True Oncocytic Acinic Cell Carcinoma: A Case Image

A 67-year-old female with a history of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) presented with right sided otalgia and a 2-3 cm firm, tender right posterior parotid mass. Fine needle aspiration biopsy (FNAB) established a diagnosis of acinic cell carcinoma (AciCC). Further workup demonstrated lung nodules which were confirmed by FNAB to represent metastatic AciCC. A right radical parotidectomy with sacrifice of the facial nerve, segmental mandibulectomy, and selective neck dissection (levels II-IV) was performed. Microscopically, the tumor displayed an infiltrative border with a solid multinodular growth pattern and fibrosclerotic septation. The tumor was composed mainly of uniform cells with abundant eosinophilic granular cytoplasm with round nuclei with prominent nucleoli. Nuclei were fairly monomorphic, mitotic counts were 3-4 per 2mm² and there was no necrosis despite the aggressive growth pattern. An anti-mitochondrial immunohistochemical stain showed strong reactivity in the tumor cells, with an internal positive control of adjacent striated ducts. An immunohistochemical stain for NR4A3 demonstrated strong nuclear reactivity in the tumor cells. Electron microscopy highlighted the tumor cells with numerous mitochondria and distinctive electron dense intramitochondrial inclusions. Concurrent CLL/SLL was identified on histologic examination of the lymph nodes, but they were free of AciCC. After eight weeks of follow-up, she tolerated the surgery well and is currently receiving radiation therapy to the parotid and neck. In this illustrative case, we justify the oncocytic designation of AciCC by morphology, immunohistochemistry, and electron microscopy.