Primary Smooth Muscle Tumor of Breast: An Unusual Case Presentation

Abstract

Abstract Introduction Primary smooth muscle tumors (SMT) of the breast are rare with leiomyosarcomas representing less than 0.1% of all malignant breast tumors. Case Presentation A 58-year-old female with no significant past medical history noted on screening mammography to have a circumscribed 6-mm nodule in the right breast, upper outer quadrant. Core needle biopsy showed a spindle cell neoplasm with smooth muscle differentiation. The excisional biopsy showed a 6-mm lesion composed of atypical hyperchromatic spindle cells in fascicles, marked nuclear pleomorphism, and 5 mitoses per 10 high-power fields. By immunohistochemistry, the spindle cells were positive for smooth muscle actin, desmin, and negative for S-100 and cytokeratin AE1/AE3. Positron emission tomography/computed tomography of head/neck, chest, abdomen, and pelvis did not identify other neoplasms. Despite the lesion size, findings were supportive of a leiomyosarcoma. Discussion Breast SMTs have nonspecific clinical or imaging features. Histologically, these present as spindle cell tumors with smooth muscle differentiation. Initial workup starts with distinction between benign and malignant neoplasms. The malignant SMTs are usually large tumors with cytologic atypia and mitotic activity used as diagnostic criteria. In addition, the distinction between primary and metastasis is important and frequently relies on clinical history and exclusion of other primary origins by radiographic survey. In the current case, although the tumor size is unusually small, the histological features and absence of other primary malignancies support the diagnosis of a leiomyosarcoma. Conclusion Primary leiomyosarcoma of breast is extremely uncommon with less than 70 cases reported in the literature. Although they are usually large tumors, this diagnosis should be included in the differential diagnosis when smooth muscle differentiation, significant atypia, and mitoses are encountered in a spindle cell tumor of the breast.