Photopenic defects on O-(2-[18F]-fluoroethyl)-L-tyrosine PET - Clinical relevance in glioma patients.

Abstract

BACKGROUND\nO-(2-[18F]-fluoroethyl)-L-tyrosine (FET) PET has a sensitivity of more than 90% to detect gliomas. In the remaining small fraction of gliomas without increased tracer uptake, some tumors even show photopenic defects whose clinical significance is unclear.\n\n\nMETHODS\nGlioma patients with a negative FET PET scan prior to neuropathological confirmation were identified retrospectively. Gliomas were rated visually as i) having indifferent FET uptake, or ii) photopenic, if FET uptake was below background activity. FET uptake in the area of signal hyperintensity on the T2-/FLAIR-weighted MRI was evaluated by mean standardized uptake values (SUV) and mean tumor-to-brain ratios (TBR). The progression-free survival (PFS) of photopenic gliomas was compared with that of gliomas with indifferent FET uptake.\n\n\nRESULTS\nOf 100 FET-negative gliomas, 40 cases with photopenic defects were identified. Fifteen of these 40 cases (38%) had WHO grade III and IV gliomas. FET uptake in photopenic gliomas was significantly decreased compared to both the healthy-appearing brain tissue (SUV, 0.89±0.26 vs.1.08±0.23; P<0.001) and gliomas with indifferent FET uptake (TBR, 0.82±0.09 vs. 0.96±0.13; P<0.001). Irrespective of the applied treatment, IDH-mutated WHO grade II diffuse astrocytoma patients with indifferent FET uptake (n=25) had a significantly longer PFS than IDH-mutated diffuse astrocytomas (WHO grade II) with photopenic defects (n=11) (51 vs. 24 months; P=0.027). The multivariate survival analysis indicated that photopenic defects predict an unfavorable PFS (P=0.009).\n\n\nCONCLUSION\nPhotopenic gliomas in negative FET PET scans should be managed more actively as they seem to have a higher risk of harboring a higher-grade glioma and an unfavorable outcome.