Very little research has been done to learn how radiation causes injury to the brain. It is believed there are three ways injury can occur. The diagram (Figure 1.) shows how brain radiation necrosis is caused by the three types of radiation injury.

A. Brain Is Inflamed
Radiation makes the brain inflamed; much like skin becomes inflamed when it is burned. This inflammation damages the normal protective barrier of the brain. When this protective barrier, known as the blood-brain barrier, is damaged, more fluid enters the brain than normal. This fluid change triggers more swelling in the brain which continues to get worse. This is the most consistent finding with brain radiation injuries. Because the swelling (edema) causes the pressure in the brain to build up, blood flow to the brain is slowed. Due to the slow blood flow, the brain operates without enough oxygen, and so, the parts of the brain without adequate amounts of oxygen begin to die.

B. Damage to Blood Vessels
Radiation can directly damage the blood vessels in the brain. Damage to the cells lining the blood vessels is common in radiation treatment. This damage can lead to blockage of the arteries, preventing enough blood and oxygen from getting to the brain. As these blood vessels become blocked, swelling can occur, causing increased pressure and restricted blood flow.

C. Damage to Brain Cells
Radiation can directly damage the nerves in the brain, leading to death of the affected parts of the brain. This causes swelling around the injury site. The edema produces increased pressure and decreased blood flow, depriving the brain of enough oxygen.

Figure 1. Possible mechanisms of injury in brain radionecrosis.
ENLARGE

The injury caused by brain radiation is similar to the injury that occurs when other parts of the body are treated with radiation. The big difference is the swelling that occurs. The brain does not have a way of dealing with this edema because the brain is encased in bone, the skull. When radiation injury happens in other parts of the body, treatments are given to help blood flow through the injured tissue, promoting healing of the injured tissue. Treatment of radiation injuries to the brain has been focused on controlling the swelling, and not on helping restore blood flow back into the injured areas of the brain.