Inclusion Criteria:

• 1. Surgically inoperable participants with clinical diagnosis of PHEO/PGL who also have demonstrated disease histologically consistent with pheochromocytoma or paraganglioma (preferably confirmed by research site pathology review if initial pathology was done outside of research site, but not mandatory).
• 2. Progressive disease by RECIST with or without symptoms within the last 12 months. NOTE: Untreated participants with existing histologic diagnoses are eligible if progression can be demonstrated.
• 3. PHEO/PGL that is not associated with any known susceptibility genetic mutations for PHEO/PGL except SDHx mutation (a.k.a. "apparent sporadic"), based on documented genetic testing results obtained prior to study enrollment. PHEO/PGL that is associated with non-SDHx mutations such as VHL, NF1, and RET will not be eligible for this study.
• 4. Patient is or will be enrolled on protocol 00-CH-0093, Diagnosis, Pathophysiology, and Molecular Biology of Pheochromocytoma and Paraganglioma (NIH only).
• 5. Both metastatic and inoperable primary-only participants are eligible.
• 6. Must have presence of SSTR+ disease as documented by positive Ga-68-DOTATATE PET scan within 12 weeks of anticipated treatment.
• NOTE:
• * Positivity of Ga-68-DOTATATE PET scan defined as having at least one lesion that is greater than or equal to 10 mm in diameter with uptake that is higher than or equal to liver and is qualitatively higher and distinguishable from background activity.
• * Measurable disease as defined by RECIST 1.1
• 7. Age greater than or equal to 18
• 8. Karnofsky Performance Score greater than or equal to 60 or ECOG Performance Status of 2 or better.
• 9. Able to understand and willings to sign informed consent
• 10. Ability and willingness to obtain all required scans per study schedule.
• 11. Negative serum pregnancy test for women of child bearing potential or NOTE: A female is not of childbearing potential if a prior history of hysterectomy with bilateral oophorectomy or other procedure has render the participant surgically sterile, or \>2 years since last menstruation.
• 12. Female participants of childbearing potential and male participants who are not surgically sterile or with female partners of childbearing potential must agree to use effective, non-hormonal means of contraception (intrauterine contraceptive device, barrier method of contraception in conjunction with spermicidal gel) prior to study entry, for the duration of study participation and for 6 months (10 half-lives of Lu-177) after the last dose of Lu-177- DOTATATE.
• 13. Must have outside endocrinologist/medical oncologist who can follow the participant after receiving PRRT (NIH only requirement).
• 14. Patients with secreting tumors must be receiving adequate pharmacologic catecholamine blockade as determined by the treating physician.
• 15. Ineligible, unable to or unwilling to receive standard first line therapy for PHEO/PGL.

Exclusion Criteria:

• 1. Creatinine clearance \<50 mL/min calculated by the MDRD method, eventually confirmed by measured creatinine clearance (or measured glomerular filtration rate (GFR) using plasma clearance methods.
• 2. Serum albumin less than or equal to 3.0 g/dL unless prothrombin time is within the normal range.
• 3. Liver dysfunction as evidenced by Child s Class C Liver Disease or worse Alternatively, AST or ALT \> 2.5 times institutional upper limit of normal (ULN) unless liver metastases are present, in which case up to 5 times ULN would be allowed.
• 4. Hb \< 8.0 g/dL; WBC \< 2.0 x 10\^9/L (or Absolute Neutrophil Count \< 1000); Platelets \< 100 x 10\^9/L
• 5. In participants with symptoms of congestive heart failure, New York Heart Association (NYHA) classification of grade III or IV
• 6. Pregnancy or lactation.
• 7. Prior anti-tumoral radionuclide therapy with unsealed sources. Prior therapy with sealed radioactive sources such as brachytherapy will be allowed.
• 8. Prior local radiation therapy would be allowed as long as there is at least one non-irradiated index lesions.
• 9. Known brain metastases, unless these metastases have been treated and stabilized for at least 24 weeks, prior to enrollment in the study. Patients with a history of brain metastases must have a head CT or MRI scan with contrast to document stable disease for at least 24 weeks prior to enrolment in the study.
• 10. Other known co-existing malignancies except non-melanoma skin cancer and carcinoma in situ of the uterine cervix, unless definitively treated and proven no evidence of recurrence for 5 years.
• 11. Patients who participated in any therapeutic clinical study with an investigational agent within the last 30 days.
• 12. Patients may be on somatostatin analogue therapy (e.g. but not only limited to sandostatin or lanreotide therapy). However, therapy with somatostatin analogues should not be initiated or altered within 3 months of study enrolment. Patients on short term octreotide may have dose held for 24 hours prior to Lu-177-DOTATATE therapy. Those on long acting octreotide therapy will receive treatment at 1 to 5 days prior to their next cold octreotide dose, in order to prevent competition for the receptor.
• 13. Patient weight \> 400 lbs (table limit for PET scanner) or per local institutional standard for non-NIH sites.
• 14. Uncontrolled inter-current illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, hypertension (\>180/110), arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.
• 15. Inability to tolerate at least one modality of diagnostic anatomic imaging, such as CT or MRI.