New Mediastinal and Hilar Lymphadenopathy After Adjuvant Pembrolizumab in a Patient With Stage III Clear Cell Renal Cell Carcinoma

The advent of immune checkpoint inhibitors (ICIs) has transformed the management of advanced and high-risk renal cell carcinoma (RCC). In the adjuvant setting, ICIs, such as pembrolizumab, aim to reduce the risk of recurrence following potentially curative nephrectomy. However, this therapeutic approach introduces unique challenges, particularly related to immune-related adverse events (irAEs). The sarcoidosis-like reaction is a particularly rare immune-related adverse event that can be a diagnostic challenge because of its broad clinical symptoms and potential to mimic metastasis. We present a case of a 50-year-old patient who developed mediastinal and hilar lymphadenopathy during adjuvant pembrolizumab therapy following nephrectomy for stage III RCC, which was initially suspected to be metastatic renal cell carcinoma. Endobronchial ultrasound (EBUS) biopsy revealed non-caseating granulomas without malignancy, leading to a diagnosis of pembrolizumab-induced sarcoidosis-like reaction (SLR). The patient was treated with corticosteroids due to progressive fatigue, leading to a complete resolution of his constitutive symptoms and mediastinal lymphadenopathy. This study highlights diagnostic and therapeutic challenges, the importance of distinguishing irAEs from cancer progression, and strategies to avoid cognitive bias in clinical decision making.