Merkel Cell Carcinoma Masquerading as Dermatitis

Merkel cell carcinoma (MCC) is a rare, highly aggressive neuroendocrine skin carcinoma that typically presents as a rapidly enlarging, firm, painless nodule on sun-exposed areas in elderly patients.This malignancy is characterized by high rates of local recurrence, regional spread, and distant metastasis.We report a case of a woman in her 80s who presented with an erythematous patch on the left anterior thigh, a non-sun-exposed site, initially diagnosed as inflammatory dermatitis and treated with topical corticosteroids.After two months of failed therapy and interval growth, a punch biopsy was performed.Histopathology revealed a dermal infiltrate of small to medium-sized cells with "salt and pepper" chromatin and numerous mitotic figures.Immunohistochemistry confirmed a diagnosis of MCC.Staging with positron emission tomography/computed tomography (PET/CT) revealed an incidental pancreatic lesion with benign cytology and subsequent imaging stability but no evidence of metastatic disease.The staging workup was complicated by acute pancreatitis for which she was hospitalized, and during which she developed newonset atrial fibrillation and congestive heart failure.After discharge, the patient underwent wide local excision; however, sentinel lymph node biopsy was deferred due to the patient's acute comorbidities.In conjunction with the patient, the decision was made for close surveillance rather than adjuvant radiation or systemic therapy.At the most recent follow-up, 16 months after excision, there was no evidence of recurrence.This case highlights an atypical presentation of MCC as a dermatitic-appearing patch in a nonsun-exposed area and underscores the importance of biopsy in atypical lesions that do not respond to treatment, as well as the need to individualize treatment in elderly patients with significant competing comorbidities.