Phase I dose escalation study of 211At-MABG in adults with advanced pheochromocytoma / paraganglioma (PPGL) or other NET-overexpressing cancers (as evidenced by positive MIBG imaging) who are refractory to, lacking, or ineligible for approved treatments. Phase 1 dose-escalation will follow a standard 3+3 design with an expansion cohort at the recommended phase two dose (RP2D).

Inclusion Criteria:

1. Adult patients, at least 18 years of age

2. Advanced neuroendocrine cancers requiring systemic therapy and refractory to, ineligible for, declining, or lacking standard treatments.

3. I MIBG imaging indicating MIBG-avid disease (radiotracer uptake above background in at least one tumor site) per Investigator/Sub-Investigator assessment.

4. Participants must provide written informed consent prior to study-specific procedures.

5. ECOG performance status ≤ 2.

6. Adequate organ function including:

   1. Hemoglobin ≥ 9 g/dL
   2. Absolute neutrophil count ≥ 1,500/mm³
   3. Platelet count ≥ 75,000/mm³
   4. Measured or estimated GFR ≥ 60 mL/min
   5. Serum bilirubin ≤ 1.5x upper limit of normal
   6. ALT/AST each ≤ 2.5x upper limit of normal

7. Life expectancy at least 3 months as judged by treating physician

Exclusion Criteria:

1. Women who are pregnant or breast-feeding will not be eligible for this study.
2. Inability to tolerate study procedures in the opinion of the investigator or treating physician.
3. Serious or unstable medical, psychological, or social conditions that, in the opinion of the investigator, would compromise the subject's safety or successful participation in the study.
4. Uncontrolled brain metastasis (Participant must be at least 4 weeks since CNS-directed therapy and no longer requiring corticosteroid therapy).
5. Anticancer therapy, except hormonal therapy or bone supportive therapies, within 14 days of cycle 1 day 1.
6. Has a known additional malignancy (other than the disease under study) that has required active systemic treatment within the past 2 years AND for which the natural history or recent/ongoing treatment could likely interfere with study endpoints or safety of the study treatment per Investigator and Medical Director assessment.