NIMG-47. PHOTOPENIC DEFECTS ON O-(2-18F-FLUOROETHYL)-L-TYROSINE PET - CLINICAL RELEVANCE IN GLIOMA PATIENTS

Abstract

In PET imaging, a considerable number of gliomas shows no O-(2-18F-fluoroethyl)-L-tyrosine (FET) uptake compared to the healthy-appearing unaffected brain, including lesions with even less uptake (photopenic defects). The clinical relevance of photopenic defects remains to be elucidated. Glioma patients with a negative FET PET scan prior to histological confirmation were retrospectively identified in two university centers. Gliomas were visually rated as having indifferent FET uptake when no difference was found in comparison to the background activity, and as photopenic when FET uptake was below background acitivity. For the semiquantitative analysis, 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). In patients without treatment (“watch and wait” strategy), the progression-free survival (PFS) of photopenic gliomas was compared with that of gliomas with indifferent FET uptake. Of 104 FET-negative gliomas (2 WHO grade I, 75 WHO grade II, 23 WHO grade III, and 4 WHO grade IV), 36 cases with photopenic defects (35%) were identified (23 WHO grade II, 12 WHO grade III, and one glioblastoma). FET uptake in photopenic defects was significantly decreased compared to both the healthy-appearing brain tissue (SUV, 0.88 ± 0.23 vs. 1.10 ± 0.26; P<0.001) and gliomas with indifferent FET uptake (TBR, 0.83 ± 0.10 vs. 1.03 ± 0.11; P<0.001). In patients without treatment (all WHO grade II; stereotactic biopsy only), PFS of those with indifferent FET-uptake (n=16) was significantly longer than with photopenic defects (n=10) (31 vs. 23 months; P=0.026). The age and rate of IDH mutations did not differ significantly in both groups (P>0.05). Around one-third of FET-negative gliomas exhibit photopenic defects. These photopenic gliomas should be managed more actively as they might have a higher risk for harbouring a higher-grade glioma and an unfavorable outcome compared to gliomas with indifferent FET uptake.