Abstract
Histiocytic sarcoma (HS) is a rare disease with a poor prognosis. The efficacy of immune checkpoint inhibitors for this disease has not been established. A 13-year-old boy with HS refractory to conventional chemotherapy was treated with pembrolizumab, an immune checkpoint inhibitor. After treatment, the primary lesion and the bone metastases showed improvement; however, new metastatic lesions also occurred. This case suggests that the effect of immune checkpoint inhibitors might depend not only on programmed death ligand-1 expression and the ratio of tumor mutational burden, but also on other factors, such as the tumor microenvironment. Evaluation of more cases is required to identify biomarkers that define the efficacy of immune checkpoint inhibitors.