This phase III trial compares the effect of radiation therapy combined with hormonal therapy versus hormonal therapy alone in treating patients with low risk, early stage breast cancer with Oncotype Dx Recurrence =< 18. Oncotype DX is a laboratory test which results in a score that is used to help predict whether breast cancer will spread to other parts of the body or come back. Radiation therapy uses high doses of radiation to kill cancer cells and shrink tumors but may result in some side effects. Hormones called estrogen and progesterone may contribute to the growth of breast tumor cells. Hormone therapy, also called endocrine therapy, may stop the growth of tumor cells by blocking or removing these hormones. This clinical trial may help researchers understand if patients with low-risk, early stage breast cancer who have Oncotype recurrence score of =< 18 can safely omit radiation therapy and only be treated with hormonal therapy without losing any radiation treatment benefit.

This phase II trial studies the effects of niraparib in combination with dostarlimab prior to surgery in treating BRCA-mutated or PALB2-mutated stage I-III breast cancer. Niraparib is a PARP inhibitor, which means that it blocks an enzyme (proteins that help chemical reactions in the body occur) in cells called PARP. PARP helps repair deoxyribonucleic acid (DNA) when it becomes damaged. Blocking PARP may help keep cancer cells from repairing their damaged DNA, causing them to die. PARP inhibitors are a type of targeted therapy. Dostarlimab stimulates the immune system by blocking the PD-1 pathway. The PD-1 pathway controls the bodys natural immune response, but for some types of cancer, the immune system does not work as it should and is prevented from attacking tumors. Dostarlimab works by blocking the PD-1 pathway, which may help your immune system identify and catch tumor cells. Giving niraparib in combination with dostarlimab may work better against the tumor and maximize tumor shrinkage before surgery.