Genetic factors are responsible for only a small percentage of cases.
However, the environmental factors responsible for this variation are at present unknown. Age is the most recognised risk factor for breast cancer, and incidence increases with age. The risk of breast cancer is doubled among women with a first-degree relative diagnosed with breast cancer before the age of 40 years. Genetic and familial factors are described in greater detail below. Medical history Women with biopsy-confirmed benign breast disease are at increased risk of subsequent breast cancer.