Metastatic small cell osteosarcoma to the liver: a diagnostic pitfall for fine-needle aspiration cytology.

We report the fine-needle aspiration cytology of a case of metastatic small cell osteosarcoma to the liver with rosette formation, originating from distal femur, in a 36-year-old female. The aspirate of the liver metastases revealed a relatively monomorphic population of mitotically active small blue round cells arranged in clusters with prominent rosette formation simulating a neuroendocrine carcinoma or other rosette-forming small round cell tumors such as Ewing sarcoma. No extracellular mineralized matrix material was present. Comparison of the liver aspirate with the biopsy from the distal femoral lesion was crucial in reaching the correct diagnosis of metastatic small cell osteosarcoma with rosette formation. This is a potential diagnostic pitfall, particularly if interpreted without the knowledge of a suspected primary bone tumor.