BLOOD-BRAIN BARRIER 

A microscopic structure in the brain that separates blood capillaries from nerve cells. This prevents some substances from entering the brain. Many cancer chemotherapeutic agents, for example, cannot be used to treat brain tumors because they cannot pass through the barrier. Some others (CCNU, BCNU) are designed to penetrate it. There are techniques that can eliminate the barrier temporarily to allow the use of other antitumor drugs.