Prevalence and Incidence

Home > Clinical Concepts In Radiation Oncology > Patient Evaluation > Epidermic and Etiology > Prevalence and Incidence

Prevalence:

   Cancer incidence refers to the number of new cancer cases arising in a specified period of time, cancer prevalence refers to the number of people who have previously received a diagnosis of cancer and who are still alive at a presented time point. Some of these patients will have been cured and others will not. Therefore prevalence reflects both the incidence of cancer and its associated survival pattern.

Overall statistics:

   In 2008, it was estimated that there are just over two million people living with or beyond cancer in the UK who had previously been diagnosed, and this is predicted to rise by more than 3% a year1. Prevalence figures are influenced by both incidence and survival. Thus, the most prevalent types of cancer are those with a relatively high incidence rate and a good prognosis. In the UK the most prevalent cancer in males is prostate cancer and in females it is breast cancer.

The latest analysis shows that at the end of 2006, there were over 200,000 prevalent cancer patients in the UK who were alive one year after their diagnosis. In total, there were 1.13 million cancer survivors in the UK who were alive up to 10 years from diagnosis at the end of 2006.2
These latest estimates are much higher than previous forecasts of cancer prevalence. This is mainly because incidence has been rising whilst the death rates have continued to fall, leading to better survival. This trend is expected to continue over the coming years as a result of a number of factors, including an ageing population, earlier detection of cancer and continued improvements in treatment.

Incidence:

   A cancer incidence rate is the number of new cancers of a specific site/type occurring in a specified population during a year, usually expressed as the number of cancers per 100,000 population at risk. That is,

Incidence rate = (New cancers / Population) × 100,000

The numerator of the incidence rate is the number of new cancers; the denominator is the size of the population. The number of new cancers may include multiple primary cancers occurring in one patient. The primary site reported is the site of origin and not the metastatic site. In general, the incidence rate would not include recurrences. The population used depends on the rate to be calculated. For cancer sites that occur in only one sex, the sex-specific population (e.g., females for cervical cancer) is used.

An age-adjusted rate is a weighted average of the age-specific rates, where the weights are the proportions of persons in the corresponding age groups of a standard population. The potential confounding effect of age is reduced when comparing age-adjusted rates computed using the same standard population.

Questions:

1. What is the specified population to calculate the incidence rate?