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

We investigated the value of -(2-F-fluoroethyl)-l-tyrosine (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. We retrospectively identified 40 patients with 107 BMs secondary to melanoma ( = 29 with 75 BMs) or non-small cell lung cancer ( = 11 with 32 BMs) treated with ICI or TT who had F-FET PET ( = 60 scans) for treatment monitoring from 2015 to 2019. Most patients ( = 37; 92.5%) had radiotherapy during the course of the disease. In 27 patients, F-FET PET was used to differentiate treatment-related changes from BM relapse after ICI or TT. In 13 patients, 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 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. A TBR threshold of 1.95 differentiated BM relapse from treatment-related changes with an accuracy of 85% ( = 0.003). Metabolic responders to ICI or TT on F-FET PET had a significantly longer stable follow-up (threshold of TBR reduction relative to baseline, ≥10%; accuracy, 82%; = 0.004). Furthermore, at follow-up, time to peak in metabolic responders increased significantly ( = 0.019). F-FET PET may add valuable information for treatment monitoring in BM patients treated with ICI or TT.