Inclusion Criteria:

• Males or females age ≥ 18 years
• Patient has non-metastatic, histologically confirmed primary or locoregionally recurrent estrogen receptor (ER) ≤ 10%, progesterone receptor (PR) ≤ 10%, and HER2-negative breast cancer (triple negative breast cancer [TNBC]) either using the baseline biopsy specimen or the post-neoadjuvant chemotherapy (NAC) residual surgical specimen and residual cancer burden (RCB)2 or RCB3, as defined by Symmans et al., 2007, and received neoadjuvant anthracycline and/or taxane-based chemotherapy OR patient has non-metastatic, histologically confirmed primary ER > 10% and/or PR > 10%, HER2-negative breast cancer with RCB3 and received neoadjuvant anthracycline and/or taxane-based chemotherapy OR patient has locoregionally recurrent TNBC or ER > 10% and/or PR > 10%, HER2-negative breast cancer.
  • Note: The RCB can be calculated at http://www3.mdanderson.org/ap/medcalc/index.cfm?pagename=jsconvert2
  • Note: Results from any CLIA-certified lab are acceptable for the purpose of determining study eligibility.
  • Note: For patients with primary breast cancer, there is no minimum number of neoadjuvant cycles required provided the patient received an anthracycline or taxane preoperatively. Patients with locoregionally recurrent breast cancer are not required to have received preoperative chemotherapy.
• Patient has undergone total mastectomy or wide local excision with axillary staging, and the margins of the resected wide local excision or mastectomy specimens are free of invasive tumor and ductal carcinoma in situ (DCIS) or patient has undergone axillary surgery for regionally recurrent breast cancer. Unresected axillary level III, internal mammary, and supraclavicular nodal disease is permitted.
  • Note: For patients who have undergone mastectomy, immediate reconstruction is allowed.
• Patients must have completed their final breast surgery including re-excision of margins for invasive cancer and DCIS, or received their last adjuvant chemotherapy infusion,  within 90 but not fewer than 21 days prior to registration unless patient received postoperative chemotherapy in which case patients must have completed their adjuvant chemotherapy within 90 days but not fewer than 28 days prior to registration. but no sooner than 21 days prior to the initiation of RT for surgery or 28 days for chemotherapy. Post-radiotherapy adjuvant systemic therapy (e.g. adjuvant capecitabine) may not be completed less than 21 days prior to the first fraction of RT or begin less than 28 days from the last fraction of RT.
• Patient must have recovered from surgery with the incision completely healed and no signs of infection prior to registration.
• Patients must be proceeding with breast/chest wall and regional nodal irradiation including internal mammary node treatment. BFor patients with bilateral breast cancer is permitted provided that , RT ismust be indicated and administered only to one side.
• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2
• Willing to provide tissue and blood samples for correlative research:
  • Leukocytes ≥ 3,000/mcL;
  • Absolute neutrophil count ≥ 1,500/mcL;
  • Platelets ≥ 100,000/mcL;
  • Total bilirubin ≤ 1.5 x institutional upper limit of normal (ULN);
  • Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase [SGOT])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase [SGPT]) ≤ 3 x institutional ULN;
  • Creatinine ≤ institutional ULN; OR
  • Glomerular filtration rate (GFR) ≥ 60 mL/min/1.73 m^2 for patients with creatinine levels above ULN.
• Human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral therapy with undetectable viral load within 6 months are eligible for this trial.
• For patients with evidence of chronic hepatitis B virus (HBV) infection, the HBV viral load must be undetectable on suppressive therapy, if indicated.
• Patients with a history of hepatitis C virus (HCV) infection must have been treated and cured. For patients with HCV infection who are currently on treatment, they are eligible if they have an undetectable HCV viral load.
• Negative urine or serum pregnancy test for individuals of childbearing potential.
  • Note: The effects of berzosertib on the developing human fetus are unknown. For this reason and because DNA-damage repair inhibitors as well as radiation used in this trial are known to be teratogenic, women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry and for the duration of study participation, and for 6 months after completion of M6620 administration. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately.  Men treated or enrolled on this protocol must also agree to use adequate contraception prior to the study, for the duration of study participation, and 6 months after completion of berzosertib administration.
• Ability to understand and the willingness to sign a written informed consent document.

Exclusion Criteria:

• Patients who have had chemotherapy within 4 weeks prior to entering the study.
• Prior RT to the ipsilateral chest wall or ipsilateral breast or thorax. Individuals with prior RT to the contralateral breast or chest wall are eligible.
• Patients who have not recovered from adverse events due to prior anti-cancer therapy (i.e., have residual toxicities > grade 1) with the exception of alopecia and grade 1-2 taxane-induced neuropathy which will be permitted.
• Patients who are receiving any other investigational agents or concomitant anti-neoplastic treatment, except endocrine therapies and bisphosphonates which are permitted without restriction even during protocol treatment. Postoperative chemotherapy is allowed but must be discontinued >28 days prior to registration.
• History of allergic reactions attributed to compounds of similar chemical or biologic composition to berzosertib.
• Berzosertib is primarily metabolized by CYP3A4; therefore, concomitant administration with strong inhibitors of CYP3A4 (e.g., ketoconazole, itraconazole, clarithromycin, ritonavir, indinavir, nelfinavir, and saquinavir) or inducers of CYP3A4 (e.g., rifampin, phenytoin, carbamazepine, phenobarbital, St. John's wort) should be avoided. Patients requiring any medications or substances that are strong inhibitors or inducers of CYP3A during the course of the study and for 14 days prior to enrollment are ineligible. Because the lists of these agents are constantly changing, it is important to regularly consult a frequently-updated medical reference. As part of the enrollment/informed consent procedures, the patient will be counseled on the risk of interactions with other agents, and what to do if new medications need to be prescribed or if the patient is considering a new over-the-counter medicine or herbal product.
• Patients with uncontrolled intercurrent illness. This includes but is not limited to, ongoing uncontrolled serious infection requiring IV antibiotics at the time of registration, symptomatic congestive heart failure, unstable angina pectoris, symptomatic/uncontrolled cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.
• Active systemic lupus, scleroderma, or dermatomyositis with a CPK level above normal.
• Pregnant women are excluded from this study because berzosertib as a DNA damage repair inhibitor may have the potential for teratogenic or abortifacient. Because there is an unknown but potential risk for adverse events in nursing infants secondary to treatment of the mother with berzosertib, breastfeeding should be discontinued if the mother is treated with berzosertib.
• Patients with known hereditary syndromes predisposing to radiosensitivity such as Li Fraumeni syndrome and ataxia telangiectasia are excluded from the study. Patients with mutations in breast cancer predisposition genes such as BRCA1, BRCA2, PALB2, CHEK2, and ATM are eligible.
• Patients with a prior or concurrent malignancy, excluding non-melanoma skin cancers and non-invasive cancers whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen.