It's not clear exactly what causes breast cancer in men, although a number of things can increase your risk of developing the condition.

Age

Like many cancers, men have an increased risk of developing breast cancer as they get older. Most cases are diagnosed in men aged 60-70. It's rare for young men to develop the condition.

Genetics and family history

A genetic mutation is a permanent alteration in the DNA sequence that makes up a gene. The result is that one or more of the body’s processes may not work in the way they should.

There are a number of genetic mutations known to increase your risk of developing breast cancer. The most significant mutation identified is known as the BRCA2 mutation. Faulty genes are believed to be the cause of male breast cancer in around one or two in every 10 cases.

There's also evidence that breast cancer can run in families, especially in men who have a first-degree relative, such as a mother or sister, who has developed breast cancer.

Routine testing for the faulty genes that cause breast cancer in men isn't usually carried out on the NHS, unless specifically requested by a specialist. However, some private clinics may offer gene testing. Tests can be expensive, with prices ranging from around £2,000 to £3,000.

Oestrogen

There's evidence that high levels of the hormone oestrogen, or prolonged exposure to it, can increase the risk of breast cancer in men.

Compared to women, men tend to have low levels of oestrogen, but there are circumstances that can increase the levels of oestrogen in men. These include:

• hormone treatments – man-made (synthetic) versions of oestrogen are often used to treat prostate cancer and are also given to transgender patients undergoing a male to female sex change
• obesity – obese men have higher levels of oestrogen than normal
• cirrhosis – this is scarring of the liver, often caused by long-term alcohol misuse

There is also a rare genetic condition affecting males called Klinefelter syndrome. This is a congenital condition (boys are born with it) and it means affected males produce less of the hormone testosterone than usual.

As testosterone usually helps to limit the effect of oestrogen, men with Klinefelter syndrome are more likely to develop breast cancer than the general male population.

Occupational risks

There's some evidence that men who work in hot environments are twice as likely to develop breast cancer compared with men who work in cooler environments. Environments linked to an increased risk of breast cancer in men include:

• blast furnaces
• steel works
• rolling mills – a factory where metal (usually steel) is shaped using rollers
• car manufacturing plants

One hypothesis to help explain the link between working environment and the increased risk of developing breast cancer is that excessive heat may damage the testicles, which could lead to an increase in oestrogen levels.

Another hypothesis is that working in hot environments usually involves exposure to certain chemicals that may increase the risk of developing breast cancer in men.

Rates of breast cancer are also unusually high in men who manufacture perfumes and soaps. They are seven times more likely to develop breast cancer than the male population at large. The reason for this increased risk is still unclear. Exposure to certain chemicals seems to be an obvious possibility, but this hasn't yet been proven.

Radiation

Exposure to radiation has been linked to an increased risk of developing breast cancer in men.

Research has found that men who received a course of radiotherapy (where high energy X-rays are used to kill cancerous cells) directed at the upper chest were seven times more likely to develop male breast cancer compared to the population at large.

However, even a seven-fold increase means the chance of developing breast cancer is still very low.