We described one case of leiomyoma and one case of atypical leiomyoma in breast parenchyma. Smooth muscle tumors of the breast are relatively rare, and the diagnostic criteria are not yet elucidated. The biological behavior and recurrence rate of morphologically borderline leiomyoma or atypical leiomyoma is not particularly clear. In our reported case of atypical leiomyoma, the patient experienced no recurrence after lumpectomy while her medical history of spindle cell tumor in bilateral breast highly suggest that the atypical leiomyoma recurred from previous lesions.
Smooth muscle tumor of uterine is common and the histologically borderline leiomyoma has been classified using an entity of "smooth muscle tumor of uncertain malignant potential" (STUMP) by World Health Organization (WHO)[7–11]. STUMPs of uterine has well-defined diagnostic criteria including focal/multifocal or diffuse nuclear atypia and 6–9 mitoses/10HPF; more than 15 mitoses/10HPF without cytological atypia or necrosis; tumor with diffuse nuclear atypia and uncertain mitoses which is often due to brisk karyorrhexis. The recurrence rate of STUMPs in uterine was reported to be lower than 2% [4].
Leiomyosarcoma of the breast share similar morphology and diagnostic criteria with those in other parts of the body, including irregular infiltrative borders, prominent nuclear atypia, pleomorphism and mitotic activity[12–14]. In the condition of multiple or recurrent breast leiomyoma, the condition of hereditary leiomyomatosis and renal cell cancer should be considered which was caused by germline mutation in fumarate hydratase (FH) gene [15], although in our reported case patients reported no such family history. Breast leiomyoma belongs to breast spindle cell tumors and its differential diagnosis include fibroblastic and myofibroblastic tumor, spindle cell lipoma, nodular fasciitis and nerve sheath tumor[16]. Usually, positive stain of SMA and desmin by IHC could confirm a smooth muscle origin. The cell origin of breast parenchymal leiomyoma was not clear yet and several hypotheses exist including through smooth muscle metaplasia of myoepithelial cell, through smooth muscle cells surrounding blood vessel and through differentiation from multipotent mesenchymal cells[17].