This research is being done to see if proton beam radiation therapy (PBT) results in fewer changes to a participant's heart measured with MRI-imaging than conventional or "photon" radiation therapy (XRT) for participants with non-metastatic left sided breast cancer.

The names of the two study groups in this research study are:

• Proton Radiation Therapy (PBT)
• Conventional or "Photon" Radiation Therapy (XRT)

This is a randomized phase II trial comparing participants with stage II-III breast cancer treated with accelerated Proton Beam Radiation Therapy (PBT) versus accelerated conventional photon radiation therapy (XRT) inclusive of the regional lymph nodes.

Participants will be randomized into one of two study groups: proton beam radiation therapy versus photon radiation therapy. Randomization means that participants are put into a group by chance.

Study procedures includes screening for eligibility, study treatment visits, Cardiac Magnetic Resonance Imaging (MRI), blood tests, and questionnaires.

The National Cancer Institute, American Society of Clinical Oncology, and Claflin Grant are supporting this research by providing funding for the research study.

Inclusion Criteria:

• ≥18 years of age
• Non-metastatic Breast Cancer patients who are scheduled to receive conventional left-sided or bilateral breast/chest wall RT inclusive of treatment to the internal mammary lymph nodes (IMNs)
• Prior chemotherapy is permitted
• Ability to understand and the willingness to sign a written informed consent document
• No contraindication to MRI
• Patients with right-sided breast cancer or patients with left-sided patients not requiring treatment to the IMNs, but where cardiac anatomy is determined to be unfavorable by the study PI, will be considered eligible.

Exclusion Criteria:

• Person who is pregnant or breastfeeding.
• Patients receiving any other investigational agent will not be excluded from study eligibility, unless the patient is already enrolled in an interventional study evaluating cardiac toxicity. No cytotoxic therapy or radiotherapy may be used during radiation therapy.
• Contra-indication to gadolinium contrast (e.g., chronic kidney disease)
• Contra-indication to radiotherapy (e.g., scleroderma, p53 mutation)

All women of all races and ethnic groups are eligible for this trial.

The Dana-Farber / Harvard Cancer Center encourages and supports the responsible and ethical sharing of data from clinical trials. De-identified participant data from the final research dataset used in the published manuscript may only be shared under the terms of a Data Use Agreement. Requests may be directed to: [contact information for Sponsor Investigator or designee]. The protocol and statistical analysis plan will be made available on Clinicaltrials.gov only as required by federal regulation or as a condition of awards and agreements supporting the research.

Data can be shared no earlier than 1 year following the date of publication

Contact the Partners Innovations team at http://www.partners.org/innovation