Drug provides another treatment option for an early form of breast cancer

Ductal carcinoma in situ (DCIS) is described as a very early form of breast cancer, where cancer cells are present in milk ducts, but have not spread to the surrounding breast tissue. It is estimated that approximately a fifth of all screen-detected breast cancers are DCIS, with around 4,800 people diagnosed with DCIS in the UK each year.

Lead author Professor Jack Cuzick from QMUL said: "We found that anastrozole and tamoxifen had similar overall efficacies, with slightly better outcomes for those who took anastrozole. But more importantly, because of their very different side-effects, anastrozole can be offered as an alternative for patients who may not tolerate tamoxifen as well or have previous illnesses making tamoxifen unsuitable.

"Now we know that anastrozole is effective for treating hormone sensitive ductal carcinoma in situ, women will have a greater choice of treatments to suit their own previous medical histories and tolerability of medications."

The research was funded by Cancer Research UK, and the results are being presented at the 2015 San Antonio Breast Cancer Symposium on 11th November and published in The Lancet.

Professor Peter Johnson, Cancer Research UK's chief clinician, said: "Breast cancer is the single most common cancer in the UK and we continue to learn more about how to prevent and treat it through our research. We already know that tamoxifen can reduce the risk of cancer developing in some women who have early changes in the breast, but some get unacceptable side effects. This trial shows that there is an equally good alternative in anastrozole, which may suit some people better. It's important we continue to gain more insight into the side effects of different drugs and understand who is at higher risk and who is most likely to benefit."

The study looked at 2,980 postmenopausal women with DCIS in 14 countries, who were either given anastrozole or tamoxifen for five years after surgery. After a median follow-up of 7.2 years, 144 participants developed breast cancer, and 69 died, of which four were due to breast cancer.

The groups had a similar number of cases of the disease recurring, whether they took tamoxifen or anastrozole. Those who took anastrozole had an 11 per cent lower rate of recurrence of DCIS or invasive cancer than those who took tamoxifen, but this difference was not significant.

The women in the two groups had different side effects. Women who took anastrozole experienced fewer womb and ovarian cancers and non melanoma skin cancers, and fewer deep vein thromboses and gynecological issues, compared with those who took tamoxifen. However, more strokes, fractures and musculoskeletal issues were seen among those receiving anastrozole.

The trial was limited by its lower-than-expected event rate, which adds uncertainly about the lack of statistical significance of some of the small differences seen. The authors say it is too early to assess the effect of these treatments on mortality, and long-term follow-up and further research is planned to study these issues.

The study was also funded by National Health and Medical Research Council Australia, Breast Cancer Research Fund, and AstraZeneca.

Following on from IBIS-II, the IBIS 3 Feasibility study will begin recruitment in 18 UK sites in January 2016, with the aim of comparing further treatments for 300 breast cancer survivors at high risk of late recurrence.