Adamantinoma-Like Ewing Sarcoma Mimicking Merkel Cell Carcinoma in the Parotid Gland: A Diagnostic Pitfall.

Adamantinoma-like Ewing sarcoma (ALES) is a newly described rare entity, which shows EWSR1::FLI1 rearrangement characteristic of Ewing sarcoma. This can be diagnostically challenging as it manifests histologically with epithelial differentiation and has diffuse keratin expression as well as p40 and p60 positivity. We hereby report a case of ALES in a 33-year-old woman with a past medical history of breast carcinoma who presented with a right-sided parotid mass. CT scan of the neck showed a heterogenous mass within the superficial lobe, measuring 17 mm in diameter for which the patient underwent superficial parotidectomy. Histopathology of the mass revealed a malignant neoplasm formed of solid nests, cords and sheets of cells with minimal cytoplasm and monomorphic nuclei with granular chromatin and indistinct nucleoli. Brisk mitotic activity and tumor necrosis were also present. The tumor showed strong and diffuse reactivity for pankeratin (clone AE1/AE3) and keratin 20, both in a dot-like pattern, raising the suspicion of metastatic Merkel cell carcinoma; however, molecular studies showed EWSR1::FLI1 rearrangement, supporting the diagnosis of ALES. In summary, it is prudent to have knowledge about this entity to avoid its misdiagnosis as other malignancies of the head and neck region which exhibit a different clinical course, prognosis and hence treatment modalities.