Inclusion Criteria:

          -  Have a histologically confirmed diagnosis of ACC (Weiss score of >= 3).
             (LinWeiss-Bisceglia system will be used for oncocytic ACC).

          -  Have a high risk of relapse defined as: Stage I-III ACC (according to the European
             Network for the Study of Adrenal Tumors [ENSAT] classification) within 90 days of
             surgical resection of primary tumor with curative intent with either microscopically
             complete resection (R0, defined as no evidence of microscopic residual disease
             according to surgical reports, histopathology, and perioperative imaging),
             microscopically positive margins (R1), or undetermined margins (RX, based on surgical
             or pathological reports without unequivocal evidence of metastasis in the
             perioperative imaging). Each participating center will determine the pathological
             stages and resection margins AND Ki67 > 10% (to be determined by an experienced
             pathologist in each participating center and preferably via quantitative imaging
             analysis).

          -  Have perioperative imaging (computed tomography [CT] with contrast, magnetic resonance
             imaging [MRI] of the chest/abdomen/pelvis, or fluorodeoxyglucose positron emission
             tomography [FDG-PET] CT) without unequivocal evidence of disease within 8 weeks before
             randomization. Patients with indeterminate non-specific nodules (< 1 cm for soft
             tissue lesions and < 1.5 cm in the short dimension for lymph nodes) will be permitted
             to participate in this study.

          -  Have an Eastern Cooperative Oncology Group (ECOG) performance status 0-2.

          -  Be able to comply with the protocol procedures.

          -  Provide written informed consent.

        Exclusion Criteria:

          -  The time between primary surgery and randomization > 90 days.

          -  Gross residual disease after surgery (R2 resection)

          -  High suspicion for metastatic disease on perioperative imaging

          -  They have undergone repeated surgery for recurrence of disease.

          -  They have a history of recent or active prior malignancy, except for cured
             non-melanoma skin cancer, cured in situ cervical carcinoma, breast ductal carcinoma in
             situ, or other treated malignancies where there has been no evidence of disease for at
             least 2 years.

          -  They have renal insufficiency (estimated glomerular filtration rate [GFR] < 50
             mL/min/1.73 m^2).

          -  They have significant liver insufficiency (serum bilirubin > 2 times the upper normal
             range)

          -  They have significant liver insufficiency (serum alanine aminotransferase [ALT] or
             aspartate aminotransferase [AST] > 3 times the upper normal range)

          -  Impaired bone marrow reserve (neutrophils < 1000/mm^3)

          -  Impaired bone marrow reserve (platelets < 100,000/mm^3)

          -  Pregnancy or breast feeding.

          -  They have known congestive heart failure (ejection fraction < 45%). The extent of
             cardiac testing will depend on the judgment of the local principal investigator (PI).
             In general, in patients with a history of cardiac disease, it is recommended to obtain
             a baseline two-dimensional echocardiogram as standard of care to document ejection
             fraction. In patients without prior cardiac disease, a baseline electrocardiogram
             (EKG) is sufficient if there is no evidence of acute ischemic changes or prior
             evidence of myocardial infarction. If EKG results are abnormal (ischemic changes,
             significant arrhythmia, or suggestion of prior myocardial infarction), a
             two-dimensional echocardiogram will be obtained to assess ejection fraction. Cardiac
             imaging and EKG may not be needed in patients assigned to mitotane who do not have
             prior cardiac history and have low suspicion for cardiac symptoms to reflect standards
             of clinical practice. Similarly, utilizing cardiac imaging and EKG within the past 12
             months is permitted if there is no suspicion for cardiac issues.

          -  They have preexisting grade 2 peripheral neuropathy.

          -  They underwent previous or current treatment with mitotane or other antineoplastic
             drugs for ACC.

          -  They underwent previous radiotherapy for ACC.

          -  They have any other severe acute or chronic medical or psychiatric condition or
             laboratory abnormality that would, in the judgment of the investigator, pose excess
             risk associated with study participation or administration of the involved drugs or
             that, in the judgment of the investigator, would make the patient inappropriate for
             entry into this study.