Simplify the following text for a Grade 10 native English-speaking person.
Original: Metastatic breast cancer is a complex multi-step process involving the expansion of cancerous cells from the breast to other areas of the body . It is a serious complication of breast cancer , as metastatic disease in breast cancer is often fatal , with treatments mainly limited to palliation .
Simplified: Metastatic breast cancer is Breast cancer that has spread beyond the primary tumor .