Long-term trends in cancer mortality for Indigenous Australians in the Northern Territory

Falling mortality rates over recent decades for stomach cancer, lung cancer in men, cervical cancer and, more recently, breast and colorectal cancers are evidence of the effectiveness of cancer prevention and treatment for most Australians.1 To what extent these trends apply to Indigenous Australians is uncertain, because until now studies of Indigenous cancer mortality have been for restricted periods of time, or time trends have not been examined in detail.2,3 This is the first detailed report of long-term trends in Indigenous cancer mortality.

There were 484 cancer deaths in NT Indigenous residents in 1991–2000. During this period, cancer mortality rates in the NT Indigenous population were higher for cancers at several sites, and lower for a small number of sites, compared with those for the total Australian population; mortality rate ratios ranged from 9.4 for thyroid cancer to 0.1 for kidney cancer (Box 1). There were no deaths from melanoma in the Indigenous population. For most cancer sites, the mortality rate ratio was higher in younger than older people for most cancers, this difference being statistically significant for six cancers (Box 1).

Between 1977 and 2000, cancer mortality in the NT Indigenous population increased for eight cancer sites and fell for three; the rise was statistically significant for cancers of the oropharynx, lung and pancreas (Boxes 2, 3 and 4). Lung cancer was by far the commonest registered cause of cancer death in NT Indigenous people and mortality from lung cancer doubled between 1977 and 2000. In contrast, lung cancer mortality in all Australians increased by 55% in women and fell by 26% in men over the same period (Box 3).

The breast cancer mortality rate in NT Indigenous people was lower than the total Australian rate, except in 1997–2000, and showed no clear trends (Box 4). In contrast, the NT Indigenous mortality rate for cancer of the cervix was much higher than the total Australian rate, but fell in 1997–2000 to under half the previous rate, although the 95% confidence interval for the annual percentage change was wide (Box 2).

Mortality rates in the NT Indigenous population for all smoking-related cancers more than doubled from 57.4 per 100 000 in 1977–1982 to 117.3 per 100 000 in 1997–2000, showing a greater increase than mortality rates for all non-smoking-related cancers combined (Box 5). Lung cancer, which was the cause of over half of the smoking-related cancer deaths, alone accounted for 21% of all NT Indigenous cancer deaths in 1977–1981, increasing to 26% in 1997–2000, compared with a decrease from 21% to 19% for the total Australian population.

Higher cancer mortality rate ratios for younger than older Indigenous people are particularly marked for smoking-related cancers, and probably a consequence of a comparatively recent uptake of smoking by NT Indigenous people,9,10 and the higher levels of tobacco consumption by Indigenous teenagers and young adults in recent decades.11 A “healthy survivor” effect may also play a part if early mortality from other smoking-related diseases lowers the proportion of smokers, particularly heavy smokers, who survive past 65 years of age.

On a more positive note, Indigenous cancer mortality is persistently low for cancers of the colon and rectum, kidney, prostate and melanoma. As Indigenous lifestyles change, mortality from some of these cancers might increase, but this is not yet apparent in the NT.

There are several potential sources of inaccuracy in our findings. Some Indigenous people may be misclassified in death records as non-Indigenous, resulting in incorrectly low Indigenous mortality rates. However, comparison of the cancer mortality rates in the non-Indigenous population of the NT with those in the total Australian population (data not shown) shows that the rates are, if anything, only a little higher than would be expected. Thus, any misclassification of Indigenous status is small and with no apparent time trend. While misclassification of cause of death might also have affected the accuracy of our data,12 it is unlikely that it would account for the large differences between Indigenous and total Australian mortality rates reported here. Indigenous people were included in total Australian mortality rates when these differences were estimated, but they comprise only 2% of the Australian population and their inclusion would have minimal effects on the differences.

The increasing cancer mortality rates, and the more pronounced differences between NT Indigenous and total Australian cancer mortality rates in younger than older adults, demonstrate that cancer is an important and increasing health problem for Indigenous Australians. As cancers affecting Indigenous Australians to a greater extent than other Australians are largely preventable, the major challenges in cancer control lie in prevention — particularly by reducing tobacco consumption, increasing the coverage of Pap smear testing and follow-up treatment, and maintaining high hepatitis B vaccination levels.2 Efforts to prevent these cancers have been increasingly successful over several decades for other Australians. It is high time Indigenous Australians shared in these successes.

3: Lung cancer, 1977–2000

Mortality rate trends for lung cancer for the NT Indigenous population and the total Australian population, by sex, 1977–2000. Mortality rates are age- and sex-standardised to the 1991 Australian population distribution.

4: Cancer mortality rate trends, 1977–2000

Cancer mortality rate trends for the NT Indigenous population and the total Australian population, 1977–2000. Mortality rates are age- and sex-standardised to the 1991 Australian population distribution.

5: Smoking-related cancers versus other cancers, 1977–2000

NT Indigenous mortality rates for smoking-related cancers and other cancers, 1977–2000. Mortality rates are age- and sex-standardised to the 1991 Australian population distribution.