This study is for older adults with colorectal cancer. The purpose is to determine if low muscle mass plays a role in side effects from chemotherapy.

This study is for patients that have been diagnosed with Merkle Cell Carcinoma. The investigational drug in this study is pembrolizumab. The purpose of this study is to compare the usual approach of post-surgery observation with or without radiation to using pembrolizumab with or without radiation after surgery. Participants can expect to be on this study for a total of 5 years.

This study is for patients that have been diagnosed with Merkel cell carcinoma. The investigational drug in this study is pembrolizumab. This study will compare any good and bad effects of using radiation along with an immunotherapy treatment [MK-3475 (pembrolizumab)] compared with MK-3475 (pembrolizumab) alone. Participants can expect to be in this study for up to 7 years.