Pemetrexed Disodium in the Cerebrospinal Fluid of Patients With Leptomeningeal Metastases

DISEASE CHARACTERISTICS: * Diagnosis of systemic malignancy (solid tumor or hematologic malignancy) or primary CNS lymphoma with leptomeningeal metastases (LM) as documented by MRI, cerebrospinal fluid, or both * Patients may have brain metastases in addition to LM * Patients with clinically significant interstitial fluid with effusion controlled by drainage are eligible PATIENT CHARACTERISTICS: * Karnofsky performance status 60-100% * Life expectancy \> 2 months * Creatinine clearance ≥ 45 mL/min * Bilirubin \< 1.5 times upper limit of normal (ULN) * Transaminases \< 3.0 times ULN (5 times ULN for hepatic metastasis) * WBC \> 3,000/mm³ * Neutrophil count \> 1,500/mm³ * Platelet count \> 100,000/mm³ * Hemoglobin \> 10 g/dL * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception during and for 3 months after completion of study treatment * Able to undergo lumbar puncture (i.e., no noncommunicating hydrocephalus or spinal block) or has an Ommaya reservoir in place * Able to take steroids, cyanocobalamin (vitamin B12), and folic acid * No other active cancer except nonmelanoma skin cancer or carcinoma in situ of the cervix * Patients with prior malignancies who are in complete remission and are off all therapy for that malignancy for ≥ 3 years are eligible * No significant medical or psychiatric illness that would interfere with study compliance PRIOR CONCURRENT THERAPY: * More than 2 weeks since prior radiotherapy and recover…