Testing MK-3475 (Pembrolizumab) as Adjuvant Therapy for Triple Receptor-Negative Breast Cancer

Inclusion Criteria: * STEP 1 REGISTRATION * Patients must have histologically confirmed estrogen receptor (ER)-, progesterone receptor (PR)- and HER2-negative (triple-negative, TNBC) or ER-, PR- weakly positive and/or HER2- equivocal status and must not have received and not be planning to receive adjuvant anti-HER2 or endocrine therapies after completion of neoadjuvant chemotherapy; patients who are HER2-positive by American Society of Clinical Oncology (ASCO) College of American Pathologists (CAP) guidelines are ineligible; HER2-negative and HER2-equivocal cases as per ASCO CAP guidelines that do not receive HER2-targeted therapy are eligible; patients with weakly ER or PR positive disease, defined as ER and/or PR less than or equal to (=\<) 5% by immunohistochemistry, are eligible if the treating physician considers the patient not eligible for adjuvant endocrine therapy; residual disease must be \>= 1 cm in greatest dimension, and/or have positive lymph nodes (ypN1mi, ypN1, ypN2, ypN3) observed on pathologic exam * NOTE: If the ER and/or HER2 results are discordant between the initial, pre-chemotherapy, and post-chemotherapy surgical tissue, the receptor status of the residual disease has to be used to determine eligibility. Immunohistochemistry (IHC)-positive isolated tumor cells in the lymph node (N0 \[i+\]) are not considered node-positive and these patients also must have \>= 1 cm residual invasive cancer in the breast to be eligible. * Patients must not have meta…