Merkel cell carcinoma brain metastasis with radiological findings mimicking primary CNS lymphoma: illustrative case

Abstract

Background: Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine tumor with a high likelihood of distant metastasis. Approximately 30 cases of MCC brain metastasis have been reported. The authors report a case of MCC brain metastasis with imaging findings mimicking primary central nervous system lymphoma.

Observations: A 69-year-old asymptomatic White female with a past medical history of rheumatoid arthritis and MCC of the right cheek with no known regional or distant spread presented with a right frontal lobe lesion discovered incidentally on a surveillance scan. Brain magnetic resonance imaging revealed a vividly enhancing homogeneous lesion with restricted diffusion on diffusion-weighted imaging and corresponding apparent diffusion coefficient maps. Imaging characteristics suggested a highly cellular mass consistent with primary central nervous system lymphoma; however, given the likelihood of metastasis, resection was recommended. An intraoperative frozen section suggested lymphoma. However, further examination revealed positive cytokeratin 20 staining for a tumor, and a final diagnosis of MCC brain metastasis was made.

Lessons: Imaging characteristics of MCC brain metastasis can vary widely. A high level of suspicion should be maintained in a patient with a known history of MCC. Aggressive resection is recommended, regardless of appearance on scans or pathology of frozen sections, because MCC can mimic other intracranial pathologies.

Keywords: Merkel cell carcinoma; brain metastasis; imaging findings; primary CNS lymphoma.

FIG. 1.

A: Noncontrast CT shows a hyperdense inferior parasagittal right frontal lobe mass with surrounding hypodense edema. B: Axial T1-weighted MRI sequence shows a T1 isointense mass with surrounding edema. Postcontrast axial (C) and coronal (D) T1-weighted MRI sequences depict avid enhancement of the mass.

FIG. 2.

Axial fat-suppressed T2-weighted (A) and fluid-attenuated inversion recovery (FLAIR)-weighted (B) MRI sequences show a T2/FLAIR isointense inferior parasagittal right frontal lobe mass with surrounding T2/FLAIR hyperintense edema and regional sulcal effacement. Axial DWI (C) and the associated ADC map (D) depict restricted diffusion within the mass, suggesting high cellularity.

FIG. 3.

A–C: Hematoxylin and eosin (H&E) and immunohistochemical staining. A: Intraoperative H&E-stained frozen section of MCC initially diagnosed as primary CNS lymphoma. Original magnification, 40×. B: Immunohistochemistry showing diffuse uptake of cytokeratin 20, a biomarker for MCC. Original magnification, 100×. C: Histology of primary CNS lymphoma for comparison. Original magnification, 20×. Histology shows a diffuse infiltrate of closely packed intermediate to large mononuclear cells with scant cytoplasm. Used with permission from PathologyOutlines.com and Drs. Courville and Young.

FIG. 4.

MCC brain metastasis diagnostic algorithm. * Multiple cases of patients with MCC brain metastasis without a primary lesion have been reported. DDX = differential diagnosis.