This study shows that in recent years, there are minor differences between the incidence rates presented for the total population of Norway and the rates that are shown if only the Norwegian-born are included. So far, the national incidence rates have provided a good picture of the cancer risk in the Norwegian-born portion of the population. For all types of cancer as a whole, the Norwegian-born have somewhat higher rates than the total population. We saw the greatest differences for melanoma and cancer of the cervix uteri. Since the early 2000s there has been an increase in the incidence of liver cancer in the population, and there is speculation as to whether this increase could be attributable to immigration (3). In this study we saw that there has also been an increase in the incidence of liver cancer among the Norwegian-born, even though the rates were lower than in the total population.
Our previous study has shown that the incidence rates for all types of cancer as a whole are lower among immigrants than in the rest of the population, while immigrants have a higher incidence of lung cancer in men and liver cancer in both genders (2). This study shows that in spite of this observation, the national incidence rates have so far provided a good picture of the cancer risk among the Norwegian-born. The increasing differences between the rates for the Norwegian-born and the total population towards the end of the period nevertheless show that this might change, both because the proportion of immigrants is increasing and because the age composition is changing, whereby the number of elderly people in this group is increasing.
There has been some speculation as to whether the decreasing incidence rate for lung cancer among men could partly be ascribed to a fall in the proportion of men who are smokers, caused by an increase in the proportion of non-smoking immigrants (6). This is not likely, however, since the incidence rate for lung cancer is higher among immigrant men in general (2), and this study shows that the rates of lung cancer declined towards the end of the study period, both in the total population and among Norwegian-born men.
During the period of study, there has been an increase in the incidence rates for cancer of the liver and thyroid gland. Hepatitis B and C are known risk factors for liver cancer (7), and increasing immigration from countries with a high prevalence of this type of cancer has been put forward as a possible explanation of the increasing incidence in liver cancers. However, our results show only a minor difference when comparing the Norwegian-born and the total population. This means that the increase in the national rates for liver cancer is a reality also among the Norwegian-born, and cannot be attributed to increased immigration alone. The incidence of liver cancer is increasing also in the United States, and this increase is observed across ethnicities (8).
Over the last decade, increasing incidence rates for cancer of the thyroid gland have also been observed in the other Nordic countries (9). Our results showed a higher incidence rate for cancer of the thyroid gland in the total female population, but as for liver cancer, we could also see an increase in the Norwegian-born portion of the population. We assume that this increase can be explained by higher rates of detection, and to a lesser extent can be attributed to immigration. Over the period of study, the differences between the Norwegian-born and the total population increased with regard to many of the types of cancer. In the same period, immigrants have accounted for an increasing proportion of the population of Norway. On 1 January 2018, immigrants accounted for 33.1 % of the population of Oslo (10), and it is reasonable to assume that cancer incidence in this group may have an effect on incidence rates in the total population. For example, a review of cancer rates by county in 2013 showed especially low rates of prostate cancer in Oslo (11). This may possibly be due to the city's large immigrant population, which has a low prevalence of prostate cancer (2). This study shows only a negligible difference between the national rate of prostate cancer and the rate among the Norwegian-born.
Although the national rates have until now provided an adequate picture of cancer incidence in Norway, it will be crucial to take the composition of the population into account in future presentations of cancer incidence at the national level, thus to be able to analyse the development of cancer risk in sub-groups of the population. To date, we have had little knowledge about cancer incidence in the Norwegian-born portion of the population. Existing knowledge has been based on stand-alone studies, because country of birth and national background have not been registered as variables in the Cancer Registry of Norway. In light of a proposal for an amendment to the Regulations concerning the Cancer Registry of Norway (12), the Ministry of Health and Care Services has now revised Section 1–7 of the regulations to meet precisely this need (13).
This study population is large, based on the total population of Norway over a given period of time. Data from the Cancer Registry of Norway have been shown to be near-complete and have a high validity (14). They will thus provide a reliable picture of actual cancer incidence among the Norwegian-born as well as the total population. The population in this research study nevertheless differs somewhat from the population that the Cancer Registry of Norway uses in its annual publication of cancer incidence rates in Norway, and the incidence rates in this study are somewhat higher that those that were presented at the national level in 2017 (3).
The incidence of some types of cancer may be low and thus render the rates vulnerable to random variations. We have therefore presented the development in incidence rates over the last 27 years.