Metastasis is the process by which cancer cells break away from the original (primary) tumor, travel to other parts of the body, and form new tumors there. These new tumors are called metastatic tumors and are made of the same type of cancer cells as the original cancer (for example, breast cancer that has spread to the bone is still breast cancer, not bone cancer).

Simple definition

  • Metastasis means the spread of cancer cells from where they started to a distant part of the body.
  • When cancer has spread in this way, it is often called metastatic cancer or stage 4 cancer.

How metastasis happens

  • Cancer cells can break off from the primary tumor, invade nearby tissue, and enter blood vessels or lymphatic vessels.
  • They travel through the bloodstream or lymphatic system, settle in another organ (like lungs, liver, bone, or brain), and start growing into a new tumor there.

Why metastasis matters

  • Metastasis is one of the main reasons cancer can be life-threatening, because it means the disease is no longer confined to one spot and becomes harder to treat completely.
  • Treatment for metastatic cancer often focuses on controlling growth, relieving symptoms, and maintaining quality of life, rather than cure in many cases.

A quick way to picture it

  • Imagine the primary tumor as a “root” tumor and metastatic tumors as “seeds” that have been carried to new “soil” (organs) where they can grow again.
  • Even in a new organ, the metastatic tumor keeps the identity of the original cancer type (for instance, metastatic lung cancer in the brain is still lung cancer under the microscope).

If you’re asking because of a real medical situation, it is important to speak with a doctor or oncology team, who can explain what metastasis means for that specific cancer type and stage.