Inclusion Criteria:  Patient over 18 years of age, Suffering from a diffuse primary B lymphoma of the mediastinum, newly diagnosed locally on a biopsy with anatomopathological analysis according to the recommendations of the WHO 2016 classification of hematological malignancies, All stages (I-IV) All IPI scores (0-5) With mediastinal involvement, Untreated (apart from emergency corticosteroid therapy less than 2mg/kg/day for 7 days), Treatment with R-CHOP-14 or R-ACVBP with PET-CT guided strategy (delta SUVmax) to be initiated, Tumor fixation above liver background on pre-treatment FDG PET/CT/CT (Deauville score ≥4), Having signed the informed consent prior to any study procedure Affiliated or beneficiary of a social protection plan.  Exclusion Criteria:  Patient who has already started chemotherapy treatment, Contraindication to FDG PET-CT, No mediastinal involvement, Positive HIV serology, Positive hepatitis B or C serology with positive viral load, Protected adult (under guardianship or curatorship), Pregnant or breastfeeding women, Patient unable to understand the study for any reason or to comply with the constraints of the trial (language, psychological, geographical problems, etc.).