Metastasis
Metastasis happens when cancer cells break away from the original (primary) tumor, travel through the bloodstream or lymphatic system, and form a new tumor in another organ or tissue. A metastatic tumor is still named for, and treated based on, where the cancer originated — a breast cancer that has spread to the bone is still breast cancer, not bone cancer. Whether metastasis is present is one of the three factors (alongside tumor size and lymph-node involvement) that TNM staging measures.