This clinical trial is focused on studying a type of cancer called Stage III-IVA Resectable Locoregionally Advanced Head and Neck Squamous Cell Carcinoma (LA HNSCC). The study is investigating the use of a treatment called pembrolizumab, which is also known by its code name MK-3475. Pembrolizumab is being tested as a therapy given before surgery (neoadjuvant therapy) and in combination with standard treatments after surgery (adjuvant therapy). The standard treatments include radiotherapy, which uses high-energy rays to kill cancer cells, and may also include a drug called cisplatin, which is a type of chemotherapy that helps stop the growth of cancer cells.

The purpose of the study is to compare the effectiveness of pembrolizumab combined with radiotherapy and possibly cisplatin, against the standard treatment of radiotherapy with or without cisplatin. The study will look at how long participants remain free from cancer events, which means the cancer does not return or worsen. Participants in the study will receive their treatments through an intravenous infusion, which means the medication is given directly into a vein.

Throughout the study, participants will be monitored to see how their cancer responds to the treatment and to check for any side effects. The study aims to understand if adding pembrolizumab to the standard treatment can improve outcomes for patients with this type of head and neck cancer. The trial will also assess other factors such as overall survival, which is the length of time participants live after starting the study, and changes in quality of life, which refers to the overall well-being of participants during and after treatment.