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Recent data suggest that diffuse gliomas carrying mutations in codon 34 of the H3 histone family 3A protein represent a very rare, distinct subgroup of IDH–wild type malignant astrocytic gliomas. However, characteristics detectable by MRI and 18F-FET PET in H3-G34-mutant gliomas are unknown.We report on MRI and 18F-FET PET findings in 8 patients from 4 German centers with H3-G34-mutant diffuse gliomas. MRI analyses included multifocality, contrast enhancement, necrosis, cysts, hemorrhages, calcification, and edema. 18F-FET PET characteristics were evaluated on the basis of static 18F-FET PET parameters, such as maximal tumor-to-background ratio (TBRmax) and biological tumor volume (BTV), as well as the minimal time-to-peak (TTPmin) obtained from dynamic 18F-FET PET data.MRI showed multifocal lesions in 2 of 8, contrast enhancement in 6 of 8, necrosis in 3 of 8, cysts in 3 of 8, hemorrhage in 1 of 8, and calcifications in 1 of 8 patients. None of the tumors showed marked peritumoral edema. However, all 8 H3-G34-mutant gliomas were characterized by a high uptake intensity on 18F-FET PET with a median TBRmax of 3.4 (range, 2.5–11.7) and a relatively diffuse uptake pattern leading to a large BTV (median, 41.9 mL; range, 7.5–115.6). Dynamic PET data revealed a short median TTPmin of 12.5 minutes.MRI features of diffuse gliomas with H3-G34 mutation may present very heterogeneously with some cases not even fulfilling the imaging criteria of high-grade glioma. In contrast, in 18F-FET PET, these tumors show an extensive and diffuse tracer uptake resulting in large BTV with a high TBRmax and a short TTPmin, thus resembling PET characteristics of aggressive high-grade gliomas, namely, glioblastomas.