Inclusion Criteria:

        Subjects must meet all of the following criteria to participate in this study:

          1. Written informed consent and HIPAA authorization for release of personal health
             information of subject or subject's legally authorized representative.

          2. Age ≥ 65 years at the time of consent

          3. Eastern Cooperative Oncology Group (ECOG) performance status of 0 - 3 within 14 days
             prior to day 1 of treatment

          4. Histological or cytological confirmation of PCNSL, CD20 positive by
             immunohistochemistry

          5. Received at least 2 cycles of high-dose methotrexate (HD-MTX) containing induction
             chemotherapy per institutional standard (R-MPV preferred; see Appendix VI ) without
             evidence of progressive disease. No less than 4 weeks and no more than 8 weeks may
             have elapsed between last dose of HD-MTX containing induction chemotherapy and day 1
             of study treatment

          6. Recovered from all reversible acute toxic effects of prior therapy (other than
             alopecia) to ≤ Grade 1 or baseline)

          7. Measurable disease at the time of diagnosis (i.e. prior to pre-study HD-MTX containing
             induction chemotherapy) including lesions that can be accurately measured in 2
             dimensions by CT or MRI of brain and with a greatest transverse diameter of ≥ 1 cm.
             The following disease assessments must have been obtained prior to initiation of
             pre-study HD-MTX containing induction chemotherapy: MRI of the brain (and spine if
             indicated)

          8. Deemed poor candidate for whole brain irradiation (WBI) or autologous stem cell
             transplant (ASCT) due to advanced age, ECOG performance status of 2, or in the opinion
             of the treating physician, subject would not tolerate the administration of WBI or
             ASCT for other reasons

          9. Life expectancy of at least 3 months

         10. Demonstrate adequate organ function as defined below (all screening labs to be
             obtained within 14 days prior to day 1 of treatment):

               1. Absolute Neutrophil Count (ANC) ≥ 1000K/mm3

               2. Platelet Count ≥ 75 K/mm3

               3. Hemoglobin (Hgb) ≥ 8 g/dL

               4. Serum creatinine ≤ 1.5 mg/dL OR creatinine clearance ≥ 50 cc/minute as measured
                  by a 24-hour urine collection or estimated by the Cockcroft and Gault formula

               5. Bilirubin ≤ 1.5 x upper limit of normal (ULN) (except subjects with Gilbert
                  Syndrome who must have a total bilirubin level of < 3.0 x ULN)

               6. Aspartate aminotransferase (AST) ≤ 3 x ULN

               7. Alanine aminotransferase (ALT) ≤ 3 x ULN

         11. Females of childbearing potential (FCBP) must have a negative serum pregnancy test
             within 3 days prior to day 1 of treatment. NOTE: Females are considered of child
             bearing potential unless they are surgically sterile (have undergone a hysterectomy,
             bilateral tubal ligation, or bilateral oophorectomy) or are postmenopausal (at least
             12 consecutive months with no menses without an alternative medical cause).

         12. FCBP must be willing to use a highly effective contraceptive method (i.e., achieves a
             failure rate of <1% per year when used consistently and correctly) from the time of
             informed consent until 5 months after treatment discontinuation. Contraceptive methods
             with low user dependency are preferable but not required.
             (http://www.hma.eu/fileadmin/dateien/Human_Medicines/01-About_HMA/Working_Groups/CTFG/
             2014_09_HMA_CTFG_Contraception.pdf)

         13. Male subjects must be willing to use condoms from the time of treatment initiation
             until 7 months after last dose of nivolumab. For a non-pregnant FCBP partner,
             contraception recommendations should also be considered.

         14. As determined by the enrolling physician, ability of the subject to understand and
             comply with study procedures for the entire length of the study.

        Exclusion Criteria:

        Subjects must not meet any of the following criteria:

          1. Documented or suspected ophthalmologic involvement at the time of enrollment as
             determined by the investigator. Subjects with ophthalmologic involvement prior to or
             during pre-study induction are allowed if there is no evidence of ophthalmologic
             involvement prior to enrollment as determined by the investigator.

          2. Any concurrent systemic involvement by lymphoma outside CNS or intraocular lymphoma
             without evidence of brain disease

          3. Any previous chemotherapy or radiation therapy for PCNSL except for treatment with a
             HD-MTX containing induction chemotherapy. Subjects treated with corticosteroids for
             PCNSL are allowed.

          4. Pregnant or breastfeeding (NOTE: breast milk cannot be stored for future use while the
             mother is being treated on study)

          5. Has a known additional malignancy within the past 5 years that is active and/or
             progressive requiring treatment; exceptions include basal cell or squamous cell skin
             cancer, in situ cervical or bladder cancer, or carcinoma of the prostrate with a
             current PSA value of <0.5 ng/mL or other cancer for which subject has completed
             treatment, been disease-free for at least five years, and is considered by
             Sponsor-Investigator to be at <30% risk of relapse, or on hormonal therapy for a
             history of either prostate cancer or breast cancer, provided that there has been no
             evidence of disease progression during the previous three years.

          6. Treatment with any investigational drug (including drugs not FDA-approved for the
             indication for which they are given) within 28 days prior to day 1 of treatment

          7. Subjects with active, uncontrolled infections (subjects must be afebrile for >48 hours
             off systemic antibiotics).

          8. Uncontrolled intercurrent illness including, but not limited to, ongoing or active
             symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or
             psychiatric illness/social situations that would limit compliance with study
             requirements as determined by the investigator.

          9. Major surgery and/or radiotherapy within 14 days prior to initiation of study
             treatment

         10. Known history of positive test for human immunodeficiency virus (HIV) or known
             acquired immunodeficiency syndrome (AIDS). NOTE: Testing for HIV must be performed at
             sites where mandated locally.

         11. Active infectious hepatitis, type B or C. Subjects with a past or resolved HBV
             infection (defined as the presence of hepatitis B core antibody [anti-HBc] and absence
             of HBsAg) may be included if HBV DNA is undetectable.

         12. Subjects with active interstitial pneumonitis.

         13. Subjects with active, known or suspected autoimmune disease. Subjects with vitiligo,
             type I diabetes mellitus, residual hypothyroidism due to autoimmune condition only
             requiring hormone replacement, psoriasis not requiring systemic treatment, or
             conditions not expected to recur in the absence of an external trigger are permitted
             to enroll.