The first year counts: cancer survival among Indigenous and non-Indigenous Queenslanders, 1997–2006

Aboriginal and/or Torres Strait Islander people (referred to here as Indigenous Australians) have poorer rates of cancer survival than non-Indigenous people.1 Because ascertainment of Indigenous status in routinely collected data sources is incomplete, analysis of population-based cancer registry data has been largely restricted to registries known to have high-quality data ascertainment, such as those in Queensland, Northern Territory, South Australia and Western Australia.2 The limited data that are available show a consistent picture of similar overall cancer incidence, but lower incidences of some cancers that have a better prognosis (such as melanoma) and higher incidences of cancers with poorer prognoses.3

Recent studies have highlighted significantly lower survival among Indigenous patients with cancer compared with non-Indigenous patients.1,4-8 Although Indigenous people are more likely to be diagnosed at advanced stages with certain cancers, or to receive less treatment, this does not completely explain the survival disadvantage.5-7 A matched case–case study in Queensland found that Indigenous patients treated in the public health system were 30% more likely to die from their cancer than non-Indigenous patients after adjusting for stage, cancer treatment and comorbid conditions.8

A higher proportion of Indigenous Australians than non-Indigenous Australians live in remote areas.9 Many remote areas are also characterised by socioeconomic disadvantage, with both remoteness10,11 and area-socioeconomic disadvantage12 associated with lower cancer survival. Currently, however, there is limited information on how the Indigenous survival differential varies across categories of remoteness and area-socioeconomic disadvantage. In this population-based study, we sought to address this lack of knowledge, and thus inform further research, policy and clinical priorities aimed at redressing the survival inequalities currently experienced by Indigenous people with cancer.

Data were provided by the population-based Queensland Cancer Registry (QCR),13 under an agreement between Cancer Council Queensland and Queensland Health allowing access to non-identifiable data. For these analyses, all people of known Indigenous status aged 15 years and over who were diagnosed with a primary invasive cancer (International classification of diseases for oncology, 3rd edition [ICD-O-3] codes, C00–C80) during 1 January 1997 to 31 December 2006 were included. We excluded people whose age or residential location at diagnosis was unknown, or whose diagnosis was based on the death certificate or autopsy report only. Cases included in the study were followed to 31 December 2007, with matching to the National Death Index. Those still alive at 31 December 2007 were censored at that date, while those who died from a cause other than the diagnosed cancer were censored at their date of death.

Of the original 180 095 people aged 15 years and over who were diagnosed with invasive cancer in Queensland between 1997 and 2006, 14.6% were of unknown ethnicity (Box 1). A further 2.5% were excluded because they were either diagnosed at death (total, 1.5%; Indigenous, 2.9%; non-Indigenous, 1.5%), did not have information on the SLA of residence (total, 0.8%; Indigenous, 0.7%; non-Indigenous, 0.8%) or the number of days between diagnosis and death (total, 0.2%; Indigenous, 0.3%; non-Indigenous, 0.2%), or did not have a SEIFA value assigned (total, 0; Indigenous, 0.4%; non-Indigenous, 0). The final cohort included 150 059 individuals of known ethnicity, of whom 1819 (1.2%) were Indigenous (Box 1). The most common cancers among Indigenous people were lung, breast, colorectal, prostate and cervical cancers. Among non-Indigenous people, the most common cancers were colorectal, breast, prostate and lung cancers and melanoma. A slight majority (55.5%) of people diagnosed with cancers were males.

In this population-based study of cancer in Queensland, we found significant disparities between the survival outcomes for Indigenous and non-Indigenous people after their diagnosis. These differences remained after accounting for remoteness, area-socioeconomic disadvantage, age group, sex and mix of cancers. However, this survival disparity was modified by time since diagnosis, with the comparative risk of death decreasing as the time from diagnosis increased. This varying time effect has not been previously noted in studies examining cancer survival among Indigenous people in Australia,4,19 apart from a brief mention of time-varying comparative Indigenous and non-Indigenous survival in patients with colorectal cancer in the Northern Territory.20

This is important information for Indigenous patients with cancer to know, because the longer they survive the greater chance they have of continuing to survive. Most of the currently published literature has focused on the poorer survival outcomes of Australian Indigenous people diagnosed with cancer. This is justifiable, and has helped galvanise a concerted government, clinical and research effort to reduce this disparity in cancer survival. However, the perspective that Indigenous cancer patients have a continued poorer prognosis cannot help but limit their own personal optimism and outlook for the future. Thus, our results, based on the total Queensland population, provide an avenue for increased optimism among Indigenous cancer patients that they can successfully complete their cancer journey.

However, this optimism must be constrained by a strong call to action to understand what is causing the very wide disparity in survival within the first 1 to 2 years after diagnosis. A recent Queensland study showed a survival differential for Indigenous patients with breast cancer that remained even after adjusting for spread of disease.21 This suggests that other factors, such as the impact of poorer general health and increased comorbid conditions among the Indigenous compared with the non-Indigenous population, also play an important role. There may be a healthy cohort effect, as Indigenous patients who survive beyond 2 years after diagnosis may have fewer comorbid conditions or better general health than those who died earlier. Alternatively, Indigenous patients with cancer in Queensland (all cancers) are less likely to undergo treatment for their cancer than other patients.8 Indigenous patients who use health services and receive adequate treatment may have better rates of survival.22 Until Australian cancer registries standardise the collection and recording of stage and treatment data, it will be impossible to explore these factors appropriately.

We found no evidence that the differential in Indigenous versus non-Indigenous survival varied according to geographical area of residence. As the use of any area-based measure of socioeconomic status is likely to over-estimate the affluence of Indigenous people,23 this reinforces the lack of evidence. This may indicate the Indigenous survival differential is not primarily related to access to treatment or socioeconomic barriers, but that other as yet unknown factors are more relevant, including those related to culture and general health, and that these other factors have similar impact across geographical locations. Clearly, having an almost 50% differential in cancer survival within the first 12 months of diagnosis is not acceptable, and our findings should increase the motivation for further efforts in this area. Greater emphasis and research focus should therefore be placed on identifying the factors responsible for the early disparity in survival.

Limitations of our study include the relatively small numbers of Indigenous cases, which gave us limited capacity to investigate differences for specific types of cancer. We were also unable to separate the effects of early diagnosis from other factors, including those of treatment differentials. As it is possible that not all cases of cancer in Indigenous people were identified, there is potential misclassification of true Indigenous status. However, this misclassification is thought to be small,8 and ascertainment is considered high.2

In conclusion, our findings provide some cause for cautious optimism, as Indigenous patients in Queensland who have already survived 2 years after a cancer diagnosis are likely to have a similar outlook to that of non-Indigenous patients. However, there is still an urgent need to address the unacceptable disparity in survival outcomes early after diagnosis. This important health issue must remain a continuing priority.

1 Flow chart illustrating case ascertainment for the study

ARIA+ = Accessibility/Remoteness Index of Australia. IRSAD = Index of Relative Socioeconomic Advantage and Disadvantage. * Includes ethnicity, ARIA+, IRSAD and dates at death and/or diagnosis. † Includes diagnosis by death certificate or at autopsy, and missing information.

2 Comparison of survival between Indigenous and non-Indigenous people aged ≥ 15 years diagnosed with invasive cancers in Queensland, 1997–2006