Inclusion Criteria:

Men and women age ≥ 18 years old (unless otherwise specified) Willing and able to provide written informed consent/assent ECOG Performance Status 0 - 1

Patients with:

Newly diagnosed breast cancer with tumor size < 5 cm who are not eligible for I-SPY2 (to prevent recruitment competition), or not recommended to undergo standard of care neoadjuvant chemotherapy with at least one of the following features:

• Triple negative breast cancer (TNBC) defined using the ASCO CAP guidelines with the following modification supported by a recent publication as ER ≤ 10%, PR ≤ 10%, HER2- determined by immunohistochemistry and/or fluorescence in situ hybridization analyses and with tumor size ≤ 2.5 cm;
• HR+ HER2- breast cancer regardless of nodal status and age of diagnosis ≥ 50
• HR+ HER2- breast cancer and age of diagnosis <50 with tumor size ≤ 2.5 cm and clinically node (+)
• HR+ or HR- and HER2+ breast cancer with tumor size ≤ 2.5 cm
• Ductal carcinoma in situ (DCIS) with microinvasion

OR

• Locally recurrent breast cancer of any receptor subtype with no prior radiation, not recommended to receive neoadjuvant chemotherapy and expecting surgical excision as part of treatment.

A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies:

1. Not a woman of childbearing potential (WOCBP) as defined in Appendix 2 of the study protocol OR
2. A WOCBP who agrees to follow the contraceptive guidance in Appendix 2 during the treatment period and for at least 120 days for study treatments with risk of genotoxicity after the last dose of study treatment.
   • Female participants of childbearing potential should be willing to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the study through 120 days after the last dose of study medication. Medically accepted methods of birth control include a diaphragm, cervical cap, latex condoms, surgical sterility, intrauterine devices (IUDs), hormonal implants, injectable contraceptives, or birth control pills. Participants of childbearing potential are those who have not been surgically sterilized or have not been free from menses for > 1 year.
   • Ability to tolerate radiation therapy (e.g., lie flat and hold position)
   • Demonstrate adequate hematologic, renal, hepatic, thyroid, and bone marrow function

Inclusion Criteria:

• Men and women age ≥ 18 years old (unless otherwise specified)
• Willing and able to provide written informed consent/assent
• ECOG Performance Status 0 - 1
• Patients with:
  • Newly diagnosed breast cancer with tumor size < 5 cm who are not eligible for I-SPY2 (to prevent recruitment competition), or not recommended to undergo standard of care neoadjuvant chemotherapy with at least one of the following features
    • Triple negative breast cancer (TNBC) defined using the ASCO CAP guidelines with the following modification supported by a recent publication as ER ≤ 10%, PR ≤ 10%, HER2- determined by immunohistochemistry and/or fluorescence in situ hybridization analyses and with tumor size ≤ 2.5 cm;
    • HR+ HER2- breast cancer regardless of nodal status and age of diagnosis ≥ 50
    • HR+ HER2- breast cancer and age of diagnosis <50 with tumor size ≤ 2.5 cm and clinically node (+)
    • HR+ or HR- and HER2+ breast cancer with tumor size ≤ 2.5 cm
    • Ductal carcinoma in situ (DCIS) with microinvasion

OR

• Locally recurrent breast cancer of any receptor subtype with no prior radiation, not recommended to receive neoadjuvant chemotherapy and expecting surgical excision as part of treatment.

  • A female participant is eligible to participate if she is not pregnant (see Appendix 2), not breastfeeding, and at least one of the following conditions

  applies
  1. Not a woman of childbearing potential (WOCBP) as defined in Appendix 2. OR
  2. A WOCBP who agrees to follow the contraceptive guidance in Appendix 2 during the treatment period and for at least 120 days for study treatments with risk of genotoxicity after the last dose of study treatment.

     • Female participants of childbearing potential should be willing to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the study through 120 days after the last dose of study medication. Medically accepted methods of birth control include a diaphragm, cervical cap, latex condoms, surgical sterility, intrauterine devices (IUDs), hormonal implants, injectable contraceptives, or birth control pills. Participants of childbearing potential are those who have not been surgically sterilized or have not been free from menses for > 1 year.

     • Ability to tolerate radiation therapy (e.g., lie flat and hold position)
     • Demonstrate adequate hematologic, renal, hepatic, thyroid, and bone marrow function

Exclusion criteria:

          • A WOCBP who has a positive urine pregnancy test within 72 hours prior to
          allocation. If the urine test is positive or cannot be confirmed as negative, a
          serum pregnancy test will be required.
          • A history of prior radiotherapy to the ipsilateral breast/chest wall that
          precludes delivery of hypofractionated radiotherapy.
          • Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti PD L2 agent
          or with an agent directed to another stimulatory or co-inhibitory T-cell
          receptor (eg, CTLA-4, OX 40, CD137).
          • Has received prior systemic anti-cancer therapy including investigational
          agents within 4 weeks
          • Has not adequately recovered from major surgery or has ongoing surgical
          complications
          • Has a known additional malignancy that is progressing or requires active
          treatment. Exceptions include basal cell carcinoma of the skin, squamous cell
          carcinoma of the skin, or in situ cervical cancer that has undergone
          potentially curative therapy.
          • Has active autoimmune disease that has required systemic treatment in the
          past 2 years except replacement therapy (eg., thyroxine, insulin, or
          physiologic corticosteroid)
          • Participants with vitiligo or resolved childhood asthma/atopy would be an
          exception to this rule. Participants that require intermittent use of
          bronchodilators or local steroid injections would not be excluded from the
          study. Participants with hypothyroidism stable on hormone replacement or
          Sjogren's syndrome will not be excluded from the study. Steroid prep due to dye
          allergies prior to staging scans or use in anti-emetic prophylaxis is allowed.
          • Has a history of (non-infectious) pneumonitis/interstitial lung disease that
          required steroids or has current pneumonitis/interstitial lung disease.
          • Has an active infection requiring systemic therapy.
          • Has a history or current evidence of any condition, therapy, or laboratory
          abnormality that might confound the results of the trial, interfere with the
          participant's participation for the full duration of the trial, or is not in
          the best interest of the participant to participate, in the opinion of the
          treating investigator.
          • Has known psychiatric or substance abuse disorders that would interfere with
          cooperation with the requirements of the trial.
            -  Is pregnant or breastfeeding or expecting to conceive or father children
               within the projected duration of the trial, starting with the
               pre-screening or screening visit through 120 days after the last dose of
               trial treatment.
            -  Has a known history of human immunodeficiency virus (HIV) (HIV 1/2
               antibodies).
            -  Has a known history of Hepatitis B (defined as Hepatitis B surface antigen
               [HBsAg] reactive) or known active Hepatitis C virus (defined as HCV RNA
               [qualitative] is detected) infection.
          Note: no testing for Hepatitis B and Hepatitis C is required unless mandated by
          local health authority.
          • Has received a live vaccine or live-attenuated vaccine within 30 days before
          the first dose of study intervention. Administration of killed vaccines is
          allowed.
          Note: please refer to Section 5.7 for information on COVID-19 vaccines • Has
          severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its
          excipients.
          • Has had an allogenic tissue/solid organ transplant.