Effects of MK-3475 (Pembrolizumab) on the Breast Tumor Microenvironment in Triple Negative Breast Cancer

This is an open-label, single-arm, single institution pre-operative "window of opportunity study" of pembrolizumab in subjects with newly diagnosed triple-negative, \< 3 cm, node negative breast cancer. Subjects may not have received any prior neo/adjuvant chemotherapy, definitive surgery or radiation treatment prior to enrollment. The goal of this project is to determine if immune modulation through treatment with MK-3475 (Pembrolizumab) in triple negative breast cancer (TNBC) patients will alter expression of immune tolerant markers \[including PD-L1\], within the primary tumor. Investigators hope to determine if MK-3475 exposure will change the molecular signature of breast stroma from "normal" adjacent breast tissue (non-tumor tissue) obtained prospectively at the time of breast conserving surgery (BCS) in TNBC patients. Finally, investigators will investigate how pembrolizumab alters breast stromal response to high dose radiation delivered by intraoperative radiation therapy (IORT).