A trial looking at letrozole and megestrol acetate for breast cancer (PIONEER)

This trial compared a short, 2 week treatment with letrozole and megestrol acetate before surgery for early breast cancer. This was compared to having letrozole on its own.

It was for women who:

• had a high number of oestrogen receptors on their breast cancer cells (oestrogen receptor positive or ER positive)

• had been through the menopause

Women could join the trial between 2017 and 2022. The team published the results in 2025.

The trial looked at changes in cancer cells after 2 weeks of treatment. It did not look at how treatment worked in the longer term.