Inclusion Criteria:

          -  Documentation of disease

               -  Histologic documentation: Histologically-proven advanced (metastatic or
                  unresectable primary) pheochromocytoma or paraganglioma

               -  Stage: Advanced (metastatic or unresectable primary) disease

               -  Tumor site: Histologically-proven pheochromocytoma or paraganglioma

               -  Radiographic evaluation: Radiographic evidence of disease progression by Response
                  Evaluation Criteria in Solid Tumors (RECIST) version (v) 1.1 in the 12 months
                  prior to registration

          -  Measurable disease

               -  Lesions must be accurately measured in at least one dimension (longest diameter
                  to be recorded) as >= 1 cm with computed tomography (CT) or magnetic resonance
                  imaging (MRI) (or >= 1.5 cm for lymph nodes). Non-measurable disease includes
                  disease smaller than these dimensions or lesions considered truly non-measurable
                  including: leptomeningeal disease, ascites, pleural or pericardial effusion,
                  lymphangitic involvement of skin or lung

          -  Prior treatment with other chemotherapy, radiotherapy (including peptide radionuclide
             receptor therapy [PRRT]), or surgery must be completed >= 28 days prior to
             registration. Patients must have recovered from any effects of any major surgery prior
             to registration

          -  Prior treatment with radiolabeled metaiodobenzylguanidine (MIBG) must be completed >=
             12 weeks prior to registration and lifetime cumulative 131I-MIBG dose must be < 1000
             MBq kg-1 (36 mCi kg-1)

          -  Prior treatment with antibiotics must be completed >= 7 days prior to registration

          -  Contraception

               -  Therapy utilized in this trial is associated with medium/high fetal risk

               -  Women of childbearing potential and their partners, who are sexually active, must
                  agree to use two highly effective forms of contraception in combination. This
                  should be started from the time of registration and continue throughout the
                  period of taking study treatment and for at least 1 month after last dose of
                  study drug(s), or they must totally/truly abstain from any form of sexual
                  intercourse

               -  Male patients must use a condom during treatment and for 3 months after the last
                  dose of study drug(s) when having sexual intercourse with a pregnant woman or
                  with a woman of childbearing potential. Female partners of male patients should
                  also use a highly effective form of contraception if they are of childbearing
                  potential. Male patients should not donate sperm throughout the period of taking
                  study drug(s) and for 3 months following the last dose of study drug(s)

          -  Eastern Cooperative Oncology Group (ECOG) performance status: 0-2

          -  Absolute neutrophil count >= 1,500/mm^3

          -  Platelet count >= 100,000/mm^3

          -  Hemoglobin >= 10 mg/dL

               -  In the absence of transfusion within the previous 24 hours

          -  Total bilirubin =< 1.5 x upper limit of normal (ULN)

               -  Except in the case of Gilbert's syndrome, then total bilirubin must be =< 3.0 x
                  ULN

          -  Aspartate aminotransferase (AST)/alanine aminotransferase (ALT) =< 3.0 x ULN

          -  Creatinine < 1.5 x ULN OR calculated (calc.) creatinine clearance > 50 mL/min

               -  Calculated by Cockcroft-Gault equation

          -  Patients with human immunodeficiency virus (HIV) positivity are allowed if CD4 count >
             250 cells/uL and they have an undetectable HIV viral load within 6 months of
             registration

        Exclusion Criteria:

          -  No prior treatment with temozolomide, dacarbazine, or a poly ADP ribose polymerase
             (PARP) inhibitor

          -  No prior allogeneic bone marrow transplant or double umbilical cord blood
             transplantation (dUCBT)

          -  Not pregnant and not nursing, because this study involves an agent that has known
             genotoxic, mutagenic, and teratogenic effects. Therefore, for women of childbearing
             potential only, a negative pregnancy test done =< 7 days prior to registration is
             required

          -  No indication of uncontrolled, potentially reversible cardiac condition(s) as
             determined by investigator (e.g., unstable ischemia, uncontrolled symptomatic
             arrhythmia, congestive heart failure, QTcF prolongation > 500 msec, electrolyte
             disturbances, etc.) and no known congenital long QT syndrome

          -  No extensive bilateral lung disease or pneumonitis

          -  No abnormal organ or bone marrow function =< 28 days prior to registration

          -  No active infection

          -  No history of myelodysplastic syndrome (MDS) (or any dysplastic leukocyte morphology
             suggestive of MDS) or acute myeloid leukemia

          -  No known gastrointestinal condition(s) that might predispose for drug intolerability
             or poor drug absorption

          -  No known medical condition causing an inability to swallow oral formulations of agents

          -  No history of allergic reaction attributed to compounds of similar chemical or
             biologic composition to PARP inhibitors

          -  Concurrent use of combination antiretroviral therapy (ART) is not permitted

          -  Chronic concomitant treatment with strong or moderate CYP3A4 inducers or inhibitors is
             not allowed. Patients must discontinue the agent(s) >= 21 days prior to registration;
             enzalutamide and/or phenobarbital must be discontinued >= 5 weeks prior to
             registration