Primary Breast Leiomyosarcoma Presenting as Recurrent Spindle Cell Lesions: Diagnostic Pitfalls, Immunohistochemical Insights, and Multidisciplinary Management of a Rare Tumor

Abstract Background Primary leiomyosarcoma of the breast is extremely rare, accounting for less than 1% of breast tumors. Diagnosis is often difficult because it can mimic other spindle cell lesions, and standard treatment guidelines are lacking. Reporting such cases helps improve the understanding and management of these uncommon tumors. Case Presentation: A 44-year-old woman presented with a rapidly growing, firm lump in her left breast. She had previous surgeries elsewhere with diagnoses of leiomyoma and desmoid-type fibromatosis. Imaging showed a large lobulated mass (14.8 cm) without lymph node involvement or distant metastasis. Core needle biopsy revealed a spindle cell tumor with moderate atypia and mitotic activity, but it was not definitive. Excisional histopathology demonstrated intersecting fascicles of pleomorphic spindle cells, necrosis, and dermal invasion. Immunohistochemistry confirmed smooth muscle origin (SMA and desmin positive), excluding epithelial, neural, and vascular tumors. The tumor was graded as Grade 2. The patient underwent a modified radical mastectomy followed by adjuvant radiotherapy and planned chemotherapy. Conclusion Primary breast leiomyosarcoma is rare and can present with diagnostic challenges. Core biopsy and cytology alone may be misleading. Immunohistochemistry was essential to reach the correct diagnosis and to differentiate from other spindle cell tumors. Management requires a multidisciplinary approach, including surgery with clear margins and tailored adjuvant therapy. This case adds to the limited literature and offers lessons on diagnosis, management, and follow-up of rare spindle cell breast tumors.