Merkel Cell Carcinoma of the Thigh Presenting as a Hemorrhagic Mass: A Rare Case Report and Literature Review

BACKGROUND: Merkel cell carcinoma (MCC) is a rare, aggressive primary cutaneous neuroendocrine carcinoma with a marked propensity for early regional lymph node metastasis. Although MCC most often arises on sun-exposed head and neck skin in older adults, tumors of the lower extremity are uncommon and may be mistaken for benign hemorrhagic lesions. CASE PRESENTATION: A 54-year-old woman developed a rapidly enlarging, hemorrhagic mass in the left suprapatellar thigh. Magnetic resonance imaging demonstrated an extracompartmental subcutaneous soft-tissue mass without quadriceps muscle invasion. Wide local excision including the quadriceps fascia was performed. Histopathologic examination showed a dermal/subcutaneous small blue round cell neoplasm with brisk mitotic activity. Immunohistochemistry demonstrated diffuse cytoplasmic synaptophysin positivity, paranuclear dot-like CK20 reactivity, chromogranin A positivity, and negative MCPyV staining; TTF-1, S100, melan-A, HMB-45, and hematolymphoid markers were negative. Staging positron emission tomography/computed tomography identified ipsilateral inguinal nodal involvement. Therapeutic inguinal lymph node dissection revealed metastatic MCC in one of four lymph nodes without extranodal extension. The final stage was pT3 pN1b cM0 (AJCC 8th edition), corresponding to stage IIIB disease. Adjuvant radiotherapy (57 Gy in 20 fractions) was delivered to the primary bed and ipsilateral inguinal basin. The patient remains disease-free at 5-year follow-up. CONCLUSIONS: Lower-extremity MCC can mimic hemorrhagic or post-traumatic lesions, contributing to diagnostic delay. Persistent or rapidly enlarging "hematoma-like" lesions warrant early biopsy, and timely pathologic nodal staging is essential. Multimodal management can achieve durable control even in node-positive disease.