the investigators hypothesise that 18F-FDOPA PET kinetic parameters are good biomarkers to characterise suspected LGG brain lesions that exhibit no contrast on MRI, for identifying aggressive lesions. These parameters could constitute diagnostic biomarkers for this indication. This new diagnostic tool could enhance patient care in the short term in an evolving pathology affecting socially active subjects with a poor prognosis

Diffuse low-grade gliomas (LGGs) without any contrast enhancement on MRI are rare (15% of gliomas, 700 cases/year in France), have a poor prognosis (median overall survival from 5 to 15 years) and affect young, socially active subjects (median age 40 years). Among these lesions, 30% present with high grade histopathological criteria or with poor prognostic molecular characteristics, according to the 2021 WHO Classification of Tumors of the Central Nervous System (lack of IDH [Isocitrate DeHydrogenase] mutation, CDKN2A/B deletion). These high-grade types of tumours progress within 6 months and their diagnosis and management represent a public health issue. Moreover, the care of LGG patients is currently not standardised.

Although treatment is based on surgery and the complete excision of the lesion, as far as this is possible, and/or first-line chemotherapy ±radiotherapy, the optimal time to begin treatment remains controversial.

Aggressive forms should be diagnosed as soon as possible to allow immediate surgery to improve survival, whilst strategies allowing the maintenance of an optimal quality of life, more often with functional surgery alone, are recommended for non-aggressive forms. The main hurdle to standardised patient management is the lack of amenable non-invasive biomarkers to identify aggressive LGG forms.

18F-FDOPA positron emission tomography (PET) is promising to diagnose initial gliomas with conventional Standardised-Uptake-Value (SUV) parameters. Our team recently demonstrated the potential of 18F-FDOPA PET kinetics to better characterise gliomas. Two parameters are determined from the 30-minute dynamic acquisition curve of the tumour: the time-to-peak SUV (TTP), and the SUV slope. In our previous studies, limited by their monocentric and retrospective nature, molecular characteristics were mainly predicted by TTP: long TTP for an IDH-mutation and short TTP for IDH-wildtype tumours. A prospective multicentric study is needed to confirm our preliminary results in a specific population of suspected LGGs without any contrast enhancement on MRI.

The investigator hypothesise that 18F-FDOPA PET kinetic parameters are biomarkers which lead to improved care because they characterise aggressive forms of gliomas exhibiting no contrast on MRI.

Inclusion Criteria:

• Age between 18 and 75 years old
• WHO general condition ≤2
• Identification of a unifocal brain tumour at the initial diagnosis with no contrast in the MRI and suspected to be a LGG, with biopsy/surgery envisaged within 6 months of the PET scan
• MRI performed a maximum of 3 weeks before inclusion and comprising the conventional morphological sequences (T1, T1 sequences with injection of contrast agent and T2 FLAIR).
• Subject affiliated to or beneficiary of a social security plan
• Subject having received complete information on the organisation of the research and having signed the informed consent form.

Exclusion Criteria:

• Multifocal brain lesions
• Contraindication to 18F-FDOPA PET
• Pregnant, parturient women or nursing mothers under Article L1121-5
• Women of childbearing age who do not have effective contraception under Article L1121-5
• Monitoring not possible
• Persons deprived of their liberty by a judicial or administrative decision under Article 1121-8, persons undergoing psychiatric treatment under Articles L. 3212-1 and L. 3213-1.
• Patients cannot simultaneously participate in an interventional research trial for the duration of the KING study