Men with naturally high levels of the female hormone estrogen have a greater risk of developing breast cancer, according to according to an epidemiology paper published in the Journal of Clinical Oncology.

This is the first time a link between estrogen levels in the blood and male breast cancer has been identified, despite its connection to breast, womb and ovarian cancers in women, according to study author Professor Tim Key, Cancer Research UK's hormone and nutrition expert at the University of Oxford. Men with the highest levels of estrogen were two and a half times more likely to develop breast cancer than men with the lowest levels of the hormone.

Still, male breast cancer is very rare, with one man in every 100,000 diagnosed with breast cancer each year in the UK. That is 350 male cases in the UK each year compared with nearly 50,000 cases of breast cancer in women. The symptoms, diagnosis and treatment of male breast cancer are very similar to breast cancer in women. The main risk of developing the disease in men is age and almost eight in 10 cases are diagnosed in those aged 60 and older.

The aim was to study a large international pool of men with breast cancer. The research compared estrogen levels in 101 men who went on to develop breast cancer with 217 healthy men.

Key says, "We've shown for the first time that just like some forms of the cancer in women, estrogen has a big role to play in male breast cancer. So now the challenge is to find out exactly what this hormone is doing to trigger this rare form of the disease in men, and why some men have higher levels of estrogen in their blood. Our discovery is a crucial step forward in understanding the factors behind male breast cancer."