The number of people living with a type of cancer in a given population, either at a point in time or over a period of time. Prevalence includes both incidence (new cases) and survival (pre-existing cases) - it determines how many people are living with and beyond cancer.
Prevalence can be expressed as counts or proportions. Counts let us know the total number of people in the population who have cancer or had it in the past, while proportions let us know the percentage of people in the population who are have or have had cancer. How the prevalence proportion is reported depends on how common the disease is. For example, prevalence proportions for common diseases may be reported as a percentage (per 100 people) or per 1,000 people, while rarer conditions will be reported per 1 million people. Commonly, cancer prevalence proportions are reported per 100,000 people.