Nivolumab-Induced Pneumonitis in Patient With Metastatic Melanoma Showing Complete Remission on 18F-FDG PET/CT A 49-year-old patient with metastatic melanoma was treated with nivolumab (Opdivo). An early 18F-FDG PET/CT after 2 cycles showed a progressive metabolic disease. A 4-month optimal follow-up 18F-FDG PET/CT showed a complete metabolic response. The treatment was stopped after 22 cycles because of immunotherapy-related pneumonitis. After discontinuation of treatment, PET/CT examinations demonstrated a metabolic complete remission during 2 years. The metabolic pattern on early PET was suggestive of pseudoprogression, which is a rare phenomenon reflecting an activation of inflammatory cells within the tumor microenvironment causing lesions to increase in size and to accumulate FDG until a sufficient immune response is developed. Received for publication March 13, 2019; revision accepted May 29, 2019. Conflicts of interest and sources of funding: none declared. Correspondence to: Micheline Razzouk-Cadet, MD, Department of Nuclear Medicine, Centre Hospitalier Universitaire de Nice, Archet, 151, Route de Saint-Antoine de Ginestière 06200 Nice Cedex 3, France. E-mail: razzouk-cadet.m@chu-nice.fr. Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.