What do the terms incidence and prevalence stand for? What do they measure? What are they used for?
Incidence and prevalence are two statistical measures of disease frequency that apply to any and every disease, although here we’ll just be concerned with cancer.
The World Health Organization (WHO) defines cancer incidence as “the number of new cancers of a specific site/type occurring in the population per year, usually expressed as the number of new cancers per 100,000 population”. Prevalence, on the other hand, includes not only new cases, but all existing cases of a given cancer in a given population at a specified point in time or during a given period.
In fact, the incidence of a type of cancer in a population represents the risk for individuals in that population to develop that cancer, whereas its prevalence reflects the cancer’s burden in that population. Accordingly, epidemiologists use these values for different purposes: incidence can be a tool for identifying risk factors for a cancer in a population, and prevalence can guide the planning of health care facilities based on the number of patients that may exist simultaneously.
Incidence (the risk measure), which relates more to individuals, can vary according to where we live and our lifestyle (environmental risk factors), but also to age, sex, and different genetic factors.
We cannot talk about cancer statistics and not mention cancer mortality, the number of deaths per year per 100,000 population for a given cancer – which can also be subdivided by place of residency, age, sex, etc. Since cancer mortality increases as the population grows older, epidemiologists turn instead to the so-called “age-adjusted mortality” in which the age factor has been taken out of the equation, as an indicator of the progress against cancer, according to the US’s National Cancer Institute.
Every year, the WHO’s Global Cancer Observatory (GCO) platform (formerly known as GLOBOCAN) publishes a rich set of worldwide cancer incidence statistics concerning the previous year, in order to inform cancer control and cancer research. It also provides estimates into the future for all cancers in general and for each type of cancer in particular, for instance by country. “The data presented in the GCO are considered the best available in each country worldwide”, they write on their website, gco.iarc.fr. “The GCO provides global cancer statistics and estimates of incidence and mortality in 185 countries for 36 types of cancer, and for all cancer sites combined”.
This allows experts to rank cancers. Just to give an example, in 2020 the most common cancers worldwide, in terms of new cases (incidence), for both sexes and all ages, were: breast (2.26 million cases), lung (2.21 million cases), colon and rectum (1.93 million cases), prostate (1.41 million cases), skin (non-melanoma) (1.20 million cases),, and stomach (1.09 million cases). For Portugal, only five types appeared at the forefront, and in a different order: colon and rectum (10,501 cases), breast (7,041 cases), prostate (6,759 cases), lung (5,415 cases), and stomach (2,950 cases).
By Ana Gerschenfeld, Health & Science Writer of the Champalimaud Foundation.
Reviewed by: Professor António Parreira, Clinical Director of the Champalimaud Clinical Center.