PNET-like features of synovial sarcoma of the lung: A pitfall in the cytologic diagnosis of soft-tissue tumors

Fine-needle aspiration (FNA) cytology of soft-tissue tumors is evolving. As more experience is gained, we are becoming aware of potential pitfalls. We describe 2 cases of synovial sarcoma of the lung, primary and metastatic, in patients who had FNA biopsy performed on a lung mass. The cytologic smea rs showed extremely cellular groups of malignant small round cells, interse cted by small blood vessels, with numerous loose single cells, in a backgro und of macrophages and mature lymphocytes. The tumors displayed monomorphic cells forming rosettes and displaying occasional mitoses. A diagnosis of n euroendocrine tumor/primitive neuroepithelial tumor (PNET) was suspected. F urthermore, this suspicion was supported by immunohistochemical stains, whi ch showed positivity for a neuroendocrine marker, Leu 7 (case 1), and for a neural marker, CD 99 (O 13 or HBA 71) (both cases); and negativity for cyt okeratins (case 1). The resection specimen of case 1 had mostly tightly pac ked small round cells, with occasional rosettes, similar to the FNA biopsy, and focal areas composed of spindle cells, organized in a focal fibrosarco ma-like and hemangiopericytoma-like pattern. A balanced translocation betwe en chromosomes X and 18, demonstrated by both karyotyping and fluorescent i n situ hybridization (FISH), enabled us to make a diagnosis of synovial sar coma, which was histologically classified as poorly differentiated. Case 2 was a metastatic biphasic synovial sarcoma of the arm, with a prominent epi thelial component. Synovial sarcoma, when composed mainly of small round ce lls on cytologic smears, is a great mimicker of neuroendocrine/PNET tumors, with light microscopic and immunohistochemical overlap. Awareness of this potential pitfall may aid in preventing a misdiagnosis. Its recognition is of major concern, especially for the poorly differentiated variant, because it is associated with a worse prognosis. (C) 2001 Wiley-Liss, Inc.