Study of Pemetrexed to Treat Recurrent or Progressive Primary Central Nervous System Lymphoma

Inclusion Criteria: * Histological confirmation of Primary Central Nervous System Lymphoma * Male or female \> 18 years of age or older * Negative pregnancy test (if of childbearing potential) * Any number of previous recurrences will be allowed * Karnofsky Performance Status \> 60 * Hematocrit \> 30,000 * Platelet \> 100,000 * Absolute Neutrophil Count \> 1,500 * Bilirubin \< 1.5 x upper limits of normal * Transaminases (ALT and AST) \< 1.5 x upper limits of normal * Creatinine \< 1.5 x upper limits of normal * Creatinine Clearance \> 45 mL/min * Adequate medical health to participate in this study * Adequate documentation of menopause (natural/surgical) or patient commitment to routine use of reliable birth control (barrier/hormonal) * Ability to read and understand the patient informed consent form * Ability and willingness to follow all requirements of the study including following all directions, taking medication as prescribed, and completion of all diaries and forms Exclusion Criteria: * Karnofsky Performance Status \< 60 * Hematocrit \< 30,000 * Platelet \< 100,000 * Absolute Neutrophil Count \< 1,500 * Bilirubin \>1.5 x upper limits of normal * Transaminases (ALT \& AST) \> 1.5 x upper limits of normal * Creatinine \> 1.5 x upper limits of normal * Creatinine Clearance \< 45 mL/min * Inability to undergo gadolinium-contrasted MRIs, including severe claustrophobia or insufficient allergy prophylaxis