Anastrozole or tamoxifen for early breast cancer?

This page tells you about tamoxifen or anastrozole for early breast cancer. There is information about

• Hormone therapy and breast cancer
• Anastrozole
• Tamoxifen

• Research into anastrozole and tamoxifen
• Guidelines for treating early breast cancer
• Working out what is best for you

Many breast cancers are hormone sensitive. The female hormones oestrogen and progesterone encourage the cells of these breast cancers to grow. So drugs that block these hormones can help to lower the risk of early breast cancer coming back after surgery. This is called adjuvant hormone treatment.

Anastrozole (Arimidex) is a type of hormone drug for breast cancer called an aromatase inhibitor. Another name for anastrozole is Arimidex. Women who have had their menopause do not produce oestrogen from their ovaries. But they make oestrogen in the fatty tissues of the body. This process uses an enzyme called aromatase. Anastrozole blocks aromatase and so stops oestrogen production in post menopausal women.

Tamoxifen works in a different way to anastrozole and both pre and post menopausal women can take it. It stops oestrogen from attaching to breast cancer cells and is a type of therapy called a selective oestrogen receptor modorator. Tamoxifen has been used as a breast cancer treatment for more than 30 years.

Tamoxifen has been used for a long time as treatment for early breast cancer. Arimidex is a newer drug and doctors wanted to see whether it could work better than tamoxifen. A large phase 3 clinical trial compared anastrozole with tamoxifen for post menopausal women who had early stage breast cancer. The study was called ATAC (Arimidex, Tamoxifen, Alone or in Combination). The women in this trial took the hormone therapy after surgery.

The ATAC trial involved over 9,000 post menopausal women with early stage breast cancer from all over the world. After these women had surgery for their breast cancer, they took either 5 years of tamoxifen or 5 years of anastrozole or a combination of the two. The final recruits joined this trial in 2000. The average follow up time is now 10 years.

The trial showed that anastrozole was more likely to stop breast cancer coming back than tamoxifen. You may hear this called disease free survival (DFS). Tamoxifen worked well and it is one of the most effective breast cancer treatments ever developed. But in this trial, the anastrozole seemed to work slightly better. The survival rates were the same for both drugs.

In women whose cancer did come back, it came back later in the women on anastrozole than it did in women taking tamoxifen. There were also fewer secondary cancers and a significant reduction in the number of women who went on to get breast cancer in the other breast.

The trial found that these drugs cause different side effects. These side effects only happen in a small number of women. Tamoxifen can cause cancer of the womb in a small number of women and this is less likely to happen with anastrozole. There were also fewer cases of blood clots in the veins (deep vein thrombosis or DVT) with anastrozole. 

Women taking anastrozole were more likely to have weakening of their bones (osteoporosis) and fractures but less likely to have hot flushes. They were more likely to have joint pain. 

The number of other types of cancer that the women developed was similar in the two groups although the types of cancer they developed were different. The women who took tamoxifen were more likely to develop womb, ovarian and skin cancer and the women taking anastrozole were more likely to develop bowel and lung cancer. Remember that the risk of developing another type of cancer was low and lowering the risk of recurrence of the breast cancer outweighed any risk of developing another type of cancer.

In February 2009, the National Institute for Health and Clinical Excellence (NICE) recommended anastrozole or letrozole as a first line hormone treatment for women after surgery if

• They have ER positive breast cancer, and
• They have been through the menopause, and
• Their doctors think there is more than a low risk of the cancer coming back

If you cannot take an aromatase inhibitor for any reason, then you should be offered tamoxifen. If you are going to have chemotherapy for early breast cancer, you usually start hormone therapy once your chemotherapy has finished.

If you have already been taking tamoxifen for 2 to 3 years, NICE recommend that you should be offered exemestane or anastrozole.

If you have already been taking tamoxifen for 5 years, and you had cancer in your lymph nodes when you had your operation, NICE recommend that you are offered letrozole for another 2 to 3 years.

Your doctor should discuss with you which treatment is most suitable for you. And they should take into account factors such as

• The side effects and benefits of each different hormone therapy
• Whether you've already had treatment with tamoxifen
• What they think the risk of your cancer coming back might be. 

You can find out more about the side effects of each type of hormone therapy by clicking on the links above for each particular drug.

There is also information in the types of hormone therapy for breast cancer section.

Which treatment is best for you will depend on your situation. For example if you are premenopausal anastrozole won't be suitable. Remember tamoxifen is still a very good treatment for breast cancer. Talk to your doctor about what is best for you based on your particular situation.