Reactive spindle cell nodules of the breast after core biopsy or fine-needle aspiration

Reactive spindle cell nodules (RSCNs) arising postoperatively or after fine -needle aspiration (FNA) have been reported previously in the genitourinary tract and thyroid. We describe 18 cases of similar lesions in breast, asso ciated with a history of core needle biopsy or FNA. The majority of the RSC Ns (15 cases) were associated with papillary lesions or complex sclerosing lesions. The RSCNs were nonencapsulated and relatively nodular, measuring 1 .5 to 9 mm. They were composed of spindle cells with mild to moderate nucle ar pleomorphism and a low mitotic count. A network of small blood vessels, macrophages, and lymphocytes was present in all cases. Immunohistochemicall y, the spindle cells expressed smooth and specific muscle actins, supportin g a myofibroblastic origin. The association of RSCNs with needle trauma to fibrosclerotic lesions, such as complex sclerosing lesions and papillary le sions that regularly have myofibroblasts, suggests an exuberant reparative cause. Recognition of this reactive process will avoid overdiagnosis of mam mary spindle cell malignant neoplasm.