Answer: Merkel cell carcinoma

Histology: The tumor in this case is located in the subcutaneous tissue because this particular tumor is actually a recurrence. The tumor is composed of small blue cells with scant cytoplasm and a high nuclear-to-cytoplasmic ratio. The nuclei are round and vesicular and often have a somewhat glassy or fine nuclear chromatin. Small nucleoli may be observed. The tumor is associated with necrosis and mitotic figures are easily identified. In some areas the cells seem to nest together in a somewhat cohesive pattern. Nuclear molding is not seen.

Discussion: In this case the differential diagnosis is with other small round-cell tumors, including acute leukemia, lymphoblastic lymphoma, and metastatic small cell carcinoma from other sites. Immunohistochemistry can be extremely helpful in making this diagnosis. Merkel cell tumors are positive for low-molecular weight keratin, neurofilaments and neuron specific enolase. CK-20 positivity shows a characteristic perinuclear dot-like staining pattern. The consistent immunoreactivity for neurofilaments can be helpful in making the distinction between small cell carcinoma of the lung and other organs. Another helpful feature in distinguishing Merkel cell carcinoma from a small cell carcinoma, is that small cell carcinoma is usually positive for TTF-1 whereas Merkel cell carcinoma is negative. Merkel cell carcinoma is negative for the usual hematologic markers, which would be positive in leukemia and lymphoma. In addition, sometimes Merkel cell carcinoma shows focal activity for other neuroendocrine markers such as chromogranin and synaptophysin. Chromosomal abnormalities have been identified in some cases of Merkel Cell carcinoma. These abnormalities have been of chromosomes 11 and 12 and are seen in 20-30% of cases studied. These tumors are aggressive tumors, and as in this case it is a recurrence of the original presentation. In addition to recurrence, this tumor has the ability to metastasize to distant sites as well as lymph nodes. The recommended initial treatment for Merkel cell carcinoma is wide resection and lymph node dissection. Other modes of additional therapy such as radiation and chemotherapy have also been applied, particularly for recurrence and metastatic tumors.