BOX 3-1Basic Cancer Population Statistics Defined

Cancer Incidence

Cancer incidence is the number of new cases occurring in a population, expressed either as an absolute number or as a rate. The incidence rate per 100,000 people per year approximates the average risk of developing cancer in a given year and is often used to compare rates over time or across populations. Measurement of incidence rates requires the identification of all new cases of cancer in a defined population, usually in a defined geographic region. The most basic cancer incidence reporting includes information about the person (age, sex, ethnicity) and about the cancer (the date of detection, anatomic site, histology, and the most valid diagnostic method used). The stage of disease at diagnosis (i.e., the extent of disease according to standard definitions) is also a valuable piece of information.

Cancer Mortality

Cancer mortality refers to the number of deaths attributed to cancer in the population, and the cancer mortality rate is the number of deaths per 100,000 people per year. These statistics are usually reported as rates, relating the number of deaths to the underlying population (i.e., the census population). As with incidence, reports can be more and less detailed, but information about the person (age, sex, ethnicity) and the cancer (anatomic site) is very useful.

Mortality is the product of the incidence and fatality from cancer. Fatality is the proportion of people with cancer who die in a given time period, usually a year. It conveys the risk that an individual with cancer will die, while the mortality rate describes the average risk of dying from cancer in the population.

Mortality rates are often used as proxies for cancer incidence, especially where incidence data are not available. For cancers with a poor prognosis everywhere, mortality may, indeed, mirror incidence, and comparisons made across time and place may be valid. In places where people are unlikely to receive curative treatment, even for cancers otherwise considered “curable,” mortality may also be a surrogate for incidence, but comparing across areas may not be straightforward.

A statistic derived from cancer mortality is person-years of life lost (PYLL), which weights deaths at different ages: Death at a young age results in more PYLLs than death in old age. PYLL can be modified further by adding aspects of quality of life, such as a year spent in extreme pain would result in loss at a fraction of a “quality-adjusted life-year” or QALY.

Cancer Survival

Cancer survival describes the proportion of individuals with cancer who are still living for defined periods after diagnosis, often aggregated by type of cancer, age group, sex, and place of residence. This statistic is often referred to as the “survival rate,” although it actually describes an individual’s probability of being alive, and not actually a rate. Cancer survival statistics are derived by calculating the proportion of people originally diagnosed with a type of cancer who are still alive at specified points after diagnosis, such as 1-year survival. For many cancers, 5-year survival is synonymous with “cure,” because relatively few of those surviving 5 years go on to die from the cancer (breast cancer is the most important exception). Survival is influenced strongly by the stage of disease at diagnosis and the availability of effective treatment. If no adjustment for stage at diagnosis is made in calculating survival, people diagnosed at earlier stages will appear to have better survival than those diagnosed with later stage disease, regardless of treatment, but this is simply a statistical artifact. But for cancers for which effective treatments exist, early detection and treatment means a real survival advantage. If existing treatments are not very effective (e.g., for pancreatic cancer, lung cancer, and stomach cancer) or if the person does not have access to medical services, the stage of disease makes little difference in survival. Survival is, therefore, a crude measure of the effectiveness and/or availability of cancer treatment.

Cancer Prevalence

Cancer prevalence indicates the number of people alive with cancer in a population. Unlike incidence and mortality, there is no standard definition of a prevalent cancer case. The most appropriate definition may depend on how the information is going to be used. One approach is to count people with cancer who are in active treatment or follow-up, which has strong economic effects. As a practical matter, this has been interpreted by some as cases within 5 years of diagnosis. However, many cancer survivors live with long-term effects of the disease itself and the treatments, some of which require further management, so an argument could also be made for a more inclusive definition. Cancer prevalence can be calculated from cancer registries with good long-term follow-up, or the more usual, estimated from incidence and survival data.

From: 3, The Cancer Burden in Low- and Middle-Income Countries and How It Is Measured

Cover of Cancer Control Opportunities in Low- and Middle-Income Countries
Cancer Control Opportunities in Low- and Middle-Income Countries.
Institute of Medicine (US) Committee on Cancer Control in Low- and Middle-Income Countries; Sloan FA, Gelband H, editors.
Washington (DC): National Academies Press (US); 2007.
Copyright © 2007, National Academy of Sciences.

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