The Cancer Research UK study1 shows taking tamoxifen for longer than the recommended five years provides greater protection against breast cancer recurrence and reduces breast cancer deaths.
The ‘aTTom’ study looked at almost 7,000 women with breast cancer who, after five years of taking tamoxifen, either continued taking the drug for another five years or stopped treatment.
Dr Daniel Rea, clinical lead researcher based at the University of Birmingham, said: “These results are important as they establish that giving tamoxifen for longer than the current standard of five years significantly cuts the risk of breast cancer returning.
“Doctors are now likely to recommend continuing tamoxifen for an extra five years and this will result in many fewer breast cancer recurrences and breast cancer deaths worldwide. Tamoxifen is cheap and widely available so this could have an immediate impact.”
Around 75 per cent of breast cancers are oestrogen receptor positive and may benefit from hormone therapy. The female sex hormone oestrogen encourages breast cancers to grow by activating oestrogen receptors. Tamoxifen blocks these receptors, reducing the chance of breast cancer returning after surgery or developing in the other breast.
Despite the benefits of tamoxifen in preventing breast cancers from returning, it does have side effects. Women taking tamoxifen can experience side effects similar to menopausal symptoms, such as night sweats and hot flushes. Rare but serious side effects of tamoxifen include increased risk of endometrial cancer (cancer of the lining of the uterus), blood clots, and stroke.
In this study no increase in the incidence of stroke was observed with 10 years of tamoxifen therapy, though endometrial cancer risk was higher in this arm. Endometrial cancer is often detected early, when it can often be treated successfully. The researchers estimate that for every endometrial cancer death that occurs as a side effect of long-term tamoxifen, there would be 30 deaths from breast cancer prevented.
Professor Richard Gray, based at the University of Oxford and presenting the aTTom results at ASCO, said: “Five years of tamoxifen is already an excellent treatment but there have been concerns that giving it for longer might not produce extra benefits and could even be harmful. The aTTom study establishes that the benefits of taking tamoxifen for longer greatly outweigh the risks.”
Kate Law, director of clinical research at Cancer Research UK, said: “Large clinical trials like aTTom are vitally important to understand how drugs such as tamoxifen work and how best to use them. We need these sorts of studies so we can be sure the benefits from cancer drugs outweigh the side-effects that they may have.”
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1. The study also received funding from the Medical Research Council
2. 580 women from 3,468 women taking tamoxifen for ten years, compared to 672 of 3485 women who stopped tamoxifen after five years.
3. 849 deaths among the ten year tamoxifen group compared to 910 deaths in the five year tamoxifen group.