BREAST cancer tumours can produce a hormone in menopausal women which makes them incurable, a new study has found.

Oncologists have long been puzzled why some menopausal breast cancer sufferers no longer respond to treatment and see their disease return.

After surgery to remove tumours women are prescribed hormone therapy to suppress the sex hormone oestrogen which fuels the cancer’s growth.

But in some cases in menopausal women the tumours start producing aromatase, an enzyme which helps cancer cells spread by turning the hormone androgen into small amounts of oestrogen, the study led by Imperial College London and the European Institute of Oncology in Milan found.

The findings could lead to a new test to detect this and allow doctors to consider other treatments to help women live longer with secondary cancer.

Breast cancer is the most common cancer in the UK with 55,200 new cases diagnosed every year.

Around seven in 10 breast cancers are so-called ER positive, which means the cancer cells contain a receptor for the hormone oestrogen which fuels tumours.

After surgery they are usually given hormonal therapy to reduce the amount of oestrogen in the body.

Tamoxifen is given to women who haven’t been through the menopause and prevents oestrogen from binding to DNA in cancer cells.

During the menopause the ovaries stop producing oestrogen but it is still made in several other tissues by an enzyme called aromatase.

These women are given aromatase inhibitors which prevents residual oestrogen from being produced in other tissues.

However both tamoxifen and aromatase inhibitors stop working in around a third of patients, but why this drug resistance developed was until now unknown.

The new study found in one in four patients taking aromatase inhibitors, the tumours had increased production of aromatase in the cancer cells by increasing the number of aromatase genes, in a process called amplification.

Imperial’s Research Fellow Dr Luca Magnani said: “For the first time we have seen how breast cancer tumours become resistant to aromatase inhibitors.

“The treatments work by cutting off the tumour’s fuel supply – oestrogen – but the cancer adapts to this by making its own fuel supply.”