BreastVAX: Radiation Boost to Enhance Immune Checkpoint Blockade Therapy

Radiation boost (RT) 7 Gy x 1 fraction on Day -14/Day -7 (arm 1) or Day -7/Day -14

Pembrolizumab infusion flat dosing 200 mg delivered over 30 minutes.

Blood will be collected for laboratory studies and complete correlative studies to examine how the subject's immune system is responding to treatment. On day 0, samples of tumor and any lymph nodes removed from armpit will be analyzed by the UPenn Department of Pathology according to standard practice.