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Metastatic myxofibrosarcoma with durable response to temozolomide followed by atezoluzimab: A case report.

Myxofibrosarcoma (MFS) is a well-recognized histotype of soft tissue sarcomas that generally presents with localized disease. Herein, we describe the case of a patient with metastatic MFS who experienced durable response to 6th line therapy with temozolomide. Upon further progression, his tumor was notable for a high tumor mutational burden, and he was subsequently treated with 7th line immunotherapy, atezolizumab, achieving a second durable response. This case highlights the role of immunotherapy following administration of alkylating agents. Review of the literature indicates that recurrent tumors treated with alkylating agents often experience hypermutation as a means of developing resistance, and that checkpoint inhibitors are subsequently effective in these tumors. KEY POINTS: To our knowledge, this is the first patient reported with myxofibrosarcoma with high tumor mutational burden after administration of temozolomide monotherapy. Hypermutation may be a resistance mechanism for patients with soft tissue sarcoma who develop resistance to alkylating agents. Checkpoint inhibition may be effective therapy in patients with STS with high TMB as a consequence of alternate systemic therapy resistance.