Glossary of terms
Material
The first year of cancer registration in Iceland was 1954. Since it was not possible to distinguish between prevalent and new cancers for this first year, published figures start with the year 1955. The number of cancers registered in the ICR is somewhat higher than the number of individuals listed, because the same individual can be diagnosed with more than one cancer.
Incidence
Incidence is the number of new cases arising in a given period in a specified population. This information is collected routinely by cancer registries. It can be expressed as an absolute number of cases per year or as a rate per 100,000 persons per year (see age-specific rate ). The rate provides an approximation to the average risk of developing a cancer.
Mortality
Mortality is the number of deaths occurring in a given period in a specified population. It can be expressed as an absolute number of deaths per year or as a rate per 100,000 persons per year.
Relative survival
Relative survival is defined as the ratio of the observed survival in the group of patients to the survival expected in a group of people in the general population without the disease of interest, who are similar to the patients with respect to sex, age and calendar time at the time of diagnosis.
Prevalence
The prevalence of a particular cancer can be defined as the number of persons in a defined population who have been diagnosed with that type of cancer, and who are still alive at a given point in time. Complete prevalence represents the number of persons alive on a certain day who previously had a diagnosis of the disease, regardless of how long ago the diagnosis was, or if the patient is still under treatment or is considered cured.
Age-specific rate
Age-specific rate is calculated simply by dividing the number of new cancers or cancer deaths observed in a given age category during a given time period by the corresponding number of persons per year in the population at risk in the same age category. For cancer, the result is usually expressed as an annual rate per 100,000 persons at risk.
ASR (age-standardized rate)
An age-standardized rate (ASR) is a summary measure of the rate that a population would have if it had a standard age structure. Standardization is necessary when comparing several populations that differ with respect to age because age has such a powerful influence on the risk of cancer. The ASR is a weighted mean of the age-specific rates; the weights are taken from population distribution of the standard population. The most frequently used standard population is the World Standard Population:
| age | 00-04 | 05-09 | 10-14 | 15-19 | 20-24 | 25-29 | 30-34 | 35-39 | 40-44 | |
|---|---|---|---|---|---|---|---|---|---|---|
| number | 12,000 | 10,000 | 9,000 | 9,000 | 8,000 | 8,000 | 6,000 | 6,000 | 6,000 | |
| age | 45-49 | 50-54 | 55-59 | 60-64 | 65-69 | 70-74 | 75-79 | 80-84 | 85+ | Total |
| number | 6,000 | 5,000 | 4,000 | 4,000 | 3,000 | 2,000 | 1,000 | 500 | 500 | 100,000 |
The calculated incidence or mortality rate is then called World Standardized incidence or mortality rate. It is also expressed per 100,000.
Cumulative risk
Cumulative incidence/mortality is the probability or risk of individuals getting/dying from the disease during a specified period. For cancer, it is expressed as the number of newborn children (out of 100, or 1000) who would be expected to develop/die from a particular cancer before the age of 75 or (80 or 85) if they had the rates of cancer observed in the period. Like the age-standardized rate, it permits comparisons between populations of different age structures.
Pointers regarding tables and charts
Incidence and mortality in tables and charts at the Cancer Registry's homepage are age-standardized, using the World standard population (see above).
Survival-functions can be unreliable due to random variation, for sites with low incidence. For age-specific incidence, figures for the highest age-groups can be unreliable, also caused by random variation due to few cases in the corresponding age-group. Before 1996, mortality caused by leukaemia could not be subgrouped, so mortality due to acute and chronic leukaemia together is shown on mortality chart for acute leukaemia for that period. Before 1971, mortality caused by kidney cancer and carcinoma of the urinary tract could not be distinguished, so for that period the mortality for those sites together is shown on mortality chart for carcinoma of the urinary tract.