Incipient and In Situ Merkel Cell Carcinoma of the Skin: A Review

Background and Objectives: Merkel cell carcinoma (MCC) is a rare, aggressive, invasive cutaneous neuroendocrine carcinoma. It commonly affects the skin of the extremities and head and neck regions in elderly patients. In situ MMC represents MMC confined to the epidermis. Incipient MCC is a descriptive term that represents in situ MCC with early focal dermal microinvasion. In situ MCC and incipient MCC have a much better prognosis than MCC. In this study, we aimed to address the clinicopathologic features of early lesions of MCCs, including both incipient and in situ forms. Methods: We conducted a PubMed search using the following keywords: (“Merkel cell carcinoma” OR “Merkel carcinoma” OR “Merkel” OR “MCC”) AND (“in situ” OR “incipient” OR “intraepidermal”) AND (“skin” OR “cutaneous”. The inclusion criteria included (i) human studies, and (ii) case reports and series published in the English language with the above-mentioned search keywords. Studies not meeting all inclusion criteria were excluded. Results: Incipient and in situ MCCs are extremely rare events (15 case reports). They usually appear as tiny (2 mm to 6 mm) erythematous papules or nodules over the skin. Immunohistology (for CK20, EMA, and neuroendocrine markers) was required to establish the diagnosis of these lesions. Conclusions: MCCs carry a significantly high mortality rate due to their aggressive nature. However, for in situ MCC and incipient MCC, local surgical excision is usually curative, and the prognosis is excellent. Therefore, dermatologists and dermatopathologists should remain vigilant for these forms of early lesions of MCCs. This will help with early detection and prompt treatment.