4 October 2021
Internationally, 3864 women participated in the prevention trial (818 in Australia and New Zealand (ANZ), 2980 in DCIS (178 ANZ), and 1410 in the bone substudy (229 ANZ). This trial opened in ANZ in 2005, and during the subsequent 16 years until 2021, an enormous amount has been learnt. Here only the main findings will be summarised:
- Anastrozole, taken for 5 years, reduces the chance of developing breast cancer by 54% and DCIS by 59%, and continues to reduce the chance of these events for at least 5 years after completing anastrozole treatment.
- In women who have had treatment for DCIS, anastrozole and tamoxifen were both similarly effective in preventing recurrence of DCIS, or occurrence of invasive breast cancer. There was also an ongoing protective effect after stopping. Side effects were different between the two treatments and may influence treatment choice.
- In women taking anastrozole, who are at risk of low bone density, risedronate reduced the chance of anastrozole-related loss of bone density in the spine, but not in the hip. After stopping anastrozole and risedronate, bone density improved in the lumbar spine and stabilised in the hip in patients allocated to receive risedronate.
The main side effects seen with anastrozole were similar to those seen in other clinical trials and in routine practice: menopausal symptoms, arthritis/stiffness, and fractures. In the DCIS group, tamoxifen was associated with menopausal symptoms, deep vein thrombosis (uncommon), muscle spasm, and gynaecological changes. Overall, the treatments were well tolerated.
As a result of this trial, women at higher than average risk of developing breast cancer now have an additional effective option to reduce their risk. Women with treated DCIS can consider the use of either anastrozole or tamoxifen to prevent future breast cancer or DCIS. Clinical practice has changed worldwide leading to benefits flowing on to patients in the clinic. Further analyses are expected from the data that has already been collected.
The learnings from IBIS-II have been presented at multiple major international conferences and published in well-respected high impact medical journals.