A 3-year-old female patient presented with 1 month long abdominal distension. Physical examination was suspicious for an abdominopelvic mass. Laboratory studies were unremarkable, apart from a slightly elevated CA 125 (197 [N<35]). Ultrasound, contrast-enhanced CT and MRI were performed prior to surgery. Four years later, she remains disease-free.
Ultrasound showed a 17 cm well-circumscribed, heterogenous, predominantly hypoechoic mass, with minimal vascularity (Fig. 1).
On cross-sectional imaging the tumour was sharply marginated, separate from the uterus, caused displacement but not invasion of nearby organs. No ovarian tissue was visible separate from the lesion, assuming ovarian origin. On MRI it showed intermediate to low signal intensity on T1 and heterogeneous signal intensity on T2, mostly intermediate, with central areas and a peripheral halo of hypersignal (Fig. 2). Moderate ascites was present. On CT the lesion revealed extensive areas of moderate enhancement (Fig. 3).
Adnexectomy with omentectomy were performed. Microscopic evaluation revealed a tumour composed of long intersecting fascicles of monomorphic spindle cells, without necrosis and with low mitotic-index (1/10 HPFs) (Fig. 4), resembling a conventional leiomyoma. Immunostains were performed and showed positivity for alpha-smooth-muscle-actin (αSMA), muscle-specific-actin (HHF-35) and desmin, confirming the diagnosis (Fig. 5). Residual ovarian tissue was absent.
Primary ovarian leiomyoma accounts for 0,5%-1% of benign ovarian tumours. It usually affects women from 20-65 years of age, with the youngest described case at the age of 13. [1,2,3]
The most accepted theory is that they arise from smooth muscle cells in ovarian blood vessels. Other potential origins include ovarian ligaments, smooth muscle or multipotent cells in ovarian stroma, undifferentiated germ cells or cortical smooth muscle metaplasia. [1,2,4] Another possibility is that some of these tumours may arise in developmentally abnormal ovaries, supported by the absence of normal ovarian tissue histologically. [1,2]
They are usually unilateral and small, and frequently associated with uterine fibroids. Bilaterality and absence of uterine leiomyomas are more common in younger patients. [1,2,4,5]
Most of the patients are asymptomatic; however larger lesions can present with abdominal pain, ascites, Meigs syndrome, hydronephrosis and marginally elevated CA 125. [1,3,4]
Ultrasonography and MRI are the primary modalities in the evaluation of ovarian lesions. On ultrasonography, ovarian leiomyomas show similar appearance to the myometrium, appearing heterogenous with predominantly hypoechoic echotexture and minimal vascularity in the Doppler study. [1,4,6]
On MRI these tumours are solid appearing primarily, of low to intermediate signal intensity on T1W and low signal intensity on T2W sequences, like leiomyomas in other locations. Some lesions exhibit areas of high T2 signal corresponding to oedematous changes from ischaemia or cystic/myxoid degeneration. The most common pattern of enhancement is early homogeneous. Primary ovarian origin can be assumed if no ovarian tissue is identified separately from the lesion. Tracking the course of gonadal vessels also helps. [1,4,6]
Imaging can help narrow the differentials and indicate probable benign nature to avoid overtreatment. The final diagnosis is usually based on histopathologic and immunohistochemical evaluation.
Both grossly and microscopically, ovarian leiomyomas are similar to those of uterine origin. Both are well-defined elastic white masses composed by fascicles of bland spindle cells positive for smooth muscle markers. [4,5,7]
Differentials include broad ligament leiomyoma, that is seen separately from the uterus and the ovary or pedunculated subserous leiomyoma in which case identification of the site of attachment is crucial. Fibromas/fibrothecomas show similar imaging features, apart from contrast enhancement that is usually delayed and weak. Immunohistochemistry is also crucial in these cases as, contrary to leiomyomas, these are negative for smooth-muscle markers. [1,3,7]
Leiomyosarcoma is differentiated from leiomyoma microscopically, considering high mitotic activity, necrosis and nuclear pleomorphism. [2,5]
Surgical treatment is the standard of care to avoid ovarian destruction by the tumour. 
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