Merkel Cell Carcinoma Presenting as a Malignant Pleural Effusion Post-COVID-19 Hospitalization: a Case Report and Literature Review

Introduction/Objective Merkel cell carcinoma (MCC) is a rare, highly aggressive neuroendocrine carcinoma of the skin, associated with immunosuppression, UV light exposure, and the Merkel cell polyomavirus (MCPyV). Metastatic MCC diagnosed in body fluid cytology is extremely rare. Here, we report on a case of a 65-year-old male presenting with a right pleural effusion and a remote history of MCC, unknown to us during the cytologic evaluation of the effusion. To the best of our knowledge, this is the sixth case of metastatic MCC diagnosed in body fluid cytology reported in English literature, and the first one reported in a patient previously admitted for COVID-19 pneumonia. Methods/Case Report A 65-year-old male presented with dyspnea and acute hypoxic respiratory failure. Past medical history was significant for squamous cell carcinoma of the right forearm excised four months prior and ulcerative colitis. The patient had two recent hospitalizations for COVID-19 pneumonia. Chest x-ray on admission demonstrated an enlarged, loculated right pleural effusion. Emergent thoracentesis was performed and 1500 cc of bloody pleural fluid was sent for cytology. The patient’s respiratory status improved. A ThinPrep and a cell block were prepared. Both displayed clusters of small round blue cells with hyperchromatic nuclei, scant cytoplasm, and fine chromatin in a background of rare mesothelial cells, macrophages, and numerous lymphocytes. Immunohistochemical (IHC) studies showed CK20, AE1/AE3, and CAM 5.2 in a perinuclear dot-like staining pattern. CD56, chromogranin, and synaptophysin were also positive. Ki-67 proliferative index was about 40%. TTF-1 was negative, while CD45 highlighted background lymphocytes. The primary care physician was contacted and the patient’s past medical history was significant for MCC. Subsequent MCPyV and SATB2 demonstrated nuclear positivity in the tumor cells. Diagnosis of MCC was done. Results (if a Case Study enter NA) NA Conclusion MCC is a highly aggressive malignancy rarely reported in body fluids. MCC should be included in the differential diagnosis of malignant pleural effusions, especially in cases of the small blue round cells. Using IHC, like CK20 and neuroendocrine markers including the newer SATB2, can lead to an accurate diagnosis. Additional reporting of such cases may increase awareness, especially where prior history is not readily available, such as in this present instance.