PubMedDate not supplied
BACKGROUND: Cystic lesions of deep soft tissues are rare and usually are composed of a mesenchymal tumor undergoing necrosis or regressive changes. Benign cysts arising de novo are even more rare and may show features of different morphology, potentially leading to an inexact diagnosis. CASE: A 68-year-old male presented with a deep, firm mass in the upper part of the back from which a dense liquid was aspirated, with an inconclusive diagnosis. A second fine needle aspiration was performed, and the lesion was surgically biopsied. Immunohistochemical studies were also inconclusive, while ultrastructural studies suggested an origin in the synovia of the scapular bursa. CONCLUSION: The cytologic picture was suspicious for malignancy due to the presence of numerous pseudopapillary structures, reminding us of a secondary deposit from a renal or thyroid primary or mesenchymal neoplasm. However, the bland nuclear aspect suggested the benignity of the lesion, and the electron microscopic features confirmed the synovial origin.