By Emma Wilkinson
Health reporter, BBC News
Breast cancer is the commonest cancer in UK women
Lifestyle changes and screening have shifted the type of breast cancers women are diagnosed with over the past couple of decades, research suggests.
Women are now more likely to have hormone-dependent, slow-growing tumours, a comparison of tissue samples from the 1980s and 1990s shows.
The Scottish researchers also found improved survival over time, the British Journal of Cancer reported.
More than 40,000 women are diagnosed with breast cancer in the UK annually.
Previous studies have suggested that breast cancers may be more commonly hormone-dependent than in the past.
Specifically it is thought that oestrogen-receptor positive cancers may be on the rise.
It is these tumours which respond well to hormone therapy, such as tamoxifen which prevents the disease coming back.
But it has not been clear that numbers were actually on the rise as the ability to detect these types of tumours in the lab may have improved in recent years.
In the latest study, researchers re-examined actual tissue samples - 420 from between 1984 and 1986 and 653 from 1996 to 1997 - saved by two large hospitals in Glasgow.
Those diagnosed in the earlier time period had all presented with symptoms of cancer because screening by mammography had not yet been introduced.
The proportion of cancers which were oestrogen-receptor positive changed significantly from 64.2% to 71.5% over the 10-year period.
And more cancers were diagnosed as grade one - slow-growing tumours, with a decline in the number of grade three - fast-growing tumours.
There was no change over time in the proportion of progesterone or Her-2 positive cancers
It could be that screening is detecting more oestrogen-receptor positive cancers because they are slow-growing and may be detected before symptoms appear.
But another explanation could be changes in lifestyle factors which increase the risk of hormone-dependent tumours, such as women having babies at an older age, obesity after menopause and use of hormone replacement therapy.
The researchers, led by Dr Sylvia Brown at Crosshouse Hospital in Ayrshire wrote: "There is evidence that the percentage of all children being born to mothers aged 35 years and over is increasing in Scotland and that means BMI and prevalence of obesity are increasing."
She added that if there is a true increase in the proportion of these tumours it has implications for treatment decisions as many clinical trials were carried out in previous decades.
Dr Alison Ross, Cancer Research UK's senior science information officer, said: "It's plausible that lifestyle changes could be influencing the types of breast cancers that women are developing but we will need much larger studies to find out whether this trend is real.
"And it's also not clear whether these results reflect a shift in breast cancer biology or indicate that screening is better at detecting certain cancers.
"If the trend identified in this interesting study is confirmed and continues, it could have an impact on the way doctors apply results from breast cancer studies done decades ago to the treatments in use today."