• Provision of signed and dated informed consent form

• Stated willingness to comply with all study procedures and availability for the duration of the study

• Male or female, aged 18 or older

• Diagnosis of stage I-IIIB breast cancer

• Received neoadjuvant chemotherapy (minimum of 3 cycles) and surgical resection (lumpectomy and/or mastectomy)

• Discovered to have residual disease at least ypT1aNx or ypTxN1mic at surgical resection

• Candidate for adjuvant chemoradiation as part of standard clinical care

• Planned initiation of radiation within 12 weeks of their final oncologic surgery

• ECOG performance status ≤2

⁃ Adequate cardiac function, with LVEF greater or equal to 45% (only for patients who will receive TDM-1 therapy)

⁃ Adequate organ function per the following criteria within 21 days before the start of treatment. If a laboratory value required for study eligibility does not meet the below requirements, the value may be retested.

∙ Absolute neutrophil count ≥1.5 k/uL

‣ Platelets ≥100 k/uL

‣ Hemoglobin ≥ 10 g/dL

‣ Serum Creatinine ≤ 1.5 x ULN

‣ Bilirubin ≤ 1.5 x ULN (except in patients with Gilbert's disease, where bilirubin to 4x ULN is allowed).

‣ AST and ALT ≤ 2.5 x ULN

‣ Alkaline phosphatase ≤ 2.5 x ULN

⁃ For females and males of reproductive potential: agreement to use adequate contraception during study participation and for an additional 6 months after the end of chemoradiation administration or until advised by their medical oncologist

⁃ Agreement to adhere to Lifestyle Considerations throughout study duration

⁃ Subjects taking warfarin and plan to receive capecitabine will need their anticoagulant management assessed before starting treatment.