CK20-negative Merkel Cell Carcinoma with Concomitant Squamous Cell Carcinoma In-Situ and Metastasis to the Parotid Gland

Abstract Introduction/Objective This report presents a patient with an aggressive CK20-negative neuroendocrine tumor of the dermis with metastasis to the parotid gland and a concomitant squamous cell carcinoma in-situ (SCCIS) and highlights diagnostic challenges of CK20-negative tumors. Methods/Case Report A centennial female with a history of non-melanoma skin cancer and leukemia presented to the dermatology clinic with a one-year history of an enlarging skin lesion on her right cheek and a subcutaneous mass on the right central lateral neck. On examination, the lesion appeared as a rough and raised red papule with a central crust. On CT, a large heterogeneously enhancing right parotid mass with surrounding satellite nodules, adjacent necrotic adenopathy, and an ulcerating subcutaneous mass in the adjacent right face. An incidental 3 mm right upper lobe nodule was identified. The pathology report noted two distinct malignant neoplasms within the specimen. Microscopy revealed ulceration with acanthosis and full thickness keratinocyte atypia, suggesting Bowen’s disease. On microscopic examination of the separate dermal tumor, there were sheets of uniform small cells with a high nuclear to cytoplasmic ratio and scant cytoplasm. IHC staining was positive for the neuroendocrine markers AE1/AE3, CAM5.2, synaptophysin, and CD56. CK20, CK7, and TTF-1 were negative. As some MCC may be negative for CK20, CK20 was performed twice. Differential diagnoses included SCCIS with CK20-negative MCC versus a metastatic neuroendocrine carcinoma of parotid gland to the skin. The patient transitioned to hospice care soon after the diagnosis. Results (if a Case Study enter NA) NA. Conclusion Our patient presents with an interesting case of a CK20-negative MCC with metastasis to the parotid gland. Since CK20 was stained negative twice, it complicated the diagnosis of MCC vs. small cell carcinoma. We suspected a more likely diagnosis of MCC metastasis to the parotid gland due to multiple reported cases (Day et al. 2016). Our patient also had an identifiable Bowen’s disease borderline to the neuroendocrine carcinoma. It is well known of the coexistence of MCC and squamous cell carcinoma in a cutaneous lesion (Suaiti et al. 2019). Thus, a metastatic MCC to the parotid gland is more likely in this case. As MCC is an aggressive cancer that metastasizes quickly, accurate diagnosis is crucial. With recent literature reporting the association of CK20-negativity with VN-MCC, and the presence of UV signature mutations in CK20- negative MCC, genomic screening would be a useful diagnostic tool.