Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous carcinoma arising from neuroendocrine cells of the dermis. Clinically, MCC is found in sun-exposed areas such as the face or neck and presents as a small (less than 2
cm), red or violet raised nodule with intact overlying skin. The lesion is usually painless and rapidly growing. The populations most commonly affected are Caucasians over the age of 70 and the immunosuppressed [1
Microscopically, the tumor nodule of MCC is comprised of small cells with scant cytoplasm, large nuclei, finely granular chromatin, and numerous mitotic figures. MCC is best differentiated from other cutaneous tumors by immunohistochemical stains. MCC cells stain positively for cytokeratin 20 in a characteristic “dot-like” pattern and stain negatively for S-100 (positive in melanoma), TTF-1 and CK7, (positive in small cell lung carcinoma), and LCA (positive in lymphoma) [4
]. Additionally, MCC cells stain positively for neuroendocrine markers chromogranin and synaptophysin. Electron microscopy shows characteristic intracytoplasmic neurosecretory granules [1
Merkel cell carcinoma almost always arises in the dermis, but there has been one reported case in the literature of MCC arising in subcutaneous fat [2
]. Our case represents the second reported case, to our knowledge, of MCC arising in subcutaneous adipose tissue.
Treatment of MCC includes wide excision, sentinel lymph node biopsy, and local radiation. Neoadjuvant chemotherapy is not recommended. Prognosis is variable and most dependent on stage of disease at presentation. Overall, local recurrence after excision occurs in about 40 percent of cases, while regional lymph node metastasis occurs in about 30 percent of patients [4
Identified risk factors associated with MCC are: age greater than 50 years, European ancestry, ultraviolet (UV) radiation, immunocompromised state, or infection with Merkel Cell Polyomavirus (MCPyV). Frequency of MCC increases dramatically in patients after the age of 50 years suggesting an accumulation of oncogenic events. Regarding UV radiation, MCC tumors are present in sun-exposed areas such as the head and neck. Patients who have undergone prior psoralen and PUVA treatment are at higher incidence. Those of European decent are genetically more susceptible to this radiation and therefore have an increased prevalence of MCC. An association is also noted in patients who are immunocompromised; an 11-fold increase in incidence of MCC has been seen in patients with AIDS and a 5-fold increase in patients who have undergone organ transplantation. Finally, a correlation between MCPyV and MCC has been established [5
Recent research has strengthened the correlation between MCPyV and MCC. MCPyV DNA is found in the majority of MCC tumors but is also commonly found in normal tissue including saliva and tissues of the upper digestive tract. A recent study suggests that the MCPyV integrates into the Large T antigen prior to clonal expansion, and this interruption of the Large T antigen is not seen in nontumor cells [5
]. Another study indicates that lower mortality of MCC is associated with MCPyV viral integration versus those MCCs without MCPyV [6
]. This data suggests the possibility of different mechanisms for inducing oncogenesis. MCPyV DNA is ubiquitous in nature and MCC is relatively rare suggesting that this may be one of many cofactors involved in the oncogenic transformation to MCC.