DISEASE CHARACTERISTICS:

• Histologically confirmed AIDS-related primary CNS lymphoma (AR-PCNSL)

  • If tissue diagnosis is not possible, the following are accepted as confirmation:

    • Positive brain fludeoxyglucose-positron emission tomography (FDG-PET)
    • Epstein-Barr virus (EBV) detected in the cerebrospinal fluid by polymerase chain reaction

• No evidence of lymphoma outside of the CNS

  • Ocular involvement allowed

• Positive HIV serology

  • No multidrug resistant HIV not amenable to long term suppression based on either or both of the following:

    • Clinical history of poor adherence to multiple antiretroviral drugs deemed sufficient to render effective HIV control unattainable
    • HIV mutational analysis (by genotyping and/or phenotyping) that reveals high-level resistance to > 1 class of anti-HIV drugs such that a combination regimen comprising agents from ≥ 2 drug classes can not be devised to suppress HIV long term
• No immunological failure while on highly active antiretroviral therapy (HAART) for ≥ 6 months, as defined by HIV suppression < 50/mm^3 but poor recovery of CD4 cells (CD4 count < 70/mm^3)

PATIENT CHARACTERISTICS:

• ECOG performance status 0-2

• No evidence of other opportunistic diseases or complications of HIV infection, including, but not limited to, the following:

  • Aggressive symptomatic Kaposi's sarcoma requiring systemic therapy
  • Systemic and/or CNS-invasive fungal infection
• PT and/or PTT ≤ 120% of control (unless due to presence of lupus anticoagulant)
• Absolute neutrophil count ≥ 1,000/mm^3
• Platelet count ≥ 50,000/mm^3
• ALT and/or AST ≤ 2 times upper limit of normal (ULN) (unless due to antiretroviral therapy or other necessary medications, in which case ALT and AST exclusion cut-off is > 5 times ULN)
• Bilirubin ≤ 2 times ULN (unless due to protease inhibitor therapy)
• Creatinine ≤ 1.5 mg/dL OR creatinine clearance ≥ 60 mL/min
• No New York Heart Association class III or IV congestive heart failure, unresponsive to treatment, that would make hydration infeasible
• No hypersensitivity to murine proteins or any component of rituximab
• Not pregnant
• Fertile patients must use effective contraception
• No condition or circumstance that wold preclude study compliance or study participation
• No refusal to adhere to HAART

PRIOR CONCURRENT THERAPY:

• See Disease Characteristics

• No prior therapy for CNS lymphoma

  • Steroids are allowed
• No concurrent radiotherapy
• No other concurrent therapy for AR-PCNSL
• No trimethoprim-sulfamethoxazole, salicylates, nonsteroidal anti-inflammatory drugs, sulfonamide medications, or penicillin beginning > 48 hours prior to the initiation of high-dose methotrexate and continuing until serum methotrexate levels are minimal