Introduction/Background Endometrial stromal sarcoma (ESS) is a rare mesenchymal tumour that generally arises from the uterus. Ovarian origin, although infrequent, has been reported.

Ovarian ESS is so uncommon its therapeutic management is not yet established. Previous clinical reports constitute a mainstay when deciding a therapeutic approach.

Methodology We aim to describe two patients diagnosed with ovarian ESS in our referral centre during 2020. A 48-year-old woman with abdominal pain and menstrual disorder who was referred with an ovarian mass of 120x89mm. ROMA index was 41.8%. Secondly, a 78-year-old woman who presented abdominal distension and occupation due to an ovarian mass of 32.7x16.4x26.1cm. ROMA index was 76.3%.

Results First case was performed an hysterectomy, bilateral adnexectomy and peritoneal staging and a follow up conduct was decided for an ESS FIGO stage IIA.

Secondly, a radical surgical approach was conducted followed by hormonal treatment and, lately, chemotherapy because of progression.

Conclusion Experience with primary ovary ESS is limited. Clinical presentation is unspecific. Presentation typically occurs in perimenopausal or postmenopausal women.

Ovarian ESSs are divided into low-grade and high-grade. Clinical presentation is similar for both, although prognosis of high-grade ESS is poorer. Fortunately, its incidence is much lower.

For diagnosis, distinctive vasculature, small blood vessels reminiscent of the spiral arterioles of the normal endometrium are present. Also, layers of uniform small oval cells resembling endometrial stromal cells.

For subtype distinction immunohistochemistry is employed; low-grade shows diffuse positivity for CD10, ER and PR; high-grade shows absent or only focal and weak staining for these markers but shows diffuse positivity for cyclin D1.

No specific treatment guidelines are found, so uterine ESS guidelines are followed.

Surgery, involving hysterectomy and bilateral adnexectomy, remains the cornerstone of the treatment. Lymphadenectomy does not seem to improve survival.

Adjuvant therapies include radiotherapy, chemotherapy and hormonal therapy

Disclosures None.