Step by step approach to rare breast lesions containing spindle cells.

Differential diagnosis of spindle cell lesions of breast is challenging for certain reasons. The most important reason is the presence of cytological atypia and mitosis in all three conditions: reactive, benign, and malignant. Patients diagnosed with benign and malignant tumor/tumor-like lesions that had spindle cell components following the histopathological examination were included in the study. The patients' medical records were accessed to obtain the clinical history, follow-up notes, and radiological findings. Following histopathological, immunohistochemical, and clinical evaluations, the patients were diagnosed as follows: pseudoangiomatous stromal hyperplasia (PASH), bilateral desmoid-type fibromatosis (FM), adenomyoepithelioma (AME), myofibroblastoma (MFB), malignant phyllodes tumor (MF), high-grade AS, post-chemotherapy osteosarcoma (OS) + Paget's disease, and metaplastic carcinoma (MC). An algorithmic approach should be used in the diagnosis; cellular structure, presence and grade of atypia, growth pattern, mitotic activity, immunohistochemical staining, and clinical and radiological features should be evaluated together. Detection of some molecular changes can be useful in differential diagnosis.