Tamoxifen has been widely used to treat oestrogen sensitive (ER positive) breast cancer for more than 30 years. We know that it reduces the risk of the cancer coming back. However, more recently another group of drugs have been developed for ER positive breast cancer. They are called aromatase inhibitors and include anastrozole (Arimidex®), exemestane (Aromasin) and letrozole (Femara).

Tamoxifen and aromatase inhibitors work in different ways.

Tamoxifen

• blocks oestrogen from reaching oestrogen receptors on cancer cells
• can be used to treat pre-menopausal and post-menopausal women.

Aromatase inhibitors (AIs)

• block oestrogen from being made outside the ovaries. (After the menopause the ovaries stop making oestrogen but it is still made outside the ovaries).
• are only suitable for women who are postmenopausal (the ovaries are no longer producing oestrogen).

Several studies have compared aromatase inhibitors to tamoxifen. They have looked at giving 5 years of treatment with an aromatase inhibitor instead of  5 years of tamoxifen. They have also looked at giving aromatase inhibitors after a period of tamoxifen treatment.

All of these studies have shown that treatment with an aromatase inhibitor can further reduce the chance of  breast cancer coming back. They have also shown that aromatase inhibitors tend to cause fewer side-effects than tamoxifen. But, aromatase inhibitors are more likely to cause joint pains and bone thinning (osteoporosis). As aromatase inhibitors have not been used for as many years as tamoxifen the long-term effects of these drugs are not yet completely known.

Because the studies all had different designs there are now a number of hormone therapy treatment options:

• tamoxifen on its own for 5 years
• an aromatase inhibitor on its own for 5 years (anastrozole or letrozole)
• tamoxifen for 2–3 years followed by an aromatase inhibitor for a few years (exemestane, anastrozole or letrozole)
• tamoxifen for 5 years followed by an aromatase inhibitor for a few years (letrozole).

NICE (the organisation that gives advice on the use of new treatments in England and Wales) has looked at how aromatase inhibitors can be used to treat women with early breast cancer. They say that treatment with aromatase inhibitors should be considered for all post menopausal women with early oestrogen sensitive breast cancer. But, because at the moment there isn't one option that is clearly better than the others, your doctor should discuss with you which treatment approach is likely to be best for you. This will take into account:

• whether you have already had tamoxifen
• your individual risk of breast cancer coming back
• whether you have had the menopause
• the side effects and benefits of each treatment
• how these may be affected by your own health factors (such as an increased risk of blood clots or osteoporosis).