• Around 70 per cent of breast cancers are sensitive to the female sex hormone oestrogen.
  • The growth of cancer can be minimised by taking drugs that block the production and action of oestrogens in the breasts.
  • Side effects of endocrine therapy include hot flushes, joint stiffness and osteoporosis.
Breast cancer will affect one in eight Australian women before the age of 85, with around 3,430 Victorians diagnosed with the disease every year. There are many different types of breast cancer, with around 70 per cent of tumours that are sensitive to the female sex hormone oestrogen. These cancers are hormone receptor positive cancers.

However, breast cancer is more common in postmenopausal women, when the ovaries no longer produce oestrogens. It appears the reason for this is that following menopause, the body makes oestrogens in fat tissue, including within the breast. Endocrine therapy is a type of medication that stops oestrogen production and/or blocks its actions on tumour cells.

Breast cancer, oestrogen and women of childbearing age

Breast cancer in women before the age of 25 is rare. However, the risk of developing breast cancer increases with age. Hormone-positive breast tumours that develop prior to menopause are largely dependent on oestrogens produced by the ovaries for growth. In these cases, therapy includes endocrine therapy or chemotherapy, or both.

Ovarian suppression, either through surgical removal of the ovaries or by giving a gonadotropin-releasing hormone, which will lead to the temporary decrease in oestrogen production from the ovaries, may be effective treatment for hormone-positive breast tumours.

Women should make only a decision about whether or not to have chemotherapy following a full discussion with their doctors and after consideration of all available information about the benefits and risks.

Breast cancer in postmenopausal women

As breast cancer is most common in postmenopausal women, the majority of these cancers are hormone receptor positive. Breast tissue contains fat cells. These cells make an enzyme called aromatase, which produces oestrogens. In normal breast tissue, the level of aromatase is carefully regulated.

As a woman ages, the fat cells of her breasts tend to produce greater and greater amounts of aromatase, which in turn increases the amount of local oestrogens. These locally produced oestrogens appear to play a role in both the development and growth of breast cancer in postmenopausal women. Once established, the tumour acts to increase oestrogen levels to help it grow, with immune cells appearing to boost oestrogen production.

Recent studies have also identified a link between obesity and oestrogen production. Data demonstrating that obesity carries a two-fold increased risk of developing breast cancer in older women supports these findings. This makes sense considering that obese women have more of the fat cells responsible for producing oestrogens.

Breast cancer and endocrine therapy

Medications classified as endocrine therapies commonly used to block oestrogen production and/or action include the drug, Tamoxifen. Often used after surgery, these act to lower the risk of recurrence, although they can have a number of side effects. For most women, the benefits far outweigh the risks.

More recently, we have seen development of medications to stop oestrogens by blocking their production. Aromatase inhibitors (AIs), including letrozole, prevent aromatase from producing oestrogens and so reduce the amount of oestrogens within the breast. AIs have been shown to have more benefits and fewer serious side effects than tamoxifen.

Possible side effects of aromatase inhibitors include:
  • hot flushes
  • joint stiffness
  • osteoporosis.
Current research is focusing on newer compounds designed to block oestrogen production in the breast only, as the body still needs oestrogen for, among other things, healthy bones.

Breast cancer and hormone replacement therapy

Menopause can trigger unpleasant side effects such as hot flushes and vaginal dryness. Hormone replacement therapy (HRT) eases the symptoms by boosting sex hormone levels. It also reduces the risk of osteoporosis and heart disease.

Since some breast cancers depend on oestrogen, women taking HRT for a long time (more than five years) have a 0.3-fold increased risk. Women who undergo HRT for shorter periods (such as two years) have the same risk of breast cancer as women who have not used HRT. The health benefits of HRT in women for early post-menopause may outweigh the risks in many cases.

Where to get help

  • Your doctor
  • Cancer Council Victoria Information and Support Service Tel. 131 120

Things to remember

  • Around 70 per cent of breast cancers are sensitive to the female sex hormone oestrogen.
  • The growth of cancer can be minimised by taking drugs that block the production and action of oestrogens in the breasts.
  • Side effects of endocrine therapy include hot flushes, joint stiffness and osteoporosis.

More information


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A-Z of cancer conditions

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This page has been produced in consultation with and approved by: Hudson Institute of Medical Research

Last updated: January 2016

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