Testing the Addition of an Anti-cancer Drug, Berzosertib, to the Usual Treatment (Radiation Therapy) for Chemotherapy-Resistant Triple-Negative and Estrogen and/or Progesterone Receptor Positive, HER2 Negative Breast Cancer

This phase Ib trial studies the best dose of berzosertib when given together with the usual treatment (radiation therapy) in treating patients with triple negative or estrogen receptor and/or progesterone receptor positive, HER-2 negative breast cancer. Berzosertib may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. Radiation therapy uses high energy rays to kill tumor cells and shrink tumors. Giving M6620 and radiation therapy may kill tumor cells more effectively than radiation alone or shrink or stabilize breast cancer for longer than radiation therapy alone.