Breast cancer, however, can cross this natural barrier. Researchers think they may have found the genes responsible.
Spreading to the lungs seems to be the work of two genes known as COX2 and HB-EGF.
These genes appear to increase the mobility and invasiveness of cancer cells, allowing cancer to metastasize.
Cancer that has spread from its original location is said to have metastasized. For example, if breast cancer spreads to the lungs, it is not lung cancer, but rather metastasized or metastatic breast cancer.
The gene which seems to be responsible for allowing breast cancer to metastasize into the brain is called ST6GAL NAC5. It seems that this gene makes the outermost coat of breast cancer cells sticky.
The breast cancer cells then remain in the capillaries of the blood brain barrier longer and may seep through the capillary walls into the brain.
Understanding how cancer becomes mobile and moves through the body will help develop additional medications and treatment modalities.
Each year, more than half a million women, die from breast cancer, the most common cause of cancer deaths in women.