Exceptionally rapid response to pembrolizumab in a SMARCA4‐deficient thoracic sarcoma overexpressing PD‐L1: A case report

SMARCA4‐deficient thoracic sarcoma (SMARCA4‐DTS) is a new clinical entity characterized by SMARCA4 inactivation and has a dismal prognosis because of rapid growth. Effective treatments for SMARCA4‐DTS have not yet been developed. Most recently, anti‐programmed cell death 1 receptor (PD‐1) blockade has been effective for SMARCA4‐deficient lung cancer and malignant rhabdoid tumor‐like tumors. Here, we describe a patient with SMARCA4‐DTC who experienced a marked response to the administration of pembrolizumab. A 70‐year‐old female was referred to our department for treatment of SMARCA4‐DTC. Positron emission tomography‐computed tomography had revealed a left mediastinal tumor, peritoneal dissemination and multiple cutaneous metastases at diagnosis. Immunohistochemical analyses revealed 60% of tumor cells expressed programmed cell death ligand 1 (PD‐L1). The patient was given pembrolizumab as first‐line treatment. Pembrolizumab suppressed tumor growth dramatically, with only one dose leading to a partial response. Our case suggests the immunohistochemical analysis of PD‐L1 expression be undertaken for patients with SMARCA4‐DTS and that pembrolizumab treatment may be a promising strategy for PD‐L1‐positive SMARCA4‐DTS.