Metachronous Testicular Metastases from Merkel Cell Carcinoma (MCC): A Case Report and Literature Review.

BACKGROUND Merkel cell carcinoma (MCC) is a rare neuroendocrine neoplasm. The immunotherapy era has dramatically changed MCC prognosis, but unresponsive and progressive diseases after anti- programmed death-ligand 1 (PDL1) treatment still represent a challenge. MCC can metastasize in virtually every anatomical site, also during immunotherapy, and the reasons for primary resistance are debated. Testes are a rare metastatic site, accounting only 0.04% of autoptic detections in patients with more common malignancies. We report a case of a patient with metachronous and bilateral testes metastases after a previous radically treated MCC. CASE REPORT A 57-year-old man underwent radical surgery for left knee MCC. The oncologic follow-up was negative until 4 years later when the patient developed a right testis lesion. The patient underwent right orchiectomy for the suspicion of testicular primary malignancy, but the pathologist report revealed MCC metastasis. Subsequent radiologic assessment detected new bone metastases. The patient was treated with immunotherapy, experiencing a complete response. After 20 months of treatment, a further assessment revealed a new single left testis metastasis. A left orchiectomy was performed and immunotherapy was continued, maintaining a complete response. CONCLUSIONS There are few reports that MCC can be associated with uncommon metastases in testicular tissue. The present case suggests the testis represents another "sanctuary" site for the metastasizing process and host immune response. Considering the dramatic impact of immunotherapy in MCC prognosis, the study of these rare cases may aid the understanding of intrinsic resistance mechanisms to anti-PDL1, which affects a percentage of MCC patients.