Age (generally in men over 60 years, but can be found at any age)

Family history (breast cancer and/or mutations of the BRCA gene, for example, affecting females in the family)

High estrogen levels (cause by obesity, certain medications or hormone treatments)

Radiation (side effect of radiation therapy in the chest area)

Testicular issues (injuries, inflammation or removal of testicles)

Alcohol (heavy consumption and liver disease)

Klinefelter syndrome (a rare genetic condition at birth)