Circulating Tumor DNA in Primary Mediastinal Large B-cell Lymphoma (PMBL)

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.).