[PEMBROLIZUMAB-ASSOCIATED MYOCARDITIS FOLLOWED BY STEROID MYOPATHY IN A PATIENT WITH RENAL CELL CARCINOMA].

Immune checkpoint inhibitors (ICIs) are increasingly used as adjuvant therapy in high-risk renal cell carcinoma (RCC), but may cause serious immune-related adverse events, such as myocarditis. We report a rare case of myocarditis followed by steroid-induced myopathy during adjuvant pembrolizumab therapy. A 74-year-old man underwent partial and radical nephrectomy for bilateral clear cell RCC (pT1b and pT3a). Pembrolizumab was initiated postoperatively. On day 22, he developed exertional fatigue and cardiac enzyme levels were elevated. Myocardial biopsy confirmed myocarditis, and corticosteroid pulse therapy was started. Although cardiac markers improved initially, they rose again, requiring a second steroid pulse and higher maintenance doses. After 27 days of steroid therapy, the patient developed severe proximal muscle weakness. Myasthenia gravis and other autoimmune diseases were excluded. Steroid-induced myopathy was diagnosed, and steroid tapering plus rehabilitation led to functional recovery. He was later discharged without recurrence of myocarditis. This case highlights the importance of early detection and treatment of ICI-related myocarditis and awareness of secondary complications such as steroid myopathy. Multidisciplinary care is essential in managing these rare but serious conditions.