Scientists analysed the data from the UK study to try and find a link between HRT and the type of breast cancer that developed. The research found that women who took HRT had an increased risk of developing lobular cancer (affecting the cells in the ducts of the milk-producing glands) and tubular cancer. There was not such an increased risk of developing ductal breast cancer, the most common type of breast cancer that affects the cells lining the milk duct. There was no increase in the risk of medullary breast cancer, a kind of cancer that is common in women with a genetic predisposition to breast cancer.
The study demonstrated that women who had taken combined HRT (oestrogen and progesterone) had an even greater risk of developing lobular and tubular breast cancer than women on oestrogen only HRT. The researchers also discovered similar findings for women with breast cancer in situ - when the cancer has not spread to the surrounding tissues in the breast or other parts of the body. Women that took HRT had a significantly greater risk of developing lobular cancer in situ than ductal carcinoma in situ.
G. Reeves who presented the findings comments, "It is very interesting that HRT has different effects on different types of breast cancer. One possible explanation for the findings is that certain types of breast cancer are more likely than others to be hormone receptive. Further research into this topic could greatly help our understanding of the biological mechanisms underlying the development of breast cancer."
In 2003, the million women study confirmed that post-menopausal women taking combination HRT were twice as likely as non-users to develop breast cancer and women taking oestrogen-only HRT had a 30 per cent greater risk than those who haven't taken HRT.
The increase in breast cancer risk starts within one to two years of beginning either form of HRT and increases the longer it is taken. As soon as HRT is stopped, the risk begins to fall and, after five years, is the same as for women who've never taken the drug. Combined oestrogen-progesterone HRT is usually prescribed for women who still have a uterus to avoid the increased risk of cancer of the uterus caused by oestrogen-only therapy. Women are advised to discuss their concerns with their doctor.
Notes: The Million Women Study was funded by Cancer Research UK, the NHS Breast Screening Programme and the Medical Research Council. Scientists at the Cancer Research UK Epidemiology Unit in Oxford analysed data from over one million women between the ages of 50 and 64. Women joined the study between 1996 and 2001 and half were using HRT or had done so in the past. The study included 9,364 cases of invasive breast cancer and 637 breast cancer deaths, registered over 2.6 and 4.1 years of follow-up respectively.
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SS8 Hormonal replacement therapy and alternatives
Hormone replacement therapy in relation to the risk of specific histological types of breast cancer: results from the Million Women Study
G. Reeves, V. Beral
Cancer Research UK Epidemiology Unit, Oxford, United Kingdom
Background: Use of hormone replacement therapy (HRT) is associated with an increased risk of breast cancer but relatively little is known about how this risk may vary according to histological type.
Methods: The relative risk of specific histological types of breast cancer in relation to HRT use is examined among women participating in the Million Women Study. Analyses are based on 935,668 postmenopausal women, recruited into the study between 1996 and 2001 aged 50-64, with an average duration of follow-up of approximately 3 years. During the follow-up period, 12,774 breast cancers of known histological type were diagnosed of which 11,235 were invasive including 7,729 ductal cancers, 1,476 lobular cancers, 473 tubular cancers, 68 medullary cancers and 145 mucinous cancers.
Results: Current use of HRT was associated with a materially greater increase in the risk of lobular and tubular breast cancer than ductal breast cancer but there was no evidence of any increase in the risk of medullary cancer. A comparatively greater risk of lobular and tubular cancer versus ductal cancer was observed for both oestrogen-only and combined HRT but the magnitude of the corresponding relative risks were consistently larger for users of combined compared to oestrogen-only preparations. Similar patterns of risk in relation to current HRT use were observed for in situ disease in that the relative risk lobular carcinoma in situ was significantly greater than that for ductal carcinoma in situ.
Conclusions: There are substantial and, in some cases, qualitative differences in the effect of HRT use on specific types of breast cancer. Further exploration of these differences, and in particular the extent to which they may be due to differences hormone receptor status, should lead to a better understanding of the biological mechanisms underlying the development of breast cancer.