Breast cancer cells, unlike normal cells, cannot control their growth and division. Malignant or breast cancer cells can take over and destroy tissue and organs near them because they grow rapidly and without any order. As well, breast cancer cells can break away from the tumour and enter the blood system or lymphatic system (the system that fights infections). This may result in the breast cancer cells spreading to other parts of the body (metastases). Although most breast cancer occurs in women, approximately 1 percent of all breast cancer occurs in men. Treatment is similar in both sexes.

Breast cancer starts when breast cells begin to grow and multiply out of control. Over time, as the cells collect together, they form a growth or tumour.