This study looks at how a drug called pembrolizumab (MK-3475) affects breast cancer tumors in patients with Triple Negative Breast Cancer (TNBC). TNBC is a type of breast cancer that doesn't have estrogen, progesterone, or HER2 receptors. The study is for patients who have not had any treatment for their cancer yet.

The study will test if pembrolizumab changes the immune cells in and around the tumor before surgery. Patients will get two doses of the drug before their surgery to remove the tumor. Researchers also want to know if the drug affects other breast tissue and changes how the body responds to radiation during a surgery-related treatment.

• Participants must be at least 21 years old and have a tumor less than 3 cm in size.
• The study requires no prior cancer treatment and a breast size of at least a B cup for the procedure.
• Participants will need to use contraception and cannot be pregnant or breastfeeding.

This study helps understand if pembrolizumab can change how TNBC tumors behave before surgery.