Treatment of Recurrent Primary or Secondary Central Nervous System (CNS) Lymphoma With ALIMTA (Pemetrexed)

Inclusion Criteria:

* Disease state (5 categories of patients with these extremely rare disease are eligible: 1) Patients intolerant of Methotrexate-must have a diagnosis of B cell non-Hodgkin's lymphoma 2) Patients unable to receive methotrexate due to lack of drug availability-must have a diagnosis of B cell non-Hodgkin's lymphoma 3) Patients who fail to achieve a complete response to initial therapy for primary CNS lymphoma-must have a diagnosis of B cell non-Hodgkin's lymphoma 4) Recurrent Primary CNS lymphoma-If in the judgement of the treating physician therapy would be initiated without a repeat biopsy then the original biopsy with a diagnosis of B cell non-Hodgkin's lymphoma will suffice 5) Secondary CNS lymphoma-If in the judgement of the treating physician therapy would be initiated without a repeat biopsy then the original biopsy with a histologic diagnosis of B cell non-Hodgkin's lymphoma will suffice.
* Histologic diagnosis of B cell non-Hodgkin's lymphoma with measurable disease OR cytologic diagnosis of B cell non-Hodgkin's lymphoma with measurable disease OR immunohistochemical diagnosis of monoclonality (CDF) with or without measurable intracranial disease OR molecular diagnostic diagnosis of monoclonality CSF) with or without measurable intracranial disease OR Ocular-patients may have a combination of histologic confirmation of ocular lymphoma and measurable intracranial tumor. Slit-lamp examination and vitreal or retinal biopsy will be done to confirm ocular lymphoma OR neurolymphomatosis-evidence of measurable disease as nerve seeking lymphoma on MRI imaging with histologic diagnosis at any site.
* Karnofsky score of 60 or greater
* Must be able to undergo MRI scanning
* 18 years of age or older
* Life expectancy of at least 2 months
* Laboratory values as outlined in protocol
* Must be willing to practice an effective method of birth control during participation in the study whether male or female.

Exclusion Criteria:

* Definitive histologic diagnosis of low grade lymphoma without substantive clinical suspicion of transformation to an aggressive lymphoma
* Renal dysfunction defined as creatinine clearance \< 45 ml/min or serum creatinine \> 2mg/dL
* Transaminases \> 3 times above the upper limits of the institutional normal
* Acute infection, granulocytopenia or medical condition precluding surgery as judged by the caring physician and review team
* Pregnant or lactating females
* Patients must not have pre-existing immunosuppression, concurrent immunosuppressive treatment (with the exception of dexamethasone) or be a transplant recipient
* Patients must not have received prior whole brain irradiation. They can have received prior focal irradiation
* Prior participation in chemotherapy, cytotoxic therapy, immunotherapy or therapeutic protocols within 4 weeks fo enrollment. Patients unable to stop NSAIDS or Cox 2 inhibitors for two day before and after treatment as well as the day of treatment
* No other active systemic malignancy with the exception of basal cell carcinoma of the skin and cervical carcinoma in situ. Patients with a remote history (5 years or more) of malignancy are eligible for the protocol in the absence of active disease
* Clinically relevant third space fluid collection refractory to drainage
* Patient refusal to participate in the pK study