Pembrolizumab-Induced Sarcoid-Like Reaction: FDG-PET Scan Interpretation in the Era of Immunotherapy

Immunotherapy has revolutionized the treatment of malignant melanomas. Immunotherapy is associated with multi-system toxicities, which are referred to as immune-related adverse events (irAEs). Positron emission tomography (PET) with fluorodeoxyglucose (FDG) and CT is the preferred imaging modality to monitor disease progression in melanoma. FDG uptake by a sarcoid-like reaction (SLR) can mimic cancer progression, thereby posing a diagnostic and therapeutic dilemma. We present the case of a 39-year-old patient with malignant melanoma on immunotherapy who presented with PET scan findings of adenopathy with increased uptake. This case highlights the challenges in interpreting PET scan in the setting of an SLR.