115 Mucin-rich Salivary Duct Carcinoma ex Pleomorphic Adenoma of the Parotid Gland Mimicking Mucinous Adenocarcinoma

Introduction/Objective Salivary duct carcinoma (SDC) is an uncommon, aggressive malignancy, comprising <10% of salivary gland tumors. It typically affects older men and may arises de-novo or as carcinoma ex-pleomorphic adenoma (PA). A mucin-rich variant of SDC (mSDC) is exceedingly rare and may closely mimic primary mucinous adenocarcinoma or metastatic carcinoma, posing diagnostic challenge. Methods/Case Report We report a case of mSDC arising from a long-standing PA in a 76-year-old man with a history of multiple resections and radiation therapy 30 years prior. He presented with a deep-lobe parotid mass. FNA revealed adenocarcinoma with mucinous features. Total parotidectomy showed a 4.1 cm infiltrative carcinoma arising from hyalinized residual PA. No lymph node metastases were found. Histologically, the entire tumor exhibited clusters of high-grade cells, including occasional signet-ring cells, floating in abundant extracellular mucin. Immunohistochemistry showed diffuse positivity for CK7 and AR, focal expression of S100, SOX10 and p63, and negativity for CD117, DOG1, p40, and SMA. The Ki-67 index was 20%. The patient is planned to receive adjuvant chemotherapy at 4-month follow-up. Results NA Conclusion This case emphasizes the diagnostic complexity of mSDC and the importance of integrating morphology, immunoprofile, and clinical history. Broader recognition of this rare variant is essential for accurate diagnosis and management.