This is a treatment protocol designed to accompany the ongoing institutional 124I PET/CT pilot imaging study for patients with invasive breast cancer. Women whose tumors express NIS [Na+I- symporter, sodium iodide symporter] and demonstrate radioiodide uptake on 124I PET/CT scans will be eligible for 131I treatment if, (1) tumor dosimetry calculations yield a cumulative radiation dose of at least 30Gy in target tumor, (2) estimated cumulative thyroid irradiation is less than 500 cGy and, (3) the therapeutic dose of 131I is in the range of 25 to 100 mCi.

Inclusion Criteria:

• Patients with measurable locally advanced or metastatic breast cancer who completed 124I PET/CT imaging study.
• Patients who are 18 years of age or older.
• Patients must have a life expectancy of at least 3 months.
• I 124 dosimetry completed with thyroid dose to 500 cGy or less and tumor dose of 2500 cGy or more.
• I131 therapeutic dose calculated from dosimetry. Dose must be in a range of 25 to 150 mCi.
• Patients with Eastern Cooperative Oncology Group (ECOG) Performance Status 0-3 will be eligible.
• Use of concurrent systemic therapy (hormonal or cytotoxic) if associated with stable disease for at least three months prior to treatment.
• Women with locally advanced breast cancer and simultaneous metastases, even if surgery to eradicate local disease has taken place.
• Thyroid stimulating hormone (TSH) must be < 0.4 uIU/mL.
• White blood cell count >= 1,500 and platelet count >= 40,000
• Women receiving thyroid hormone supplements and methimazole.
• Ability to understand and the willingness to sign a written informed consent document.
• Premenopausal women must use contraception while receiving this treatment and during follow-up period of 42 days.

Exclusion Criteria:

• Stage 0-II breast cancer.
• Pregnant or nursing women.
• Not able to sign informed consent.
• Untreated psychiatric disorder.
• Women who have not had I124 PET/CT scan and dosimetry calculations.
• Radioiodide uptake in non-target organs or bone marrow does not exceed 25 cGy by pre-treatment dosimetry.
• Received chemotherapy less than 4 weeks before.
• History of thyroid cancer.