A Rare Case of Low-Grade Endometrial Stromal Sarcoma Invading an Old Leiomyoma.

We present the first case of low-grade endometrial stromal sarcoma (ESS) invading a leiomyoma, which was difficult to diagnose preoperatively. A 49-year-old multiparous woman was referred to our institution due to the enlargement of an old leiomyoma after menopause. Transvaginal ultrasonography revealed a 40-mm lesion in the myometrium of the uterine body with calcification and edema. Color Doppler imaging showed blood flow along the margins of the mass. Pelvic contrast-enhanced magnetic resonance imaging (MRI) revealed a 39-mm mass in the uterine body, predominantly having a low intensity on T2-weighted images, suggesting leiomyoma containing a cystic lesion with a small solid component. This solid component appeared to have a high intensity on T2-weighted images, high intensity on diffusion-weighted images, low value on apparent diffusion coefficient (ADC) map images, and contrast effect. 18F-fluoro-deoxyglucose positron emission tomography (FDG-PET)-computed tomography (CT) showed a non-significant FDG uptake in the cyst's solid component. Based on a preoperative diagnosis of cystic or myxoid degeneration of leiomyoma, laparoscopic total hysterectomy and bilateral salpingo-oophorectomy were performed. The uterus was retrieved vaginally without morcellation. Macroscopic examination of the hysterectomy specimens revealed a 55-mm white solid tumor with scattered yellow nodules. Histopathological analysis identified spindle-shaped smooth muscle cells with non-atypical nuclei, confirming a leiomyoma. However, the tumor's nodules contained slightly atypical cells with round nuclei, resembling endometrial stromal cells interspersed with small blood vessels. Immunohistochemical staining showed the nodules were negative for alpha-smooth muscle actin and positive for CD-10, estrogen receptor, and progesterone receptor. These nodules invaded the leiomyoma along vascular vessels. The final diagnosis was leiomyoma coexisting with low-grade ESS, classified as International Federation of Gynecology and Obstetrics (FIGO) stage IB. The patient received no further treatment and remains disease-free after 45 months.