Simple SummarySoft tissue sarcomas (STS) are a large group of heterogeneous mesenchymal neoplasms. There is no standard treatment for STS and locally advanced, marginally resectable primary STS remain a treatment challenge for clinicians. Identification of a molecular biomarker of the pathological response (PR) would aid in the diagnosis and treatment of this group of patients. However, the molecular biology and genetic profile of STS are still poorly understood. The study aimed to identify a biomarker for PR prediction after neoadjuvant treatment in STS. We have chosen six markers (HIF-1α, CD163, CD68, CD34, CD105, γH2AFX) for immunohistochemical staining. We found a negative correlation between the expression of HIF-1α and PR, which means poor response to therapy. Furthermore, our results showed that a high expression of γH2AFX before treatment was positively correlated with PR, providing a putative biomarker of the response to treatment.AbstractBackground: Marginally resectable and unresectable soft tissue sarcomas (STS) remain a therapy challenge due to the lack of highly active treatment. The aim of the study was to identify a biomarker to predict the pathological response (PR) to preplanned treatment of these STSs. Methods: In the phase II clinical trial (NCT03651375), locally advanced STS patients received preoperative treatment with a combination of doxorubicin-ifosfamide chemotherapy and 5 × 5 Gy radiotherapy. PR to the treatment was classified using the European Organization for Research and Treatment of Cancer–Soft Tissue and Bone Sarcoma Group recommendations. We have chosen HIF-1α, CD163, CD68, CD34, CD105, and γH2AFX proteins, rendering different biological phenomena, for biomarker study. Results: Nineteen patients were enrolled and in four cases a good PR was reported. The high expression of HIF-1α before surgery showed a negative correlation with PR, which means a poor response to therapy. Furthermore, the samples after surgery had decreased expression of HIF-1α, which confirmed the correlation with PR. However, high expression of γH2AFX positively correlated with PR, which provides better PR. The high number of positive-staining TAMs and the high IMVD did not correlate with PR. Conclusions: HIF1α and γH2AFX could be potential biomarkers for PR prediction after neoadjuvant treatment in STS.