Breast cancer is the result of genetic abnormalities which allow breast cells to grow uncontrollably. These abnormal gene "mistakes", or mutations, are usually acquired as we age. 85 - 90% of breast cancers are caused by this process, and the other 5 - 10% are due to inherited gene mutations.
How it's caused
Normally cells are created, grow, and die in a controlled way. However, when abnormal changes occur in the genes that regulate this process, normal gene function can be turned on or off. Damaged cells are then able to keep growing and dividing, and a tumour is formed.
About 90% of breast cancers arise from cells lining the milk ducts, and 10% arise from the cells lining the lobules.
Benign or Malignant?
A tumour may be benign or malignant. A benign tumour is not cancer, and not usually life-threatening, although it may cause problems as it grows. It will not spread to other parts of the body.
A malignant (cancerous) tumour, such a breast cancer, has the potential to grow and spread beyond its original site by invading healthy tissue and gaining access to lymphatic and blood vessels, which gives it the ability to spread around the body.
Learn more about breast anatomy.