Letters
Women's health Cancer Pharmacology
Volume 190 - Issue 3

Decrease in breast cancer incidence following a rapid fall in use of hormone replacement therapy in Australia

Authors:  Graham G Giles, Richard Bell, Helen Farrugia and Vicky Thursfield

Med J Aust 2009; 190 (3): 164-165. || doi: 10.5694/j.1326-5377.2009.tb02329.x
Published online: 2 February 2009
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To the Editor: We question the conclusions drawn by Canfell and colleagues1 from their analysis of trends in hormone replacement therapy (HRT) prevalence and breast cancer incidence for Australian women aged 50 years or older. Their ecological analysis lacks individual-level information on HRT use and information on tumour oestrogen receptor (ER) status, and captures only 2 years following the decline in HRT prevalence. This is an inadequate design within which to judge issues of causality; it is at best an hypothesis-generating exercise.2

The examination of only 2 years of breast cancer incidence after the decline in HRT use is unsound because of substantial unexplained annual variability in national breast cancer incidence. This is evidenced, for example, by the graph in Canfell et al’s Box 2, which identifies a fall in 1998–1999 that was not ascribed to changes in HRT prevalence. The lack of data on tumour ER status is another weakness. HRT use increases the risk of ER+ tumours, so any decline in HRT prevalence would be expected specifically to reduce ER+ tumour incidence.

We analysed Victorian Cancer Registry data for women aged 50 years or older for the period 2001–2005 — 2 years past the cut-off in Canfell et al’s analysis. Tumour ER status was available for 87% of breast cancer cases in 2001, rising to 90% in 2005. The demographic characteristics of the women for whom these data were available did not change between 2001 and 2005. Over this period, the proportion of all breast cancers that were ER+ increased from 65% to 71%, and the proportion of tumours with known ER status that were ER+ increased from 74% to 79%.

Poisson regression analysis of the age-specific incidence rates for both total and ER+ breast cancer for women aged 50 years or older estimated an average annual decline of 1.7% in the total incidence rate (P = 0.0009) and an annual increase of 0.2% in the ER+ incidence rate (P = 0.7). Our findings are illustrated in the Box. Although there was an apparent small decline in total breast cancer incidence in 2001–2003, this trend was reversed in 2004–2005. More importantly, the trend in ER+ tumour incidence was stable across the entire period.

Age-standardised incidence* of invasive breast cancer in women in Victoria aged ≥ 50 years, 2000–2005

Vertical bars represent 95% confidence intervals. ER+ = oestrogen receptor-positive.
* Standardised to World Standard Population.

These Victorian trends, covering a longer time period and including information on the tumour type most likely to be affected by changes in HRT use, provide no support for the hypothesis of Canfell and colleagues.

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