Breast cancer was studied over a 20-year period in Inuit populations in the Circumpolar region. A total of 193 breast cancers were observed in women. The incidence increased from 28.2 per 100 000 in 1969-1973 to 34.3 per 100 000 in 1984-1988. However, the incidence is low, about half what could be expected based on the rates in Denmark, Canada and Connecticut (USA). The low incidence could be explained by the Inuit diet and other lifestyle factors. These benefits should be preserved, in particular in the young, to maintain a low breast cancer incidence.
Cancer incidence data for Circumpolar Inuit populations were developed and compiled from Greenland, Canada and Alaska from 1969 to 1988 to provide the largest possible base of data for documenting the unusual patterns of cancer previously reported for these populations. Cancer incidence and population data were transferred to the Danish Cancer Registry. Coded information from various ICD-classifications and codes for the basis of diagnosis were transformed to one format, enabling joint analysis. Standard descriptive analysis was carried out with presentation of number of cases, crude incidence rates (CR), age-standardized rates (world) (ASR), cumulative rates to age 64 years, and indirectly standardized ratios (SIR) to the populations of Connecticut (USA), Canada and Denmark. The resulting database can be used to support collaborative international research among the Inuit populations.
The results of an international, collaborative study of cancer in Circumpolar Inuit in Greenland, Canada, Alaska and Russia are summarized. A total of 3 255 incident cancers were diagnosed from 1969 to 1988 among 85 000-110 000 individuals. Indirect standardization (SIR) based on comparison populations in Connecticut (USA), Canada and Denmark showed excess risk of cancer of the lung, nasopharynx, salivary glands, gallbladder and extrahepatic bile ducts in both sexes, of liver and stomach cancer in men, and renal and cervical cancer in women. Low risk was observed for cancer of the bladder, breast, endometrium and prostate, and for non-Hodgkin lymphoma, Hodgkin's disease, leukaemia, multiple myeloma and melanoma. Age-standardized incidence rates (ASRs) of cancer of lung, cervix, nasopharynx and salivary glands among Inuit were among the world's highest as were rates in women of oesophageal and renal cancer. Regional differences in ASRs within the Circumpolar area were observed for cancer of the cervix, lung, colon and rectum, liver, gallbladder and breast. The differences in the Inuit cancer incidence pattern to some extent reflect known variations in lifestyle, diet and other exposures, as well as implementation of cancer control measures. Future research addressing possible individual differences are needed to evaluate environmental and genetic factors in etiology and evaluate intervention studies.
The cancer pattern among Inuit in the Circumpolar area have shown marked differences to other populations in the world. The current paper summarises important risk factors in Greenland, including the physical environment, diet, alcohol, tobacco and other lifestyle factors. Details on population structure and history, health care and cancer registration are also included. This information is important for the interpretation of the incidence pattern for the Circumpolar Inuit collectively and for the understanding of differences between the various Inuit populations of the North.
Cancer of the oesophagus, stomach, small intestine, colon, rectum, liver, gallbladder, biliary tract and pancreas was studied in the Inuit populations of Alaska, Canada and Greenland. Indirect standardization to the populations in Canada, Connecticut (USA) and Denmark was used. High risk of oesophageal cancer was observed in both sexes with standardized incidence ratios (SIRs) of up to 7. An increased risk of colon and rectum cancer occurred among Alaskan Inuit compared with the Inuit populations in Canada and Greenland, which had lower rates. Liver and gallbladder cancer rates were high, with SIRs of 1.5 to 4.1, whereas there were no differences in pancreatic cancer in the populations compared. Dietary habits, alcohol and tobacco consumption are believed to play an important role in most of the observed cancer patterns, but for liver cancer hepatitis B virus infection is also believed to have a causal role.
Cervical cancer incidence among Inuit is high. Especially women from Greenland exhibit rates which are among the highest in the world. Compared with women in Denmark, USA and Canada, Inuit women have a 3-4 time higher cervical cancer risk. By contrast, the incidence of uterine corpus cancer is low in the Circumpolar area. Both in Greenlandic and Canadian Inuit women, ovarian cancer rates are similar to those in Danish women and non-Inuit women from Canada respectively. Only 9 cases of placenta cancer were recorded in the Circumpolar area during the 20 years of observation. Compared with available incidence rates for Denmark the incidence in Greenland was significantly higher.
In an international collaboration project we combined cancers of the male genital tract among Inuit identified from routine cancer registry systems in the Circumpolar region (Alaska, Canada and Greenland) and compared incidence rates with rates in Denmark, Connecticut (USA) and Canadian non-Inuit. We observed a low risk of prostate cancer (standardized incidence ratio (SIR) 0.2-0.3) and the incidence rate of 7.8 per 100 000 (world standard) is among the lowest in the world. Dietary and not diagnostic factors are likely explanations of this finding. Testicular cancer also occurred with low rates (SIR 0.3-0.7) although only significantly so when compared with Denmark and Connecticut (USA) which have some of the world's highest incidence rates of this cancer. Penile cancer occurred with relatively high risk (SIR 1.8-3.0) based on rates among non-Inuit. The incidence is, however, lower than anticipated considering the possibility for shared risk factors with cancer of the uterine cervix.
Cancer incidence of the nasal cavities in Inuit men are high (ASR=3.0 1984-1988), and higher than seen in Denmark, Connecticut (USA) and Canada. Lung cancer incidence is among the highest in the world, for both men and women, and larynx cancer among the lowest. The smoking pattern among Inuit, possibly combined with co-factors related to environment and diet, are believed to be the relevant causal factors.
Low rates of skin cancer, both melanoma and non-melanoma, were observed in Inuit after 20 years of observation. Tumours of the brain and central nervous system, of the thyroid, bone and connective tissues and other specified sites occurred with rates similar to those in comparison populations in Denmark, Connecticut and Canada. These findings support that neither UV and ionizing radiation from nuclear fall-out, nor pollution of herbicides and pesticides in the Arctic area have yet had any noticeable impact on cancer risk. However, unspecified and secondary neoplasms constitute 7-8% of the total Circumpolar cancer incidence and the pattern of rare cancers must be interpreted with caution. Increased diagnostic efforts with a higher precision in the future are warranted.
In an international collaboration, cancers of the buccal cavity and pharynx were combined from cancer registries in the Circumpolar region, including Alaska, Canada and Greenland. Low risk of cancers of the lip (SIR 0.2) was observed among Inuit. Increased risk of cancer of the tongue and oral cancer (SIR 2.5) were observed among Greenlandic Inuit men. Salivary gland cancer and nasopharyngeal cancer occur among Inuit with rates among the highest in the world. Environmental factors (EBV, diet) and a genetically susceptible population are believed to play a role.