Inclusion Criteria:

   - Histologically proven Primary Mediastinal Large B-Cell Lymphoma (PMLBL).

   - PMLBL without central nervous system (CNS) involvement.

   - 6 months to less than 18 years of age at the time of consent.

   - Males and females of reproductive potential must agree to use an effective
   contraceptive method during the treatment, and after the end of treatment: during
   twelve months for women, taking into account the characteristics of rituximab

   - Complete initial work-up within 8 days prior to treatment that allows definite
   staging.

   - Able to comply with scheduled follow-up and with management of toxicity.

   - Signed informed consent from patients and/or their parents or legal guardians

Exclusion Criteria:

   - Follicular lymphoma, mucosa-associated lymphoid tissue (MALT) and nodular marginal
   zone

   - PMLBL patients with CNS involvement

   - Patients with congenital immunodeficiency, chromosomal breakage syndrome, prior organ
   transplantation, previous malignancy of any type, or known positive HIV serology.

   - Evidence of pregnancy or lactation period.

   - There will be no exclusion criteria based on organ function.

   - Past or current anti-cancer treatment except corticosteroids during less than one
   week.

   - Tumor cell negative for CD20

   - Prior exposure to rituximab.

   - Severe active viral infection, especially hepatitis B.

   - Hepatitis B carrier status history of hepatitis B virus (HBV) or positive serology.

   - Participation in another investigational drug clinical trial.

   - Patients who, for any reason, are not able to comply with the national legislation.