Metastatic Breast Cancer – what you need to know Breast cancer is the most common cancer in women, affecting approximately one in eight women during their lifetime. Breast cancer mostly affects older women, but in rare cases, it may affect younger women under 45. Metastatic breast cancer is the most advanced stage of breast cancer. It is also known as advanced breast cancer, stage 4 breast cancer, or metastatic disease. Metastatic breast cancer is a serious and often life-threatening disease, affecting both women and men. Metastatic breast cancer occurs when breast cancer cells spread beyond the breast and nearby lymph nodes to