A rare case of metastatic Merkel cell carcinoma presenting without a primary cutaneous lesion: A case report.

Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine malignancy. While typically associated with a primary skin lesion, cases of occult-primary MCC are uncommon and pose significant diagnostic challenges, often leading to delays in appropriate management. A 44-year-old male presented with a painless, enlarging mass in the right groin of 1 month's duration. He denied any history of preceding or concomitant skin nodules, lesions, or rashes. Pathological examination of the right inguinal mass, via initial biopsy and subsequent wide excision, demonstrated a metastatic neuroendocrine carcinoma. Comprehensive immunohistochemical profiling (CK20+, CKpan+, Syn+, cluster of differentiation 56+, thyroid transcription factor 1-, with positive Merkel cell polyomavirus staining) confirmed the diagnosis of metastatic Merkel cell carcinoma. Staging positron emission tomography/CT and subsequent imaging identified widespread metastatic disease to the pancreas, abdomen, and mediastinum without evidence of a primary cutaneous lesion. The patient underwent surgical excision of the symptomatic right inguinal nodal mass. Upon confirmation of widespread systemic progression, he received first-line systemic therapy with the etoposide + platinum regimen (etoposide + cisplatin) combined with the PD-1 inhibitor sintilimab, followed by second-line therapy with the folinic acid + fluorouracil + oxaliplatin regimen (oxaliplatin + fluorouracil) combined with sintilimab. The disease exhibited a highly aggressive course. Transient symptomatic relief was observed after the first cycle of first-line therapy. However, the disease progressed rapidly through both first- and second-line chemo-immunotherapy regimens. The patient died approximately 14 months after initial presentation. This case underscores that MCC can present as metastatic carcinoma without an identifiable primary cutaneous lesion. This observation necessitates its inclusion in the differential diagnosis of metastatic neuroendocrine tumors, even in the absence of skin findings. Comprehensive immunohistochemistry, including cytokeratin 20 and Merkel cell polyomavirus status, is crucial for diagnosis. The rapid progression and poor response to combined chemo-immunotherapy in this young patient highlight the heterogeneous and often refractory nature of advanced MCC.