Using Incidence, Prevalence, and Mortality Statistics to Evaluate Risk and Recommend Screening Tests
Terms such as incidence and prevalence are typically used to describe an individual’s risk of developing cancer.
Incidence vs. prevalence
Incidence provides an estimate of the total number of new cancer cases that are diagnosed in a specific population group (for instance, the entire male population in the country) over a predetermined period of time (usually one year). To calculate the expected incidence of cancer cases, as relevant to the current year, researchers use the total number of cancer cases recorded over a specified number of years. The available data is then analyzed using a statistical model, allowing researchers to forecast the number of expected cancer cases for the current year. The specified time duration may be used in a different manner for different statistical reports and for varying statistical methodologies. For instance, in the Cancer Facts & Figures 2008 (published by the American Cancer Society), incidence of cancer cases for 2008 was assessed using the total number of cancer cases that occurred between 1995 and 2004.
Incidence is usually expressed as an incidence rate that describes the total number of potential cancer cases that may be diagnosed per 100,000 individuals. For instance, the incidence rate of prostate cancer for year 2008 is 161 in the United States. This implies that around 161 in every 100,000 men residing in the United States may be diagnosed with prostate cancer in any given year. The U.S. incidence rate for mesothelioma is 0.9 per 100,000. This rate is expected to peak and plateau in the next few years and then slowly deline, mirroring the trend in decreased asbestos usage and better workplace safety standards.
Incidence is usually described as an age-adjusted incidence rate. The total count of individuals belonging to different age groups varies (for instance, the total number of 30-40 year olds is greater than the total number of 80-90 year olds). This is known as age distribution. In order to compare different population groups, adjustments need to be made in the incidence rates based on the age distribution differences. For instance, there are many older adults in Florida and in comparison the population in Alaska is mostly young. Since the probability of developing cancer increases with age, Florida has a much higher annual absolute incidence rate of cancer in comparison to Alaska. However, during the period 2000 to 2004, Alaska recorded a higher annual age-adjusted incidence rate for breast cancer (total number of cases for every 100,000 women) than Florida (Alaska: 132 vs. Florida: 119.7).
Prevalence refers to the total number individuals belonging to a specific population group that have a particular form of cancer at a given point of time. In comparison to incidence that provides an estimate of the expected number of new cancer cases, prevalence can include all cancer-related cases such as newly diagnosed cases, individuals currently receiving treatment, or those who may have received cancer treatment in the past. Prevalence may be described either as an absolute number or in terms of percentage. For instance, the estimated prevalence rate for ovarian cancer in 2004 was 172,765 or approximately 0.05% in the United States. It implies that around 172,765 or 0.05% of the woman population in the United States were living with ovarian cancer or had a history of the same.
Prevalence rates describe the total number of cancer cases per 100,000 individuals. For example, in 2004, the estimated prevalence rate for ovarian cancer was 50 in the United States. It implies that 50 out of every 100,000 women, in 2004, were living with ovarian cancer or had a history of that form of cancer.
Statistics related to incidence and prevalence can be applied to large populations, for instance, the entire population of the United States, or smaller population groups, for example, only women who are 20-24 years old. Statistics related to large population groups are often based on data and information gathered from a smaller sample selected from the entire population. In case these statistics relate to any particular population groups, they are generally called “specific”. For instance, the incidence rate (age-specific) for breast cancer in the 20-24 year age group is 1.4 for every 100,000 women. Statistics that describe incidence and prevalence can also represent several different cancers combined (for example, all forms of leukemia), any particular type of cancer (for example, chronic lymphocytic leukemia), or different stages of a particular type of cancer (for example, stage III of chronic lymphocytic leukemia).
Cancer risk factors can also be described using incidence and prevalence statistics (a risk factor is something that increases an individual’s chances of developing a particular form of cancer). For instance, there is evidence that genetic mutations in any one of the genes – BRCA1 and BRCA2 – increase the risk of developing breast cancer. Further, it is known that prevalence of mutations in either one of these two specific genes is less than 1%. It implies that mutation in BRCA1 or BRCA2 gene occurs in less than 1% of women. However, among women with breast cancer, the prevalence of this specific gene mutation ranges from 5-10%. The high prevalence of this gene mutation amongst women diagnosed with breast cancer indicates that a woman with this type of gene mutation has a higher risk of developing breast cancer.
In cancer-related statistics, mortality refers to the total number of deaths that occur due to cancer over a specific period of time. The cancer mortality rate is used to describe the total number of deaths (per 100,000 individuals) that occur due to cancer over a specific period of time (generally one year). Cancer mortality rates can be evaluated for specific forms of cancer and for specific population groups (for example, children below 12 years of age, or women identified with the BRCA1 gene mutation). Similar to incidence rates, mortality rates can also be described as age-adjusted mortality rates.
With advancements in preventive measures, screening methodologies and treatment procedures, mortality rates have changed significantly over the years. For instance, in the early 1960s, the mortality rate (age-adjusted) for Hodgkin lymphoma was more than 1.55 (1.55 deaths for every 100,000 individuals) in the United States. By the 1990s this rate had dropped to less than 0.5, largely due to the introduction of new treatment procedures such as combination chemotherapy during the late 1960s.
Risk Estimation and Screening Recommendations based on Statistics
Using incidence and prevalence statistics, as relevant to various forms of cancer in several different population groups, researchers can find out which specific groups of individuals have an increased risk of developing a particular type of cancer. Statistics reveal that older women face a higher risk of breast cancer in comparison to younger women; black men are more prone to developing prostate cancer as compared to white men; and individuals who drink alcohol are more likely to develop liver cancer in comparison to those who don’t drink alcohol.
Specialists called epidemiologists use incidence and prevalence statistics along with mortality statistics to develop cancer screening recommendations. For instance, incidence and prevalence statistics indicate that colorectal cancer is one of the most commonly occurring cancers in the United States. Age-specific incidence and prevalence rates also reveal that colorectal cancer is most prevalent amongst individuals who are above 50 years of age. The mortality rate for colorectal cancer indicates that the chances of successful treatment are higher in case of an early diagnosis and not when the cancer may have spread. Using this information, doctors recommend that regular screenings for colorectal cancer should be started at age 50 in order to increase the chances of prevention or to achieve an early diagnosis. Other risk factors, for example, family history, diagnosis of other types of illnesses, and several different lifestyle factors, are also considered while making screening recommendations as relevant to an individual.
Important points to remember:
- Statistics only provide an estimate of trends prevalent amongst large population groups. They are not indicative of what actually may happen to an individual.
- Incidence, prevalence and mortality statistics for different stages of cancer, age groups, or time durations, can vary significantly. People need to talk to their doctor about the most appropriate statistics, as relevant to their specific medical condition.
- As with any type of medical information, ask your doctor to provide a clarification in case cancer-related statistics appear unclear to you.
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