical resection, predominantly oncocytic, with at least one focus prototypical of intercalated IDC. Luminal secretions were positive for mucicarmine, while PAS after diastase highlighted the latter in addition to zymogen-like cytoplasmic granules. RNA sequencing detected a NCOA4::RET fusion. This is the first report of the cyto-histologic features of the mixed intercalated duct and oncocytic subtype IDC with mucous and serous acinar-like differentiation.