• Participants' recurrences must have histologically confirmed ductal carcinoma in-situ, invasive ductal, medullary, papillary, colloid (mucinous), tubular or mixed histologies.

• Lesion size \< 3 cm treated with a partial mastectomy. Participants with invasive cancer and clinically and radiographically negative axillas do not require an axillary lymph node sampling unless they did not have prior axillary lymph node sampling (e.g. previous cancer was DCIS). Patients with DCIS as their recurrence do not require surgical assessment of the axilla. Repeat sentinel lymph node biopsy is permitted.

• Negative resection margins with at least a no tumor on ink or a negative re-excision.

• Hormonal therapy is allowed. If chemotherapy is planned, it can be delivered either prior or after the radiation is delivered. There must be at least 2 weeks between radiation and chemotherapy. HER2 directed therapy can be delivered concurrently with radiation.

• Participants must be \> 18 years of age. Because no dosing or adverse event data are currently available on the use of breast re-irradiation in participants ≤18 years of age, children are excluded from this study.

• Participants must have the ability to understand and the willingness to sign a written informed consent document.

• Performance status: ECOG Performance status ≤ 2.

• Life expectancy of ≥ 12months, in the opinion of and as documented by the investigator.

• Not based on gender; this trial is open to any gender, defined as self-representation of gender identity.