The central guardian of participants' safety and welfare in clinical trials that are federally funded (which includes NCI-sponsored trials) is the Office for Human Research Protections (OHRP), a part of the U.S. Department of Health and Human Services). OHRP is responsible for enforcing the Protection of Human Subjects part of the Code of Federal Regulations (Title 45, Part 46). If the trial is evaluating a new drug, medical device or other product that is subject to approval by the U.S. Food and Drug Administration (FDA), then the FDA's Office of Human Affairs is responsible for ensuring that it meets the Code of Federal Regulations (see the FDA's A Guide to Informed Consent).
If the trial falls into both categories, then it is subject to both sets of regulations, which actually are quite similar. Both include guidelines regarding the role of local Institutional Review Boards (IRBs), the formal body in the community charged with overseeing medical research involving people. Understanding the work of IRBs will help you see that there are outside panels of experts monitoring the investigators who conduct clinical trials -- and representing the interests of those who participate.