The patient went on to have a resection and a staging CT of the chest and abdomen.

Histology

Paraffin sections show fragments of a moderately cellular epithelial tumor. This is composed of papillary and glandular structures lined by a stratified arrangement of columnar epithelial cells. These show moderate nuclear pleomorphism. Scattered mitotic figures are noted and there are several foci of tumor necrosis. Several of the glandular spaces are distended by mucinous material.

Immunohistochemistry shows moderate diffuse cytoplasmic staining in tumor cells for Napsin A and strong lumenal membrane staining for monoclonal carcinoembryonic antigen (CEA). No staining for estrogen receptor (ER), progesterone receptor (PgR), HER-2, TTF-1, CDX2, transthyretin or cytokeratins CK7 or CK20 is seen. 

Lung non-small cell carcinoma mutation testing:

• EGFR NO MUTATION DETECTED
• KRAS NO MUTATION DETECTED

FINAL DIAGNOSIS: cerebral metastasis - moderately differentiated adenocarcinoma 

CT of the chest and abdomen

CT of the chest and abdomen (not shown) demonstrated numerous liver metastases and mesocolon lymph node enlargement without an obvious primary although the colon did appear thickened. This was considered the primary tumor. The lungs and mediastinum were normal  

Discussion

It can be difficult to distinguish a solitary metastasis form a circumscribed high-grade glioma. If non-enhancing tumor (either on T2/FLAIR or ADC or MRS or CBV) can be detected then a diagnosis of high-grade glioma can be made with certainty. When no non-enhancing tumor is identified a metastasis is generally favored but needs to be weighed against pre-test probability.