DISEASE CHARACTERISTICS:

• Histologically or cytologically confirmed non-small cell lung cancer (NSCLC)

• Must have evidence of brain metastases by MRI or CT scan

  • No single brain metastases or oligometastatic disease amenable to surgical resection or radiosurgery
• Relapsed NSCLC with brain metastases allowed
• Not a candidate for double-agent or platinum-based chemotherapy
• No leptomeningeal metastases
• No clinically relevant (defined by physical exam) pleural effusions or ascites that cannot be controlled with drainage or other procedures

PATIENT CHARACTERISTICS:

• Karnofsky performance status 70-100% OR ≥ 70 years of age
• Life expectancy > 3 months
• ANC > 1,500/mm³
• Platelet count > 100,000/mm³
• Hemoglobin ≥ 8 g/dL
• AST and ALT < 3 times upper limit of normal (ULN)
• Bilirubin ≤ 1.5 times ULN
• Creatinine clearance ≥ 45 mL/min
• No contraindication or intolerance to corticosteroid therapy
• No other malignancies within the past 5 years and disease-free OR prognosis is best defined by the NSCLC in the opinion of the attending physician
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception
• No HIV positivity
• No severe hypersensitivity to pemetrexed disodium
• Able to discontinue NSAIDs for ≥ 5 days
• No history of underlying dementia, Parkinson's disease, or Alzheimer's disease
• Able to take vitamins, folic acid, and corticosteroids

PRIOR CONCURRENT THERAPY:

• Recovered from prior oncologic or major surgery
• Prior resection of all brain metastases or only site of brain metastases allowed provided there is radiologically evaluable intracranial metastases
• No prior cranial irradiation, including stereotactic radiosurgery
• More than 30 days since prior nonapproved or investigational drug
• No other concurrent chemotherapy, immunotherapy, hormonal therapy, radiotherapy, surgery, or experimental medications