Neurofilament protein is used to distinguish MCC from oat cell carcinoma. Neurofilament protein is seen in nearly all MCCs but in few oat cell carcinomas. Other markers present with variable frequency include chromogranin, synaptophysin, vasoactive intestinal peptide, calcitonin, bombesin, corticotropic hormone, metencephalon, gastrin, and somatostatin. The absence of certain markers also helps in the diagnosis of MCC by ruling out other tumors. S-100 is seen in melanoma, whereas leukocyte common antigen is present in lymphoma. Neither of these is found in MCC.
On occasion, MCCs may be diagnosed with histology alone, but confirmation with immunohistochemistry and/or electron microscopy is always encouraged. Diagnosis by means of light microscopy alone is difficult because the appearance of MCCs is similar to that of many other undifferentiated small cell neoplasms, especially other APUD tumors, such as metastatic oat cell carcinoma. In fact, 66% of MCCs are misdiagnosed when studied with light microscopy alone.
The most common pattern is the intermediate cell type, observed in over 50% of patients with MCC. These tumors display a large nest of cells without organoid architecture or recognizable palisading. A distinct disassociation exists between cells. Areas of focal necrosis and lymphocytic invasion are typical. Cytoplasm is moderate, nuclei are vesicular, and mitoses are abundant. The trabecular cell type is observed in 25% of MCCs. In this class, the cells are arranged in organoid clusters with interconnected trabeculae separated by strands of connective tissue. Clusters may show glandlike organization. Individual cells are compactly arranged round-to-polygonal cells. Cytoplasm is abundant, and the nuclei are round, centrally located, and vesicular. Pleomorphism and mitotic activity are mild to moderate. The final and least common class is the small cell variation. This pattern consists of solid sheets and clusters of cells separated by abundant stroma, with large areas of necrosis. The cells are small with scant cytoplasm and hyperchromatic nuclei. Pleomorphism and mitoses are common.
For more on the histology of MCC, read here.
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Cite this: Elwyn C. Cabebe. Fast Five Quiz: How Much Do You Know About Merkel Cell Carcinoma? - Medscape - May 31, 2017.