A Case of Salivary Duct Carcinoma Intracranial Invasion due to Perineural Invasion Through the Facial Nerve.

BACKGROUND: Salivary duct carcinoma (SDC) is a rare parotid tumor that often develops as a rapidly growing mass with a poor prognosis. It has a high rate of distant metastases, sometimes with infiltration along nerves. We describe a case of SDC that originated outside the cranium and extended into the cranium along the path of the facial nerve. CASE DESCRIPTION: A 74-year-old man underwent magnetic resonance imaging at a local hospital, which revealed a tumor in the left internal acoustic canal; the patient was referred to our department. A left facial schwannoma was suspected, and magnetic resonance imaging was performed again 6 months later. Rapid tumor growth was confirmed, and the tumor was resected. The tumor displayed atypical epithelial cells with comedo necrosis and cribriform structure and was diagnosed as SDC. All residual intracranial tumors were removed using the middle fossa approach. The tumor, which was considered to be a primary tumor, was found near the stylomastoid foramen, and it was removed with the parotid gland. Five months after the initial surgery, metastasis to the trigeminal nerve was observed, and this was removed using a retrosigmoid approach, followed by radiation therapy. CONCLUSIONS: All 4 surgical specimens of this case were presented, and the path of tumor progression was examined in detail. Although the primary lesion was small, intracranial invasion along the facial nerve occurred. SDC should be considered as a tumor that can extend into the cranium, even with a small primary lesion.