Fine needle aspiration cytology of nodular fasciitis of the breast.

We report a case of nodular fasciitis (NF) of the breast, which was cytologically diagnosed as a spindle cell proliferation with undetermined malignant potential. Owing to small size of the lesion (5.9 × 3.7 × 4.1 mm), only fine needle aspiration (FNA) cytology was performed under ultrasound guidance. The FNA smears were cellular, rich in single/clustered spindle cells but mammary ductal epithelial/myoepithelial cells were absent. These cytologic findings suggested spindle cell growth of mesenchymal origin. Pattern-less arrangement of spindle cells, heterogeneous composition of the stromal matrix, lack of nuclear/cellular atypia, occasional mitosis but no aberrant mitotic figures, and lymphocyte infiltration indicated reactive rather than neoplastic nature of the lesion. Nonetheless, lumpectomy was conducted because the possibility of neoplasm was not completely ruled out. The histologic diagnosis of the resected nodule was NF. FNA specimens were reviewed thoroughly in an attempt to define the key cytomorphologic features of NF that are important for the correct diagnosis. Differential diagnoses from the lesions that show similar cytologic pictures are discussed in detail. Although NF arising from the breast is rare, cytopathologists should be aware of its clinical and cytopathologic characteristics. Knowledge of the possibility of NF in the breast and its cytologic findings may help cytopathologists to discern its reactive, not neoplastic, characteristics of the lesion. If the referring surgeon is alerted NF as a possibility along with other differential diagnoses, close observation would become a management option. In-depth discussion of cytologic features and a review of the pertinent literature are also included.