Treatment Monitoring of Immunotherapy and Targeted Therapy Using ¹⁸F-FET PET in Patients with Melanoma and Lung Cancer Brain Metastases: Initial Experiences

Abstract

We investigated the value of <i>O</i>-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-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 ¹⁸F-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, ¹⁸F-FET PET was used to differentiate treatment-related changes from BM relapse after ICI or TT. In 13 patients, ¹⁸F-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 ¹⁸F-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 ¹⁸F-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>¹⁸F-FET PET may add valuable information for treatment monitoring in BM patients treated with ICI or TT.