A sharp and unexpected fall in breast cancer cases may have been caused by millions of women giving up hormone replacement therapy.

In 2003, after rising steadily for 20 years, new cases of breast cancer diagnosed in the US fell by 7 per cent. About 14,000 fewer women developed the disease than in 2002. The fall was even greater among women aged 50 to 69 suffering from forms of breast cancer that are sensitive to hormones. In this group the drop was a remarkable 12 per cent.

“It is the largest single drop in breast cancer incidence within a single year I am aware of,” Professor Peter Ravdin, of the M.D. Anderson Cancer Centre at the University of Texas, told a conference in San Antonio. His team believes that the 2002 health scare that made many women abandon HRT is the most plausible explanation of the figures.

While UK statistics show nothing as dramatic as this, Professor Valerie Beral, of Cancer Research UK said there had been a slight drop in breast cancer between 2003 and 2004 in women aged between 50 and 64. As in the US, this followed years of rising incidence.

In July 2002 a study of more than 16,000 older women taking HRT was terminated early when the pill they were taking — a combination of oestrogen and progesterone — was found to increase breast cancer risks. In the scare that followed, the number of women aged over 50 taking HRT in the US fell from 30 per cent to about half that.

The team believes that this may have slowed the growth of tumours so much that they remained undetectable in routine breast cancer screening in 2003, leading to the sharp fall in new diagnoses. They admitted that they were at first very surprised by the size and speed of the response to the changes in HRT use. But the senior investigator, Proffessor Donald Berry, said that it made sense.

Breast cancer incidence had been rising steadily for 20 years. Part of the rise was caused by screening, which detects cancers earlier, but this factor could not explain all of it, he said.

“HRT had been proposed as a possible factor, although the magnitude of any effect was not known. Now the possibility is that the effect is much greater than originally thought was plausible, and that is a remarkable finding.”

Some breast cancers need hormones to grow. So cutting off their source of hormones, HRT, would slow their growth sufficiently for them to be undetectable in mammography screening the next year.

The US team, including Professors Ravdin and Berry and colleagues from the US National Cancer Institute and Harbor UCLA Medical Centre, used data from nine regions of America. They studied rates of breast cancer from 1990 onwards and found a steady increase up to 1998, when rates stabilised if adjustment for the ageing population was made.

But there was a sudden drop between 2002 and 2003. When analysed in detail, the data showed that the decline was much greater in cancers sensitive to oestrogen (8 per cent) than it was in cancers that were insensitive to the hormone (4 per cent). In the 50-69 age group, who were more likely to be using HRT, the drop in hormone-positive cancer was 12 per cent compared with 4 per cent in hormone-negative cases. “It takes breast cancer a long time to develop, but here we are primarily talking about existing cancers that are fuelled by hormones and that can slow or stop their growing when a source of fuel is cut,” Professor Berry said.

Attitudes to HRT changed abruptly when a study, part of the Women’s Health Initiative and organised by the US National Heart, Lung and Blood Institute, was terminated three years early as evidence emerged of health damage to women taking a pill called Prempro, a combination of oestrogen and progesterone.

The women were found to have a 26 per cent higher risk of breast cancer, a 29 per cent increased risk of heart attacks and a 41 per cent increase in strokes. The product, made by Wyeth, was not sold in an identical form in Britain, but a similar product, Premique, was available. The new finding was the increased heart attack risk; the increase in breast cancer risk was already known.

Times Recommends

• Karadzic found freedom in the Madhouse
• Bears kill guards at Russian mine
• Hotel is a monument to new North Korea