Inclusion Criteria:

          -  All patients must have a pathologically confirmed diagnosis of neuroblastoma, ≤ 30.99
             years of age with history of relapsed/refractory neuroblastoma.

          -  All patients must have completed upfront therapy with at least 4 cycles of aggressive
             multi-drug chemotherapy.

          -  Specific Criteria by Arm:

        Arms 1 and 2:

        Subjects with no active disease:

        i. No evidence of residual disease by CT/MRI and MIBG scan (or PET for patients who have a
        history of MIBG non-avid disease).

        o Note: Patients with residual masses detected by CT/MRI may be considered in CR if their
        MIBG is negative or if MIBG positive and evaluated by PET and found to have negative PET
        scans; biopsy confirmation may be considered if there is still reasonable concern for
        persistent disease but is not required.

        ii. No evidence of disease metastatic to bone marrow.

        Arm 3:

        Measurable or evaluable disease, including at least one of the following:

        Measurable tumor by CT or MRI; or a positive MIBG and PET; or positive bone marrow
        biopsy/aspirate in at least one site.

          -  Timing from prior therapy: Enrollment (first dose of DFMO) no later than 60 days from
             last dose of the most recent therapy.

          -  Subjects must have fully recovered from the acute toxic effects of all prior anti-
             cancer chemotherapy and be within the following timelines:

               1. Myelosuppressive chemotherapy: Must not have received within 2 weeks of
                  enrollment onto this study (6 weeks if prior nitrosourea).

               2. Hematopoietic growth factors: At least 5 days since the completion of therapy
                  with a growth factor.

               3. Biologic (anti-neoplastic agent): At least 7 days since the completion of therapy
                  with a biologic agent. For agents that have known adverse events occurring beyond
                  7 days after administration, this period must be extended beyond the time during
                  which adverse events are known to occur. The duration of this interval must be
                  discussed with the Study Chair.

               4. Immunotherapy: At least 6 weeks since the completion of any type of
                  immunotherapy, e.g. tumor vaccines, CAR-T cells.

               5. Anti-GD2 Monoclonal antibodies: At least 2 weeks must have elapsed since prior
                  treatment with a monoclonal antibody.

               6. XRT: At least 14 days since the last treatment except for radiation delivered
                  with palliative intent to a non-target site.

               7. Stem Cell Transplant:

                    1. Allogeneic: No evidence of active graft vs. host disease

                    2. Allo/Auto: ≥ 2 months must have elapsed since transplant.

               8. MIBG Therapy: At least 8 weeks since treatment with MIBG therapy

          -  Subjects must have a Lansky or Karnofsky Performance Scale score of 60% or higher.

          -  Life expectancy > 2 months

          -  All clinical and laboratory studies for organ functions to determine eligibility must
             be performed within 7 days prior to first dose of study drug unless otherwise
             indicated below.

          -  Subjects must have adequate organ functions at the time of registration:

               -  Hematological: Total absolute neutrophil count ANC ≥750/μL

               -  Liver: Subjects must have adequate liver function as defined by AST and ALT <5x
                  upper limit of normal (Normal=45), Bilirubin <1.5x upper limit normal
                  (Normal=1.0). Normal PT, PTT, fibrinogen.

               -  Renal: Adequate renal function defined as (perform one of the following):
                  Creatinine clearance or radioisotope GFR 70 mL/min/1.73 m2 or greater or a serum
                  creatinine based on age/gender

          -  Females of childbearing potential must have a negative pregnancy test. Patients of
             childbearing potential must agree to use an effective birth control method. Female
             patients who are lactating must agree to stop breast-feeding.

          -  Written informed consent in accordance with institutional and FDA guidelines must be
             obtained from all subjects (or patients' legal representative).

        Exclusion Criteria:

          -  BSA of <0.25 m2.

          -  Subjects that received DFMO at a dose higher than 1000mg/m2 BID prior to this study
             are not eligible.

          -  Subjects that received a dose of DFMO in combination with etoposide are not eligible.

          -  Investigational Drugs: Subjects who are currently receiving another investigational
             drug are excluded from participation.

          -  Anti-cancer Agents: Subjects who are currently receiving other anticancer agents are
             not eligible. Subjects must have fully recovered from hematological and bone marrow
             suppression effects of prior chemotherapy.

          -  Infection: Subjects who have an uncontrolled infection are not eligible until the
             infection is judged to be well controlled in the opinion of the investigator.

          -  Subjects who, in the opinion of the investigator, may not be able to comply with the
             safety monitoring requirements of the study, or in whom compliance is likely to be
             suboptimal, should be excluded.