What does "metastasis" refer to in a medical context?

In a medical context, "metastasis" specifically refers to the process by which cancer spreads from the original (primary) site to other parts of the body. This occurs when cancer cells break away from the primary tumor, travel through the bloodstream or lymphatic system, and form new tumors in distant organs or tissues. Understanding metastasis is crucial because it affects treatment decisions, prognosis, and the overall understanding of the cancer's progression.

The other options present concepts that are distinct from metastasis. For example, the onset of a new disease does not encapsulate the idea of cancer spread, nor does it relate to the behavior of existing cancer cells. Similarly, cancer treatment and prevention methods involve various therapies and strategies aimed at managing or preventing cancer, but they do not define the phenomenon of metastasis itself. Therefore, the correct interpretation of metastasis is directly tied to the spread of cancer, making it the accurate choice in this context.