Many people may want to know their individual risk of being diagnosed with cancer. Understanding risk can help people receive appropriate screening and possibly make certain lifestyle choices that may reduce their risk. The terms incidence and prevalence are often used to understand a person’s risk of cancer.

Incidence versus prevalence

Incidence is an estimate of the number of new cases of cancer diagnosed in a given population (for example, all men in the United States) over a specific period of time (typically one year). Expected incidence of cancer cases for the current year is calculated by using the number of cancer cases that occurred each year over a range of years and fitting those numbers to a statistical model, which can forecast the number of occurrences that are expected for the current year. The range may be used differently in different statistical reports and for different types of statistics. For example, in the American Cancer Society’s publication, Cancer Facts & Figures 2008, cancer incidence for 2008 was calculated by using the number of cancer cases from 1995 to 2004.

Incidence is frequently given as an incidence rate that states the number of new cases estimated to be diagnosed per 100,000 people. For example, the 2008 incidence rate for prostate cancer in the United States is 161, which means that almost 161 out of every 100,000 men in the United States are expected to be diagnosed with prostate cancer in 2008.

Incidence is often stated as an age-adjusted incidence rate. The number of people who fall into different age groups varies (for example, there are many more 30 to 40 year olds than 80 to 90 year olds). This is referred to as age distribution. Incidence rates can be adjusted to account for these age distribution differences so that populations can be compared. For example, Florida has a large number of older adults, while most people who live in Alaska, by comparison, are young. Because the incidence of breast cancer increases with age, the annual absolute incidence rate of invasive breast cancer is much higher in Florida than in Alaska. However, from 2000 to 2004, the annual age-adjusted incidence rate (cases per 100,000 women) for Alaska was higher than that of Florida: 119.7 in Florida versus 132 in Alaska.

Prevalence describes the number of people in a specific population that have a certain type of cancer at a specific point in time. While incidence describes the estimated number of new cases of a cancer, prevalence can describe all cases, including newly diagnosed and people who are being treated or who have been treated for cancer in the past. Prevalence can be expressed in terms of an absolute number or as a percentage: for example, the estimated prevalence of ovarian cancer in the United States in 2004 was 172,765 or 0.05%. This means, 172,765, or 0.05%, of the women in the United States were living with or had a history of ovarian cancer.

Prevalence rates express the number of cases of cancer per 100,000 people: the estimated prevalence rate for ovarian cancer in the United States in 2004 was 50. This means, in 2004, 50 out of every 100,000 women were living with or had a history of ovarian cancer.

Incidence and prevalence statistics can be given for large populations, such as all people in the United States, or for more specific population groups, such as only women ages 20 to 24. Large population statistics are usually estimates based on information collected from a smaller sample of the whole population. When these statistics describe particular population groups, they are usually referred to as “specific.” For example, the age-specific incidence rate for breast cancer in 20-year-olds to 24-year-olds is 1.4 (per 100,000 women). Incidence and prevalence statistics may also be given for several cancers combined (such as all types of leukemia), for specific types of cancer (such as chronic lymphocytic leukemiaonly),or for specific stages of a type of cancer (such as stage III chronic lymphocytic leukemia).

Incidence and prevalence statistics can also be used to describe cancer risk factors; a risk factor is anything that increases a person’s chance of developing a type of cancer. For example, it is known that genetic mutations in either one of two specific genes, BRCA1 and BRCA2, are associated with increased breast cancer risk. It is estimated that the prevalence of mutations of one of these two genes is less than 1%. This means that less than 1% of women have a mutated BRCA1 or BRCA2 gene. However, the prevalence of a BRCA gene mutation among women with breast cancer is approximately 5% to 10%. The increased prevalence of BRCA gene mutations among women with breast cancer means that a woman with a BRCA gene mutation has an increased risk of breast cancer.

Mortality

In cancer statistics, mortality is the number of deaths from cancer during a specific time period. The cancer mortality rate describes the number of deaths from cancer per 100,000 people during a specific time period, usually one year. Mortality rates can be calculated for specific types of cancer and for specific subsets of the population (such as children under 12, smokers, or women with the BRCA1 gene mutation). As with incidence rates, mortality rates can also be given as age-adjusted mortality rates.

Mortality rates can change dramatically with advances in treatment, screening, and prevention. For example, the age-adjusted mortality rate for Hodgkin lymphoma in the United States in the early 1960s was greater than 1.55 (1.55 deaths per 100,000 people). Following the introduction of combination chemotherapy in the late 1960s, the rate dropped to less than 0.5 by the 1990s.

Using the statistics: estimating risk and recommending screening

By looking at the incidence and prevalence statistics for different types of cancer in various groups of people, researchers can estimate which groups of people are at increased risk of developing certain types of cancer. Statistics tell us that older women are at higher risk for breast cancer than younger women, black men are at higher risk for prostate cancer than white men, and people who drink alcohol often are at higher risk for liver cancer than people who don’t drink alcohol.

Risk information from incidence and prevalence statistics is combined with mortality statistics to provide some of the basis for cancer screening recommendations. For example, prevalence and incidence statistics show that colorectal cancer is among the most common cancers in the United States. Age-specific prevalence and incidence rates also show that colorectal cancer is most common in people over age 50. The mortality rates for colorectal cancer show that treatment is much more successful when cancer is found early than if the cancer has spread. Therefore, combining these pieces of information, doctors recommend that routine screening for colorectal cancer begin at age 50 to increase the likelihood of prevention or early detection. Other risk factors, such as family history, presence of other illnesses, and various lifestyle factors, are also taken into account when making screening recommendations specific to an individual. To learn more about how cancer risk is estimated, read the Cancer.Net section on Risk Factors.

Points to remember

• Statistics are estimates that describe trends in large numbers of people. Statistics cannot be used to predict what will actually happen to an individual.
  
  
• Incidence, prevalence, and mortality statistics for different cancer stages, age groups, or time periods can vary dramatically. People are encouraged to ask their doctor for the most appropriate statistics based on their individual medical condition.
  
  
• As with any medical information, talk with your doctor for clarification if cancer-related statistics seem unclear.

For More Information

Understanding Survival Statistics: Guiding Prognosis and Evaluating Treatment

Risk Factors

Prevention