Brain metastasis is when malignant or cancerous cells have spread from another part of the body to the brain. About 170,000 people were diagnosed with brain metastases in 2012, and brain metastases became more common than primary tumors, such as primary brain tumors (62,000 cases), lymphoma ) (79,000) or rectal cancer (103,000). When a person has a brain tumor, it is usually because the cancer has spread from other parts of the body to the brain. The most common types of cancer that metastasize to the brain are lung cancer, breast cancer, and melanoma.

Lung cancer is the second most common cancer and the leading cause of cancer death in both men and women, accounting for nearly 25% of all cancer deaths. However, the number of new lung cancer cases is also continuing to decline, in part because of people giving up smoking and advances in early detection and treatment.