Long-Term Response Without Immune-Related Adverse Events to Atezolizumab Treatment in TMB-High Thymoma: A Case Report from the KOSMOS-II Study

<b>Background:</b> Thymic epithelial tumors (TETs), including thymic carcinomas and thymomas, are rare malignancies originating in the mediastinum. Therapeutic options remain limited for patients experiencing disease progression following platinum-based chemotherapy. High tumor mutational burden (TMB) is uncommon in thymic malignancies but may predict response to immunotherapy. We report a patient with TMB-high TET who participated in the KOSMOS-II study in South Korea and achieved a durable response to atezolizumab without developing immune-related adverse events (irAEs). <b>Case presentation:</b> A 73-year-old woman who had been treated for thymoma 20 years ago presented with a left neck mass. A biopsy of the neck mass confirmed recurrent thymoma, type B3, and her disease progressed despite platinum-based chemotherapy and subsequent pemetrexed treatment. TMB-high thymoma is very rare, but based on the next-generation sequencing (NGS) results, she was diagnosed with TMB-high (20.3 mutations/Mb) thymoma. As TMB-based immunotherapy is not approved in Korea, she was enrolled in the KOSMOS-II study and initiated on atezolizumab following molecular tumor board review. She achieved stable disease after three cycles and has remained progression-free for 14 months, completing 20 cycles without significant irAEs. Notably, her underlying myasthenia gravis did not worsen during treatment. <b>Conclusions:</b> This case demonstrates a favorable outcome with biomarker-directed ICI treatment in recurrent thymoma with limited treatment options, highlighting the importance of appropriate molecular markers to predict drug response. Although TMB-based immunotherapy is FDA-approved in the U.S., it remains unavailable in Korea, underscoring the need to explore flexible access pathways, including the potential use of immunotherapy beyond current indications, to improve treatment options for patients with life-threatening conditions.