Immunoradiotherapy holds promise for improving outcomes in patients with advanced solid tumors, including in soft-tissue sarcoma (STS). However, the ideal combination of treatment modalities remains to be determined, and reliable biomarkers to predict which patients will benefit are lacking. Here, we report the results of the STS cohort of the SABR-PDL1 phase II trial that evaluated the anti-PDL1 atezolizumab combined with stereotactic body radiation therapy (SBRT) delivered concurrently with the 2nd cycle to at least one tumor site. Eligible patients received atezolizumab until progression or unmanageable toxicity, with SBRT at 45 Gy in 3 fractions). The primary endpoint was one-year progression-free survival (PFS) rate with success defined as 13 patients achieving 1-year PFS. Sixty-one heavily pretreated patients with STS (median 5 prior lines; 52% men; median age 54 years; 28% leiomyosarcoma) were enrolled across two centers (France, Spain). SBRT was delivered to 55 patients (90%), with the lung being the most commonly irradiated site (50%). After a median follow-up of 45 months, the one-year PFS rate was 8.3% [95% CI: 3.6-18.1]. Median PFS and overall survival were 2.5 and 8.6 months, respectively. Best responses included partial responses (5%) and stable disease (60%). Immune profiling revealed increased immunosuppressive tumor-associated macrophages (e.g., IL4I1, HES1) and monocyte-recruiting chemokines in non-responders. Higher monocyte/lymphocyte ratios (MonoLR) in tumor and blood correlated with progression. PD-L1 status, lymphoid infiltration, and tertiary-lymphoid structures were not predictive. Although the primary endpoint was not met, this study highlights MonoLR imbalance as a potential biomarker to identify STS patients likely to benefit from immunoradiotherapy. EudraCT No. 2015-005464-42; Clinicaltrial.gov number: NCT02992912.

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