Potential Role of 18 F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography in Response Assessment of Rare Multisystemic Disease Like Erdheim–Chester Disease

We report the case of a 55-year-old male diagnosed with Erdheim–Chester disease, who underwent 18 F-fluorodeoxyglucose positron emission tomography–computed tomography ( 18 F-FDG PET/CT) for baseline evaluation as well as posttreatment. The patient was treated with interferon-alpha but was lost to follow-up due to COVID pandemic, 2 years later the disease status, and the response to therapy was documented using FDG-PET/CT. We would also like to highlight the varied presentations of lesions in kidneys, retroperitoneum, lungs, and paranasal sinuses with special focus on atypical finding of the kidney. Thus, FDG-PET/CT is a useful modality for demonstrating response to therapy and mapping the disease status in Erdheim–Chester disease and therefore helps in further planning of management.