Pseudoprogression is a phenomenon related to post-treatment rearrangements (including radiation necrosis). It appears early in the first year after treatment and accounts for 30 to 50% of patients followed with glioblastoma. On MRI (current gold standard with international therapeutic response evaluation criteria RANO 2010), pseudoprogression is manifested by a progression of morphological abnormalities (contrast enhancement, FLAIR hypersignal) and can simulate tumor recurrence, even though the corticosteroid improved or kept clinical symptoms stabilized. In view of prognosis, the current diagnostic tools have not enough diagnosis accuracy for differentiation between pseudo-progression and early tumor recurrence, and are based on MRI retrospective analysis (2-3 months after). Recurrence of glioblastoma, is characterized by a higher amino acid metabolism than pseudoprogression, also 11C-Methionine (11C-MET), positron emitting radiotracer, showed promising results to differentiate these two entities. To date, hybrid 11C-MET PET-MRI studies remains limited to small sample size (a few dozen patients), and none focuses exclusively on glioblastoma.

Hypothesis of our study is that 11C-MET PET-MRI may be performed as a first-line MRI for suspected pseudoprogression and may changes therapeutic decision making and also patient prognosis.

The main objective is to evaluate the performance of hybrid PET-MRI imaging with 11C-MET to differentiate pseudoprogression from glioblastoma recurrence in patients treated with surgery and radiochemotherapy, compared to multimodality MRI).

Inclusion Criteria:

• Patients aged 18 or over,
• Patient with glioblastoma treated by radiochemotherapy with temozolomide,
• Patient with suspicion of pseudoprogression between the 1st and 12th month after the end of concomitant chemoradiotherapy,
• Patient receiving a social security scheme,
• Patient for whom informed and written consent to participate has been obtained,

Exclusion Criteria:

• Subject under safeguard of justice (tutelage, curatorship),
• Minor patient,
• Patient without signs of progression MRI (stable disease or good response to treatment according to the RANO 2010 criteria),
• Clinical or radiological progression justifying a change of treatment,
• Patient not able to decide and with refusal of the family entitled to continue research.
• Pregnant woman, breastfeeding or old enough to have children but without effective contraception,
• Contraindication to performing an MRI: permanently fixed metal parts (pacemaker, cerebral clip, cochlear implant, pin or screw for recent bone fracture, dental equipment, metal splinters), claustrophobia,
• Contraindication to gadolinium according to ANSM 2017 recommendations