Apocrine epithelial-myoepithelial carcinoma with phyllodes-like morphology of the parotid gland: A case report of an unusual histology with immunohistochemical and genetic analyses using next-generation sequencing

Background: Phyllodes tumor (PT) is a relatively common tumor of the breast; however, it is known that it rarely occurs in other organs. We report a unique case of salivary PT-like carcinoma. Case presentation: A 79-year-old Japanese man noticed an elastic-hard mass in the right parotid region and consecutively underwent right superficial lobectomy. Histologically, the tumor was circumscribed with a fibrous capsule, and showed a cystic space and leaf-like architectures in the subcapsular area. The glandular epithelium showed fused glands with atypia and covered the spindle cell component with the leaf-like structures. Spindle cells harbored cellular atypia and overgrowth with a fascicular pattern. Immunohistochemically, the epithelial component was diffusely positive for cytokeratin (CK)7, androgen receptor, and gross cystic disease fluid protein-15, whereas the spindle cells were diffusely positive for vimentin, p63, SOX10, alpha-smooth muscle actin, and RAS Q61R. The latter was also partially positive for CK7 and CK14. The Ki-67 labeling index was 20.6%. Next-generation sequencing (NGS) indicated a missense mutation in HRAS (c.182A > G) and KRAS (c.35G > A,). Conclusions We diagnosed apocrine epithelial-myoepithelial carcinoma (EMC) with overgrowth of the myoepithelial carcinoma component and a PT-like morphology of the salivary gland. NGS is a very powerful tool for the diagnosis of unusual histology, like EMC.