It is a rare type of skin cancer, with a 2013 incidence of only 0.7 per 100,000 persons in the U.S. As of 2005, roughly 2,500 new cases of MCC are diagnosed each year in the United States, as compared to around 60,000 new cases of malignant melanoma and over 1 million new cases of nonmelanoma skin cancer. Similar to melanoma, the incidence of MCC in the US is increasing rapidly. Worldwide, MCC is most commonly found in regions with increased sun exposure. Australia is the country with the highest incidence of MCC but has a lower incidence of MCV-positive MCC than observed in other countries.Since 2006, it has been known that other primary cancers increase the risk of MCC significantly, especially in those with the prior multiple myeloma, chronic lymphocytic leukemia, and malignant melanoma. Immunosuppression including HIV infection or immunosuppressant therapy following organ transplant or for autoimmune disease can also increase the odds of developing MCC. History
Friedrich Sigmund Merkel (1845–1919) was a German anatomist and histopathologist who first described the Tastzellen (touch cells) in the skin in 1875. In 1878 the term Merkel cell was coined by the anatomist Robert Bonnet (1851–1921). Merkel-cell carcinoma was first described in 1972 by Cyril Toker. He reported five cases of trabecular carcinoma of the skin.
Immunosuppression
The incidence of MCC is increased in conditions with defective immune functions such as malignancy, HIV infection, and organ transplant patients, etc. Conversely, patients with brisk immune response have been shown to have improved prognoses. This is suspected to be due to the inability of the body to defend itself from infection by or reactivation of MVC. The body of data indicating the importance of immune function in MCC pathogenesis has been exploited for the development of immunotherapies discussed below. Diagnosis
Diagnosis of MCC begins with a clinical examination of the skin and lymph nodes. Following clinical exam, definitive diagnosis of Merkel cell carcinoma (MCC) requires examination of biopsy tissue to identify its histopathologic features. An ideal biopsy specimen is either a punch biopsy or a full-thickness incisional biopsy of the skin including full-thickness dermis and subcutaneous fat. On light microscopy, MCC shows basaloid tumor nests with neuroendocrine features ("salt and pepper" chromatin, scarce cytoplasm, and brisk mitotic activity). In addition to standard examination under light microscopy, immunohistochemistry (IHC) is also generally required to differentiate MCC from other morphologically similar tumors such as small cell lung cancer, the small cell variant of melanoma, various cutaneous leukemic/lymphoid neoplasms, and Ewings sarcoma. Neuroendocrine molecular markers such as synaptophysin or chromogranin A are characteristic of MCC and other neuroendocrine tumors, while other markers such as PAX5 or cytokeratin 20 can distinguish MCC from these tumors. Longitudinal imaging may also help in ruling out a diagnosis of metastatic small cell lung cancer.