Radiation Therapy

Stereotactic Irradiaition

Stereotactic techniques as described above can be combined with radiation therapy by placing radiation sources directly into the tumor (stereotactic Interstitial Irradiation) or by coupling a Linear Accelerator to a stereotactic frame. In both these procedures he tumor volume is defined in three dimensional space by stereotactic CT and MRI. In stereotactic interstitial irradiation multiple radioactive sources are placed within the tumor tissue. These sources generate a radiation dose field which has been pre-calculated to superimpose on the shape and size of the tumor volume. The problem with this therapy relates to the fact that it is invasive and about 40% of the patients may require an open operation to remove dead tissue (radiation necrosis).

In stereotactic external beam irradiation the tumor volume is placed into the center of the radiation beam from the linear accelerator. This therapy can also be fractionated similar to standard radiation treatments as described above. The problem here is that a stereotactic frame would have to be fitted for each treatment. However, there are now some techniques for holding the patient's head in a non-invasive manner.

In stereotactic radiosurgery all of the radiation can be delivered to the defined tumor target volume in one carefully calculated dose. Stereotactic radiosurgery can employ a linear accelerator which is attached to a stereotactic frame or a Gamma Knife in which highly focused weak radiation beams from 201 radioactive sources arranged in a sphere combine in the center of the sphere to deliver a very precise dose of radiation at the center of the sphere. In practice a patient's head, retained in a stereotactic headframe is moved so that the tumor is positioned in the center of the sphere. With this method the tumor receives a lethal dose of radiation while the surrounding brain tissue receives considerably less radiation.

There are several experimental radiation methods presently being investigated; These include Boron Neutron Capture Therapy (BNCT) in which a patient is given a Boron compound which is taken up by tumor cells. The patients head is then positioned at the neutron port of a nuclear reactor and neutrons are absorbed into the Boron forming gamma irradiation and an alpha particle which kills the tumor cell. In addition Bragg-Peak irradiation and Helium ion irradiation are now being studied in the treatment of some brain tumors. The results on the above are very preliminary and the disadvantage of these methods is that they are available at only a few institutions around the world.