This study investigates treatment options for patients with head and neck cancer. The purpose is to compare two different approaches used after surgery. One approach involves using radiation therapy by itself, while the other combines radiation therapy with an investigational chemotherapy medication. Head and neck cancer refers to cancers that develop in the mouth, throat, or other areas in the head and neck region.

Participants will be assigned to one of two study arms. One arm will receive only radiation therapy, and the other will receive both radiation therapy and the investigational chemotherapy medication. The treatment will last for 6 weeks, and doctors will monitor participants' conditions for 10 years after the treatment period.

• Who can participate: Adults aged 18 and older with squamous cell carcinoma of the head or neck that has been surgically removed are eligible. Individuals with recurrent disease, multiple primaries, or women who are pregnant or breastfeeding are not eligible.
• Study details: Participants will receive either radiation therapy alone or in combination with the chemotherapy medication cisplatin. A placebo is not used in this study, meaning all participants will receive active treatment.