Introduction/Background High-grade uterine sarcoma (HGUS) is a rare disease. Treatment consists of surgery, but even patients with early-stages are at high risk of recurrence. Although the benefit of adjuvant chemotherapy (AC) is uncertain, and prospective data are very limited, AC use is increasing.

Methodology Daydream is a multicentre, observational study, in patients with surgically resected FIGO stages I to IV HGUS with or without AC from 2013 to 2018. The primary endpoint is OS.

We selected potential prognostic variables based on previous evidence and performed inverse probability weighting to establish two homogeneous groups regarding the use of AC. A risk stratification into 3 groups (high, medium and low) was defined based on baseline variables associated with an increased risk of death in the multivariate analysis (p<0.05). We performed a proportional hazard Cox regression to calculate the HR of death associated to the use of AC in these 3 risk groups.

Results 746 patients from 47 centers in 22 countries were registered until December 2023. Initially 135 patients were excluded (mostly due to lack of information or inconsistencies). 156 additional patients were excluded due to missing information in the five variables we found associated with OS in the multivariate analysis (p<0.05): FIGO stage, tumor size, post-surgical histology, mitoses per 10 HPF and LSVI. OS differences of the 3 groups is shown in Table 1 Figure A. Median OS in high-risk group in patients with or without AC was 29.93 and 19.15 months respectively (HR 0.48 (0.26-0.88),p=0.02) (Table 1, Figure B). There were no differences in medium and low risk groups (Table 1, Figure C and D).

Conclusion To our knowledge, this is the first study showing a clear benefit in overall survival for a high-risk group of HGUS patients receiving AC, and suggests the potential development of a risk score to identify patients benefiting from AC.

Disclosures Attached.