Cancer types are diagnosed and treated according to what’s called their “primary” site – i.e. where the initial tumour developed. If detected early enough, a cancer is confined to its primary site. If it spreads, or “metastasises”, it leads to secondary or metastatic tumours elsewhere in the body. Even when a cancer has metastasised, doctors can usually trace its primary site with diagnostic technology. So, even though women with metastatic breast cancer have tumours outside the breast, they are registered and treated as breast cancer patients if tests show the breast was the primary site. A metastatic cancer that cannot be traced to their primary site is diagnosed as “cancer of unknown primary”. Breast cancer most commonly spreads to the lung, liver, bones and brain.