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Predicting Response to Immunotherapy by Evaluating Tumors, Lymphoid Cell-Rich Organs, and Immune-Related Adverse Events Using FDG-PET/CT

Nobashi, Tomomi, MD, PhD*; Baratto, Lucia, MD*; Reddy, Sunil A., MD; Srinivas, Sandhya, MD; Toriihara, Akira, MD, PhD*; Hatami, Negin, MD*; Yohannan, Thomas K., MD*; Mittra, Erik, MD, PhD

doi: 10.1097/RLU.0000000000002453
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Purpose To investigate whether the evaluation of tumors, lymphoid cell-rich organs, and immune-related adverse events (IRAE) with 18F-FDG PET/CT can predict the efficacy and outcome of immunotherapy.

Methods Forty patients who underwent 18F-FDG-PET/CT scans before and after therapy with immune checkpoint inhibitors from December 2013 to December 2016 were retrospectively enrolled (malignant melanoma, n = 21; malignant lymphoma, n = 11; renal cell carcinoma, n = 8). SUVmax of the baseline and first restaging scans were evaluated in tumors, spleen, bone marrow, thyroid and pituitary glands, and were correlated to best overall response in the first year after therapy; IRAE-affected areas were also evaluated.

Results Interval change between the baseline and first restaging scans showed that patients with a clinical benefit had a significant decrease in tumor parameters (P < 0.001). All patients with an increase of SUVmax in the thyroid of more than 1.5 (n = 5) on the first restaging scan had a complete response (CR) in 1 year. Patients with CR within 1 year (n = 22) were significantly associated with a favorable long-term outcome (P = 0.002). Nine patients with IRAE findings had CR at final evaluation. Among IRAE, thyroiditis was seen significantly earlier than arthritis (P = 0.040).

Conclusions The decrease of tumor parameters at early time-point PET scans was seen in patients with immunotherapy who had clinical benefit within 1 year. PET-detectable IRAE was useful for prediction of a favorable outcome. Early development of thyroiditis may particularly represent an early response indicator to immunotherapy.

From the *Division of Nuclear Medicine and Molecular Imaging, Department of Radiology, and

Division of Oncology, Department of Medicine, Stanford University, Stanford, CA; and

Division of Nuclear Medicine, Department of Diagnostic Radiology, Oregon Health & Science University, Portland, OR.

Received for publication October 1, 2018; revision accepted November 27, 2018.

Conflicts of interest and sources of funding: none declared.

T.N. and L.B. equally contributed.

Correspondence to: Tomomi Nobashi, MD, PhD, 300 Pasteur Drive, Stanford, CA 94305. E-mail: tomominobashi@gmail.com.

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