Pemetrexed and Temozolomide in Treating Patients With Relapsed Primary Central Nervous System Lymphoma （PCNSL）

Inclusion Criteria:

* Histologically confirmed primary CNS lymphoma.
* ECOG (Eastern Cooperative Oncology Group) Performance status of 0-1.
* Positive cerebrospinal fluid (CSF) cytology or immunohistochemical diagnosis of CSF monoclonality with or without measurable intracranial disease.
* Measurable (greater than 1 cm in diameter) tumor by CT scan or MRI.
* Progressed during first-line chemotherapy and/or radiotherapy OR relapsed after initial successful treatment.
* No systemic lymphoma by CT scan of the chest, abdomen, and pelvis with contrast.
* No leptomeningeal lymphoma by lumbar puncture for CNS cytology/flow cytometry.
* No ocular lymphoma by slit lamp examination.
* Must have adequate organ function as defined by the protocol: Adequate renal function: serum creatinine ≤ 1.5 mg/dl and/or calculated creatinine clearance ≥ 60 ml/min; Adequate hepatic function: bilirubin level ≤ 1.5 x ULN, ASAT \& ALST ≤ 1.5 x ULN.Adequate bone marrow reserves: neutrophil (ANC) count ≥ 1500 /mm\^3, platelet count ≥ 100,000 /mm\^3, hemoglobin ≥ 9 g/dl.
* Age \>/= 18 and \</= 75 years.
* Signed written informed consent prior to study entry.

Exclusion Criteria:

* Patients with human immunodeficiency virus seropositivity and systemic lymphoma manifestation.
* Serious uncontrolled concurrent illness.
* Previous brain radiotherapy, systemic chemotherapy.
* Concurrent chronic systemic immune therapy, targeted therapy not indicated in this study protocol.
* Any evidence of prior exposure to Hepatitis B virus.
* Unable to comprehend the study requirements or who are not likely to comply with the study parameters.
* Pregnant (confirmed by serum or urine β-HCG) or lactating.