Cancer among Circumpolar Inuit 1969-1988. Introduction and methods

Measuring cancer in indigenous populations

It is estimated that there are 370 million indigenous peoples in 90 countries globally. Indigenous peoples generally face substantial disadvantage and poorer health status compared with nonindigenous peoples. Population-level cancer surveillance provides data to set priorities, inform policies, and monitor progress over time. Measuring the cancer burden of vulnerable subpopulations, particularly indigenous peoples, is problematic. There are a number of practical and methodological issues potentially resulting in substantial underestimation of cancer incidence and mortality rates, and biased survival rates, among indigenous peoples. This, in turn, may result in a deprioritization of cancer-related programs and policies among these populations. This commentary describes key issues relating to cancer surveillance among indigenous populations including 1) suboptimal identification of indigenous populations, 2) numerator-denominator bias, 3) problems with data linkage in survival analysis, and 4) statistical analytic considerations. We suggest solutions that can be implemented to strengthen the visibility of indigenous peoples around the world. These include acknowledgment of the central importance of full engagement of indigenous peoples with all data-related processes, encouraging the use of indigenous identifiers in national and regional data sets and mitigation and/or careful assessment of biases inherent in cancer surveillance methods for indigenous peoples.

Does winter darkness in the Artic protect against cancer? The melatonin hypothesis revisited

The melatonin hypothesis states that excess exposure to environmental light may contribute to breast cancer risks via impaired pineal secretion of melatonin. A corollary, not considered previously, is that a net annual increase in oncostatic melatonin would be expected in persons who experience a light deficit during extended winter darkness periods; thus, hormone-dependent cancers should occur less frequently in people who reside north, rather than south, of the Arctic circle. Consistent with our prediction, epidemiological data indicate uniformly low risks for hormone-dependent cancers in the Arctic. The available literature on genetic, reproductive, nutritional, life-style, and environmental risk factors provides no obvious clues to the observed cancer patterns. Moreover, diurnal and 24-hour melatonin concentrations in humans living in Arctic regions were reported as high in November-January, when light intensity is low. Since these observations are consistent with our corollary and the associated prediction, we suggest that research on a melatonin-inhibited carcinogenesis in the low-risk populations of the Arctic should be pursued.

Malignant neoplasms of the lymphatic and haematopoietic system in Circumpolar Inuit

Malignancies of the lymphatic and haematopoietic system in Circumpolar Inuit were studied as part of an international collaboration combining results from cancer registries for Alaska, Canada and Greenland. Low risk was observed for all such malignancies, including non-Hodgkin lymphoma, Hodgkin's disease, multiple myeloma and the combined leukaemias. Standardized incidence ratios (SIRs) of non-Hodgkin lymphoma ranged from 0.3 to 0.5 based on comparison populations in Connecticut (USA), Canada and Denmark. SIR of Hodgkins's disease ranged from 0.1 to 0.2. Acute leukaemias accounted for 78% of all microscopically verified leukaemias and chronic leukaemias for 6%. No clear time trends were observed except for increasing rates of the combined leukaemias among women from the first to the second half of the study period. The epidemiology of haematologic and lymphoproliferative malignancies in Inuit is comparable to results from Chinese and Japanese populations in Asia and may reflect protective environmental or genetic factors for these malignancies.

Cancers of skin, bone, connective tissues, brain, eye, thyroid and other specified and unspecified sites in Inuit

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.

Cancer in Circumpolar Inuit 1969-1988. A summary

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.

Breast cancer in Circumpolar Inuit 1969-1988

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.

Cancers of the buccal cavity and pharynx in Circumpolar Inuit

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.

Cancer of the respiratory system in Circumpolar Inuit

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.

Kidney and bladder cancer in Inuit 1969-1988

The incidence of cancers of the kidney and urinary bladder in Inuit 1969-1988 was studied as part of an international collaboration combining results from cancer registries for Circumpolar Inuit residing in Alaska, Canada and Greenland. Significant high risk of renal cancer was found in Inuit women (SIRs 1.4-2.1) and the age-standardized incidence rate among women was one of the highest on record world-wide. Cancer of the urinary bladder, as opposed to renal cancer, was an uncommon malignancy in both sexes with SIRs of 0.2 to 0.4. No consistent time trend was observed for either renal or bladder cancer in contrast to high and sharply increasing lung cancer rates during the same period. Such results are more likely caused by different latency periods for renal and bladder cancer than for lung cancer following tobacco exposure, possibly combined with the absence of certain occupational exposures relevant to bladder cancer.

Cancer in Circumpolar Inuit. Background information for the cancer pattern in Greenland

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 digestive system in Circumpolar Inuit

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.

Cancer of the male genital tract in Circumpolar Inuit

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 of the female genital tract in Circumpolar Inuit

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.