## Histological Classification of Uterine Fibroids **Key Point:** Uterine fibroids are benign smooth muscle tumors classified histologically as **leiomyomas**. The term "leiomyo-" refers to smooth muscle (leio = smooth, myo = muscle), and "-oma" denotes a benign neoplasm. **High-Yield:** Leiomyomas are composed of smooth muscle cells arranged in interlacing bundles with varying amounts of fibrous tissue. They are encapsulated and well-demarcated from surrounding myometrium, though not truly encapsulated. ### Smooth Muscle Tumors of the Uterus: Benign vs. Malignant | Tumor | Classification | Prognosis | Mitotic Rate | Cellular Atypia | | --- | --- | --- | --- | --- | | Leiomyoma | Benign | Excellent | Low (<5/10 hpf) | Absent | | Leiomyosarcoma | Malignant | Poor | High (>10/10 hpf) | Present | | Atypical leiomyoma | Intermediate | Variable | Low to moderate | Moderate | **Mnemonic:** **LEIO = Smooth muscle** — Remember: Leiomyoma = benign smooth muscle tumor; Leiomyosarcoma = malignant smooth muscle tumor. **Clinical Pearl:** Malignant transformation of a benign leiomyoma to leiomyosarcoma is extremely rare (<0.1%), so the presence of a fibroid does not mandate hysterectomy solely to prevent malignancy. [cite:Robbins & Cotran Pathologic Basis of Disease 10e Ch 22]
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