18F-FET PET for Diagnosis of Pseudoprogression of Brain Metastases in Patients With Non–Small Cell Lung Cancer

Purpose To evaluate whether 18 F-fluoroethyltyrosine (FET) PET can discriminate progression from pseudoprogression of brain metastases in patients with non–small cell lung cancer undergoing immunotherapy and radiotherapy to the brain. Methods Retrospective analysis of 18 F-FET PET scans in cases with documented progression of brain metastases on MRI in a cohort of 53 patients with non–small cell lung cancer receiving immune-checkpoint inhibitors and radiotherapy of brain metastases at the University Hospital of Zürich from June 2015 until January 2019. Response to radiotherapy was assessed by MRI. In case of equivocal findings and/or radiological progression in clinically asymptomatic patients, further assessment with 18 F-FET PET was performed. Results From the cohort of 53 patients, the restaging MRI showed in 30 patients (56.6%) progression of at least 1 treated metastasis. Thereof, 18 F-FET PET was performed in 11 patients, based on the absence of neurological symptoms or presence of systemic response and physicians' decision. 18 F-FET PET correctly identified pseudoprogression in 9 of 11 patients (81.8%). In patients who did not undergo 18 F-FET PET, 5 of 19 (26.3%) were diagnosed with pseudoprogression. Conclusions Pseudoprogression of brain metastases occurred in 50% of patients diagnosed with progression on MRI. 18 F-FET PET may help differentiate pseudoprogression from real progression in order to avoid discontinuation of effective therapy or unneeded interventions.