Treatment Monitoring of Immunotherapy and Targeted Therapy Using <sup>18</sup>F-FET PET in Patients with Melanoma and Lung Cancer Brain Metastases: Initial Experiences
Norbert Galldiks, Diana S.Y. Abdulla, Matthias Scheffler, Fabian Wolpert, Jan‐Michael Werner, Martin Hüllner, Gabriele Stoffels, Viola Schweinsberg, Max Schlaak, Nicole Kreuzberg, Jennifer Landsberg, Philipp Lohmann, Garry Ceccon, Christian Baues, Maike Trommer, Eren Celik, Maximilian I. Ruge, Martin Köcher, Simone Marnitz, Gereon R. Fink, Jörg‐Christian Tonn, Michael Weller, Karl‐Josef Langen, Jürgen Wolf, Cornelia Mauch
Abstract
We investigated the value of <i>O</i>-(2-<sup>18</sup>F-fluoroethyl)-l-tyrosine (<sup>18</sup>F-FET) PET for treatment monitoring of immune checkpoint inhibition (ICI) or targeted therapy (TT) alone or in combination with radiotherapy in patients with brain metastasis (BM) since contrast-enhanced MRI often remains inconclusive. <b>Methods:</b> We retrospectively identified 40 patients with 107 BMs secondary to melanoma (<i>n</i> = 29 with 75 BMs) or non–small cell lung cancer (<i>n</i> = 11 with 32 BMs) treated with ICI or TT who had <sup>18</sup>F-FET PET (<i>n</i> = 60 scans) for treatment monitoring from 2015 to 2019. Most patients (<i>n</i> = 37; 92.5%) had radiotherapy during the course of the disease. In 27 patients, <sup>18</sup>F-FET PET was used to differentiate treatment-related changes from BM relapse after ICI or TT. In 13 patients, <sup>18</sup>F-FET PET was performed for response assessment to ICI or TT using baseline and follow-up scans (median time between scans, 4.2 mo). In all lesions, static and dynamic <sup>18</sup>F-FET PET parameters were obtained (i.e., mean tumor-to-brain ratios [TBR], time-to-peak values). Diagnostic accuracies of PET parameters were evaluated by receiver-operating-characteristic analyses using the clinical follow-up or neuropathologic findings as a reference. <b>Results:</b> A TBR threshold of 1.95 differentiated BM relapse from treatment-related changes with an accuracy of 85% (<i>P</i> = 0.003). Metabolic responders to ICI or TT on <sup>18</sup>F-FET PET had a significantly longer stable follow-up (threshold of TBR reduction relative to baseline, ≥10%; accuracy, 82%; <i>P</i> = 0.004). Furthermore, at follow-up, time to peak in metabolic responders increased significantly (<i>P</i> = 0.019). <b>Conclusion:</b><sup>18</sup>F-FET PET may add valuable information for treatment monitoring in BM patients treated with ICI or TT.