Study Evaluating Pembrolizumab +/- Olaparib in TLS Positive Selected Resectable STS Followed by Adjuvant Pembrolizumab

Soft tissue sarcomas represent a subtype of cancer that is both rare and very heterogeneous. When they are organized, their current treatment is essentially based on tumor resection surgery, +/- associated with treatment by chemotherapy and/or radiotherapy. The aim of this treatment is to reduce the risk of local recurrence (appearance of a tumor in the same region where it was first detected) and/or distant (appearance of a tumor in other regions, organs where it was first detected). Currently, no immunotherapy treatment has been approved for the treatment of patients with sarcoma. This research is based on the hypothesis that soft tissue sarcomas in which "tertiary lymphoid structures" or "TLS" are found, recognizable by a cluster of specific immune cells within the tumor, would be likely to respond better to the immunotherapy. Furthermore, the combination of immunotherapy and certain drugs targeting DNA repair has demonstrated some effectiveness in other types of cancers. Trial population :Adult patients with suspected / diagnosed soft tissue sarcoma of the extremities or trunk (Cohort 1: Undifferentiated pleomorphic sarcoma / Cohort 2: Dedifferentiated liposarcoma) The research will therefore focus on two experimental drugs : * Pembrolizumab (immunotherapy) and * Olaparib (DNA repair inhibitor). This research will make it possible to evaluate the effectiveness and safety of use of the two drugs.