Niraparib + Dostarlimab In BRCA Mutated Breast Cancer

Inclusion Criteria: * Participants must meet the following criteria on screening examination to be eligible to participate in the study. Laboratory assessments for eligibility must be completed within 14 days prior to the date of registration. Diagnostic imaging, such as MRIs and CT scans, must be performed within 28 days of the planned treatment start. * Participants must have histologically or cytologically confirmed invasive breast cancer Stage I to III with primary tumor size at least 1.0 cm defined by physical exam or imaging (whichever is larger). In the case of a multifocal, multicentric, or bilateral disease, the largest lesion must be ≥ 1.0 cm and designated as the "index" lesion for tumor evaluations. Patients with inflammatory breast carcinoma are not eligible. * Participants must have documentation of estrogen receptor (ER) and progesterone receptor (PR) testing by IHC according to local institutional guidelines in a CLIA-approved setting. Central confirmation of ER/PR status is not required. All tumors must be HER2 negative. * Arms A and B: Target lesion must be ER and PR negative (\<10% staining) by local review. * Arm C: Target lesion must be ER and/or PR positive (\>10% staining) by local review. * Participants must have documented HER2-negative invasive tumor according to local institutional guidelines in a CLIA-approved setting. Central confirmation of HER2 status is not required. HER2 negative is defined as: * 0 or 1+ by IHC, OR * Lack of gene am…