Why is metastatic breast cancer in the lung not called lung cancer?

Metastatic cancer keeps the name of the primary (original) cancer, not the place it spreads to. According to the National Cancer Institute, breast cancer that spreads to the lung is called metastatic breast cancer, not lung cancer. It is treated as stage 4 breast cancer, not as lung cancer.

The reason comes down to what the cancer cells actually are. When metastatic cancer cells are looked at under a microscope and tested in other ways, they have features like the primary cancer and not like the cells in the place where the metastatic cancer is found. In this example, the cells in the lung tumor look and behave like breast cancer cells, not lung cells.

This is also how doctors can tell that it is cancer that has spread from another part of the body, rather than a new cancer that started there.

Because the cancer is still made of breast cancer cells, treatment is based on the original cancer type. A healthcare team can explain how this applies to a specific diagnosis.