Cancer can begin in any organ or tissue of the body. The original tumor is called the primary cancer or primary tumor. It is usually named for the part of the body or the type of cell in which it begins.
Metastasis means the spread of cancer. Cancer cells can break away from a primary tumor and enter the bloodstream or lymphatic system. That is how cancer cells spread to other parts of the body.
When cancer cells spread and form a new tumor in a different organ, the new tumor is a metastatic tumor.
The cells in the metastatic tumor come from the original tumor. This means, for example, that if breast cancer spreads to the lungs, the metastatic tumor in the lung is made up of cancerous breast cells (not lung cells).
In this case, the disease in the lungs is metastatic breast cancer (not lung cancer). Under a microscope, metastatic breast cancer cells generally look the same as the cancer cells in the breast.