Radiation Therapy

Radiation therapy is the use of penetrating beams of ionizing radiation, primarily to treat cancerous/malignant tumors. Conventional radiation therapy includes photon (x-ray) and electron radiation, which is available at many clinics and hospitals. These beams are produced by electron accelerators or from radioactive sources such as cobalt. Hadron therapy includes neutrons and protons, which are generated using proton or deuteron accelerators.

The basic effect of ionizing radiation is to destroy the ability of cells to divide, by damaging their DNA strands. For photon, electron and proton radiation the damage is done primarily by activated radicals produced from atomic interactions. These types of radiation are called low linear-energy-transfer (low LET) radiation. Neutrons are high linear-energy-transfer (high LET) radiation and the damage is done primarily by nuclear interactions. If a tumor cell is damaged by low LET radiation it has a good chance to repair itself and continue to grow. With high LET radiation the chance for a damaged tumor cell to repair itself is very small.

Neutron Therapy

In general, fast neutrons can control very large tumors, because unlike low LET radiation, neutrons do not depend on the presence of oxygen to kill the cancer cells. In addition, the biological effectiveness of neutrons is not affected by the time or stage in the life cycle of cancer cells, as it is with low LET radiation. It often happens that large tumors have metastasized (spread) to other parts of the body before the patient seeks treatment. In these cases neutrons can be used to control the primary tumor, but chemotherapy must be used to limit the spread of cancer through the rest of the body.

Because the biological effectiveness of neutrons is so high, the required tumor dose to kill cancer cells is about one-third the dose required with photons, electrons or protons. A full course of neutron therapy is delivered in only 10 to 12 treatments, compared to 30 - 40 treatments needed for low LET radiation.

at Fermilab

Fermilab was built for high-energy physics research. The cancer treatment at Fermilab is possible because Fermilab's Linear Accelerator (Linac), provides beam for both high energy physics research and neutron radiation therapy. Under computer control the Linac beam was automatically switched to the treatment room as needed for patient treatment. To create the neutron beam, the space between two of the radiofrequency accelerating tanks in the linear accelerator was increased to allow room for a bending magnet. This magnet deflects protons from the Linac into the clinical beam line where they strike a beryllium target. Interactions between protons and beryllium atoms produce neutrons. An absorbing wall with an appropriately tapered hole is located between the neutron source and the patient. The opening size is adjustable, and beam-shaping devices are used to assure that radiation is delivered only to the part of the body that the physician wants to treat. The Neutron Therapy Facility at Fermilab was the highest-energy clinical fast neutron beam in the United States. This beam had the best tissue-penetrating ability and, therefore, the best capability for treating deep-seated, large tumors.