The purpose of this study is to compare the usual approach (no more treatment or additional chemotherapy and radiation therapy after surgery or radiation therapy alone after surgery), with any effects (good or bad) of receiving one year of therapy with the experimental drug MK-3475 (also called pembrolizumab) after surgery. This study will allow the researchers to know whether treatment with MK-3475 (pembrolizumab) is better, the same, or worse than the usual approach alone. There will be about 1,000 people taking part in this study. 

This study has two study groups (also called study “arms”). A computer will by chance assign you to one of the two study arms. This is called randomization. This is done by chance because no one knows if one study arm is better, the same, or worse than the other arm. Once you are put on one arm, you cannot switch to the other arm. Neither you nor your doctor can choose which arm you will be in. 

If you are in Arm 1, you will get observation with close clinical monitoring. Your doctors may recommend postoperative radiotherapy, or more post-operative chemotherapy, as part of your usual treatment. This treatment plan must be determined by you and your physician before entering the trial. If you are assigned to Arm 1, you may not receive any other treatment that is not part of the study. During the study, radiation therapy are allowed as part of your treatment plan. You will have visits with your physician every 12 weeks for one year. 

If you are in Arm 2, you will receive MK-3475 (pembrolizumab) by intravenous infusion over a 30-minute period. You will not have to be hospitalized for this infusion unless your doctor feels it is needed. You will receive MK-3475 (pembrolizumab) infusions every three weeks for one year and will be seen by your physician every six weeks during treatment. If your doctor recommends additional therapy after surgery as part of your routine care, you may also receive radiation therapy and at the same time as you receive MK-3475 (pembrolizumab).