Cancer incidence is defined as the number of new cases of cancers in a specific period of time. Incidence is often expressed as a rate (number of new cases per 100 000 persons per year). Recurrence or progression of the disease is not defined as a new case.
The Cancer Registry of Norway's purpose is, amongst others, to gather information about cancer cases in Norway in order to map the extent of cancer and keep track of changes over time. There is often an interest in the number of new cases (or frequency), also referred to as cancer incidence. This can be measured in absolute numbers within a given period of time, or as incidence rates taking the size of the population into consideration. The Cancer Registry of Norway generally gives the rate per 100 000 persons per year.
The number of cancer cases is dependent on the size and age of the population. The greater and older the population, the larger the number of cancer cases. In order to compare the incidence of cancer between periods or geographical areas, age standardised rates are used. This is to take into account the fact that the size and age structure of the populations compared, are not necessarily identical.
Although cancer is a frequent disease when looking at the entire population throughout their lifespan, the disease occurs rarely if looking at a few cancers sites, a few years and/or a smaller geographical areas. Random variations in where and when the disease occurs may cause great inequality in the incidence of a cancerous form from one year to another. In addition, the figures for the past year may be missing cases due to delayed reporting. Such random variations strongly affects rare cancer sites, small populations, short periods of time or small geographical areas the most.
In order to present larger and more stable numbers, the Cancer Registry of Norway often presents cancer incidence numbers in 5-year periods. The use of 5- or 10-year periods is recommended in order to look at statistics in smaller areas, in age groups with few cases or statistics comprising of rare or small diagnosis groups.
Taken anonymity in consideration, groups that are too small will be removed or marked as “X”.