Introduction/Background The smooth muscle neoplasms of the myometrium are the most frequent gynecological neoplasms, most of which correspond to benign uterine leiomyoma.Only 1% are malignant tumors o leiomyosarcomas. There are variants of smooth muscle neoplasm in which the neoplasm presents typical histological aspects of malignant neoplasms but not all, consequently they cannot be definitely classified as benign or malignant tumors and are considered as neoplasms with uncertain clinical behavior.Establishing the malignant potential of this type of tumors constitutes a challenge in clinical practice due to the low incidence of these variants.The aim of these study is to assess the clinicopathological aspects of uterine STUMP and atypical leiomyoma and to correlate with clinical outcomes.

Methodology We analyzed al cases of AL and STUMP diagnosed between December 1999–February 2019

Results A total of 9 cases were found, 4 labeled as STUMP and 5 as atypical leiomyoma. Mean age was 48.3 (range 31–78). Surgery was carried out in 88,8%, 4 cases underwent myomectomy and 4 hysterectomy. One patient refused surgical treatment after hysteroscopic diagnosis of atypical leiomyoma, performing uterine artery embolization. This patient developed a leiomyosarcoma 3 years later. The remaining 88.9% of cases continues asymptomatic.

Mean size of tumor was 7 cm.100% of AL presented moderate cytologic atypia (60% diffuse and 40% focal), and no mitotic activity was found. STUMP cases showed, moderate atypia associated with low mitotic index (less than 5 per 10 hpf) in 50% and moderate (between 5–9 per 10 hpf). Protein p16 was found positive in all STUMP cases, and p53 in 2 of them. The mean follow-up time was 7 years (range 2–120 months).

Conclusion The paucity of data of this type of neoplasms makes it difficult to predict their clinical behavior. Although mostly benign, the ignorance of the probable clinical course, only allows to recommend the narrow follow-up to diagnose relapses.

Disclosure Nothing to disclose.