The patient went on to have a resection. 

Histology

MICROSCOPIC DESCRIPTION:

Sections of brain show a circumscribed deposit of metastatic carcinoma. The biphasic tumor is composed of solid nests of large basaloid cells and focal areas of squamous differentiation. Central tumor necrosis is present. Basaloid tumor cells demonstrate enlarged, oval nuclei with coarsely granular chromatin and small inconspicuous nucleoli. Frequent mitoses and apoptoses are present. Squamoid tumor cells contain abundant densely eosinophilic cytoplasm and prominent intercellualr bridges. No definite keratin or gland formation is seen. 

Immunohistochemical results show tumor cells stain:

Squamous component: CKAE1/3: positive, CAM5.2: positive, CK7: positive, CK20: positive, p63: positive, p40: positive, CK17: positive, CK5/6: positive, CK14: negative.

Neuroendocrine component: CD56: positive, Chromogranin: negative, Synaptophysin: negative.

Both components: TTF1: positive (focal), NapsinA: negative, S100: negative, MelanA: negative, SOX10: negative, p16: positive, EBER-ISH: negative.

FINAL DIAGNOSIS: Metastatic poorly differentiated carcinoma with mixed squamous and large cell neuroendocrine differentiation.