BACKGROUND: Thymic carcinoma (TC) is a rare subtype of thymic epithelial malignancy. Surgical resection is a mainstay in the treatment of TC, while radiotherapy and chemotherapy are modalities used in adjuvant or palliative setting. Immune checkpoint inhibitors (ICI) including anti-PD-1 (programmed cell death 1) antibodies represent an emerging treatment modality in TC; however, their administration could be associated with life-threatening toxicity. CASE: We present a case of a 59-year-old female with grade III TC, who had received neoadjuvant chemotherapy followed by surgery and subsequent adjuvant radio-immunotherapy with an ICI, nivolumab. We provide our experience with the toxicity of an administered treatment. RESULTS: Fourteen days after the first dose of nivolumab and on 21st day after starting of radiotherapy (total dose of 40 Gy), the patient developed fulminant myocarditis with subsequent heart failure. Despite immunosuppressive therapy with high-dose glucocorticoids and mycophenolate mofetil and intensive support, the patient died within 6 days after the onset of first symptoms. CONCLUSION: Physicians should be aware of these extremely rare, but potentially fatal complications of immunotherapy.

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