my sister was a breast cancer survivor for 8 years she went...

My sister was a breast cancer survivor for eight years. She went to her specialist every six months and had digital mammograms. September 26 of this year, she passed away so suddenly. How could this have happened? The cancer spread to her liver, spine and lungs, and after a week in the hospital, she passed away.We are all devastated and angry that the cancer spread so fast without the doctor noticing it. Could she have survived? Why didn't the doctor catch this?