Mammary spindle cell proliferations (SCPs) encompass a wide range of lesions and can be challenging to accurately diagnose on core needle biopsies (CNBs). Most SCPs are excised for definitive diagnosis. In the era of minimally invasive therapy, some SCP may be followed conservatively. We aim to examine the spectrum of SCP diagnosed on CNB and evaluate if excision of benign/indeterminate SCP is always required. We identified patients with SCP across 3 institutions. The CNB were classified into benign, indeterminate, or malignant. Available excisional specimens were used to classify the lesion as benign or malignant. Clinical variables were reviewed. A total of 197 SCP met the inclusion criteria, including 100 (53%) CNB classified as benign, 52 (26%) indeterminate, and 36 (19%) malignant. Nine patients had excisions without a preceding CNB. Excision was performed in 47% of benign, 87% of indeterminate, and 86% malignant CNB. Of 123 excised SCP, 77 (63%) were benign, while 44 (36%) were malignant. Most benign lesions were not suspicious radiologically (67%), while indeterminate and malignant lesions were more likely to be suspicious (44% and 75%, respectively; P <0.001). Malignant lesions tended to present as larger, rapidly growing, masses. Most mammary SCP are benign (63% of excisions). Appropriate ancillary tests can safely exclude some malignant entities. We encourage narrowing down the differential diagnosis to pertinent entities based on clinical presentation, imaging, histology, immunohistochemistry, and molecular studies, if applicable. Patients with mammary SCP may be spared surgery provided accurate pathologic diagnosis and appropriate correlation with imaging and clinical data.
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