Incidence of metastatic breast cancer in an Australian population-based cohort of women with non-metastatic breast cancer at diagnosis

One of the major concerns for women diagnosed with early breast cancer (BC) is their risk of developing metastatic cancer (MBC).1 Information on the risk of and prognosis for patients with MBC is important to help patients make decisions about treatment and plan for the future. It is also important for health service planning and identifying research priorities. Currently, cancer registries do not routinely report this information. Consumer groups in Australia2 and internationally3 have urged for improved data collection on cancer recurrence and reporting.

Over the past 20 years, cancer registries have documented continued improvements in survival among patients with BC.4 These survival gains have been attributed to advances in treatment, particularly systemic adjuvant therapies after pivotal trials, and to a lesser extent, BC screening programs.5,6 However, the gains for women with early BC in terms of their risk of developing MBC have not been well documented. Earlier studies may no longer be relevant to contemporary populations. At present, clinicians generally rely on published clinical trials for information on these outcomes, but the extent to which trial data can be applied to the general population of women with BC is not known.

We aimed to report Australian population-based estimates of the incidence of MBC for women initially diagnosed with non-metastatic BC. We were particularly interested in the experiences of women aged 70 years and older, and of women from regional and remote areas, two populations not well represented in clinical trials.

All women aged 18 years and older diagnosed with invasive (not intraductal) localised or regional BC between 1 January 2001 and 31 December 2002 and recorded on the New South Wales Central Cancer Registry (CCR) were included. Patients were excluded if the extent of disease spread at diagnosis was recorded as distant or unknown, or if they had no hospital care recorded on the NSW Admitted Patient Data Collection (APDC).

During 2001 and 2002 , 8142 women had their diagnoses of invasive BC recorded on the CCR. After excluding women with MBC at diagnosis (546), unknown degree of spread (791) or no hospital care recorded in NSW (161), 6644 patients were included in the study. Of these, 4137 had localised BC, and 2507 had regional BC. Their median age was 59 years (interquartile range [IQR], 50–70 years). Women’s age, tumour histology and the presence of other non-breast primary cancer sites varied by degree of spread at diagnosis (Box 1).

In this Australian population-based cohort of 6644 women with non-metastatic BC, we found that about one in 20 women with localised node-negative disease and one in 6 women with regional disease at initial diagnosis developed MBC within 5 years. Clinicians can use these estimates to inform women with BC about the average risks of developing MBC.

Internationally, very few population-based studies have examined MBC incidence in patients with non-metastatic BC at initial diagnosis. European researchers recently reported a 5-year MBC risk of 10% for 18 592 patients with no distant metastases at diagnosis on the Munich Cancer Register between 1995 and 2003.11 These results are consistent with our findings of a 5-year MBC risk of 10% for the combined cohort of Australian patients with localised and regional disease. Our findings also show that, despite the known importance of underlying tumour biology in defining cancer prognosis, spread of disease at diagnosis (localised versus regional) remains an important prognostic factor in BC.

Although clinical trials can provide data to estimate the incidence of MBC for selected BC populations, it is unclear to what extent trial estimates, which are drawn from patients receiving care from participating institutions who meet trial eligibility criteria, apply to the general population with cancer. In the United States, it is documented that older individuals, those with low socioeconomic status, and those living in areas with a lower number of oncology specialists and hospital programs are underrepresented in cancer trials relative to their cancer burden.12 In examining whether these factors are also associated with MBC risk, we found a statistically significant association between socioeconomic status and MBC risk, but did not observe an association for women aged 70 years and older versus those aged 50–70 years, or for those living within versus outside a major city. The association between living in more socioeconomically disadvantaged areas and an increased risk of MBC is consistent with Australian evidence of reduced breast cancer survival for these patient groups.13 This finding is worthy of further study, and may have important implications for health service providers and health funding policymakers. The higher risk of MBC observed for patients aged less than 50 years compared with older age groups may be partly attributed to more aggressive tumour biology in younger women.14

Overall, we did not find evidence that the risk of MBC for women with node-negative BC in the general population differed substantially from that of trial populations. Our estimate of a 5-year MBC cumulative incidence of 5.3% is at least as favourable as outcomes reported in adjuvant systemic therapy trials in node-negative populations that informed practice in 2001 (5%–8% of trial patients had MBC recorded as a first event with an average 5–6 years follow-up).15,16 It is more difficult to draw comparisons between our results for patients with regional BC at diagnosis and trial-based estimates because, in our study, regional BC includes nodal metastases with the primary tumour confined to the breast as well as locally advanced BC, which confers a relatively poorer prognosis. In contrast, most trials are designed to assess interventions in one or other of these subgroups. Despite this, the 5-year MBC incidence of 18.1% that we observed for patients with regional BC falls within the upper range of MBC risk observed in adjuvant chemotherapy trials conducted in node-positive populations (14%–20% for different treatment arms).17-19

We found the annual rate of MBC notification peaks in the second year after diagnosis, and then decreases over time for both patients with localised and regional BC, and across age groups. These findings are consistent with earlier studies investigating the risk of any recurrence conducted in both trial20 and non-trial21 populations, with the highest hazard rate for early MBC being observed in women with more aggressive disease subtypes. This information may provide some reassurance for women who completed their primary BC treatment more than 2 years ago and remain recurrence free.

Our study cohort was diagnosed with BC in 2001 and 2002, so would have had access to current standard breast screening protocols and most contemporary adjuvant systemic therapies, except trastuzumab and taxanes; both of which only received broad listing for early BC on the Pharmaceutical Benefits Scheme in 2006. The National Breast Cancer Audit conducted during the period of our study indicates that tamoxifen was widely used for women with oestrogen receptor-positive BC; systemic adjuvant chemotherapy was used for 63% of pre-menopausal women and 28% of post-menopausal women (including 83% of oestrogen receptor-negative, node-positive postmenopausal cases).22 Given that the availability and use of effective systemic therapies has increased over the past decade, we anticipate that the incidence of MBC among women with a more recent BC diagnosis, especially those with BC in which the human epidermal growth factor receptor 2 protein is over-expressed (HER2-positive disease), may be even lower than our study estimates.

The strengths of our study are the size of the study population and the degree to which it is representative of the general Australian population of women with a diagnosis of non-metastatic BC. Our study shows the feasibility of using routinely collected data from the CCR and APDC to report rates of distant cancer recurrence in a timely fashion. Over time, these analyses can be extended to include longer-term data from the study cohort to assess 10-year MBC risk, and to include more recent cohorts.

Our methods have two main limitations. First, although the accuracy of the CCR in recording primary cancer events and of the hospital database in coding cancer diagnoses and procedures have both been documented to be high,23,24 the completeness and accuracy of cancer recurrence records has not been formally investigated. Further research is required to determine the proportion of MBC cases that are not notified to the CCR. There would be no notification for patients who had a clinical diagnosis of MBC made in an outpatient setting without a biopsy or any diagnostic or therapeutic hospital service. Some women with MBC will receive outpatient-based care for a number of years before requiring hospital admission for a treatment complication or disease event, and many will not have had a biopsy to confirm their diagnosis. These individuals would not have been recorded as having MBC in our study, so it is possible that the true average 5-year incidence of MBC is higher than our study estimate. Second, these administrative datasets do not provide information about prognostic factors, such as receptor status or treatments received, to develop risk estimates for specific patient groups. Cancer Institute NSW has recently implemented a clinical cancer register to help overcome these limitations.

Overall, our population-level estimates provide an important supplement to existing clinical trial data to help doctors answer questions about average MBC risk for the many women with BC in the community who may not be represented in clinical trials. This information can also inform cancer services planning, provide contemporary estimates of MBC risk to researchers planning clinical trials of treatments for early BC, and suggest priorities for research, such as investigating and improving MBC outcomes for socioeconomically disadvantaged women. Our finding that women with node-negative BC have a low risk of developing MBC is reassuring, suggesting that treatment advances demonstrated in clinical trials of adjuvant therapy are delivered to the general population of women with BC.

2 Cumulative incidence of metastatic breast cancer and annual hazard rate by degree of spread at initial diagnosis