INTRODUCTION

Spindle cell lesions are numerically the largest category of mesenchymal proliferations occurring in adults and are also the most diagnostically challenging. These lesions vary from benign reactive reparative processes to high grade malignancies including synovial sarcoma, malignant peripheral nerve sheath tumors and some forms of angiosarcoma. These lesions are associated with a significant incidence of both false positive and false negative diagnoses by both cytologic analysis and histologic interpretation of small core biopsies. Unfortunately, ancillary techniques, especially immunohistochemistry are often of little diagnostic aid in separating benign or low grade spindle cell proliferations from fully malignant spindle cell sarcomas. While some spindle cell neoplasms have characteristic age and sites of presentation, these features are of only modest help diagnostically and, in a significant percentage of cases, the cytologic diagnosis will be that of a spindle cell neoplasm followed by a recommendation for surgical biopsy. Cytomorphologic features such as high cellularity, loss of cell cohesion, and nuclear atypia support but do not unequivocally establish the diagnosis of malignancy. Some predominately spindle cell sarcomas can be definitively diagnosed by fine needle aspiration, especially when cell block material is utilized for ancillary studies. Synovial sarcoma, some benign and malignant nerve sheath tumors, and angiosarcomas may have sufficiently characteristic cytomorphology and immunohistochemical or molecular diagnostic profiles to allow definitive diagnosis based on aspirated material. Other lesions including fibrosarcomas, fibromatoses, and smooth muscle neoplasms are associated with insufficiently characteristic morphology to allow definitive diagnosis in the majority of cases.