When we talk about prevention, we are not referring to the general definition. Many dictionaries define prevention as: “the act of stopping something from happening or of stopping someone from doing something” (Cambridge Dictionary). However, each individual varies on how much risk they face in developing certain cancers. At this moment, we can't eliminate all risks- or stop all cancers from occurring. However, in medical terms, preventive intervention is defined as: “Therapeutic, nutritional, environmental, social and/or behavioral interventions at the societal, community, organizational or individual levels to reduce, modify or stop the course of a disease” (NCI thesaurus). From a medical perspective, prevention intervention is not only concerned with stopping the disease. It also focuses on the ways interventions can help reduce the risks and outcomes of diseases.
So when talking about cancer prevention and ways to reduce risks, we are not guaranteeing that such actions will 100% stop all cancers from developing in every individual. Instead, we are talking about steps you can take to help lower your chances of some sort of cancer developing later in the future (and lead to an overall healthier lifestyle as well).