Around half of all patients with breast cancer could benefit from taking the hormone progesterone alongside their treatment, indicate findings of a study funded by Cancer Research UK and published in the journal Nature.

It has long been known that women whose breast cancer tumours carry progesterone receptors as well as ones for oestrogen have a better prognosis, but the reason why has remained a mystery for many years. 

Now, scientists at Cancer Research UK’s Cambridge Research Institute and the University of Adelaide have discovered that progesterone receptors in breast cancer cells communicate with oestrogen ones to change their behaviour and slow tumour growth.

Their study showed that, in a lab setting, cancer cells were about 50% smaller when given a mix of progesterone and veteran first-line therapy tamoxifen than those hit with tamoxifen alone.

The finding, says the charity’s Jason Carroll, “provides a strong case for a clinical trial to investigate the potential benefit of adding progesterone to drugs that target the oestrogen receptor, which could improve treatment for the majority of hormone-driven breast cancers”.

Around 50,000 new cases of breast cancer are diagnosed in the UK each year and the researchers believe that around half could potentially benefit from treatment with progesterone, which is cheap and widely available.