Dermoscopy of Merkel Cell Carcinoma and Rare Tumors

Overview / Definition Merkel cell carcinoma (MCC) is a rare and highly aggressive form of skin cancer that arises from Merkel cells, which are neuroendocrine cells located in the epidermis. First described in 1972, it is associated with significant morbidity and mortality, particularly in immunocompromised individuals. The tumor is characterized by rapid growth, local invasion, and a propensity for metastasis. Epidemiology MCC is relatively uncommon, with an estimated incidence of 0.3 to 1.6 cases per 100,000 individuals per year in the United States. Risk factors include: Age: Most commonly diagnosed in individuals over the age of 50. Immunosuppression: Higher incidence in organ transplant recipients and HIV-positive patients. Sun exposure: UV radiation exposure is a significant risk factor. Merkel cell polyomavirus: Approximately 80% of MCC cases are associated with this virus. Pathophysiology / Mechanism MCC arises from the transformation of Merkel cells, which are involved in touch sensation. The pathogenesis of MCC is often linked to the Merkel cell polyomavirus, which can integrate into the host genome and lead to oncogenic transformation. Additional genetic abnormalities, such as mutations in the TP53 and RB1 genes, contribute to tumor development. Clinical Presentation Clinically, MCC presents as a painless, firm nodule on sun-exposed areas of the skin, predominantly the head, neck, and extremities. Characteristics include: Rapid growth over weeks to months. Possible overlying ulceration or erosion. Skin-colored, red, or violet hue. Regional lymphadenopathy at presentation due to metastasis. Diagnosis / Workup The diagnostic process for MCC involves: Dermoscopy: Key features include a shiny, translucent dome, blue-gray structureless areas, and a peripheral white halo. Biopsy: A punch or excisional biopsy confirms the diagnosis, typically showing nests of small, round blue cells. Imaging studies: CT scans or PET scans may be utilized to assess for lymph node