Fast neutron therapy

Fast neutron therapy
Intervention
Patient treating room for neutron radiation therapy
ICD-10-PCS	D?0?5ZZ
ICD-9	92.26
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Fast neutron therapy utilizes high energy neutrons typically between 50 and 70 MeV to treat cancer. Most fast neutron therapy beams are produced by reactors, cyclotrons (d+Be) and linear accelerators. Neutron therapy is currently available in Germany, Russia, South Africa and the United States. In the US three treatment centers operate in Seattle, Washington, Detroit, Michigan and Batavia, Illinois. The Detroit and Seattle centers use a cyclotron which produces a proton beam impinging upon a beryllium target; the Batavia center at Fermilab uses a proton linear accelerator.

Radiation therapy kills cancer cells in two ways depending on the effective energy of the radiative source. The amount of energy deposited as the particles traverse a section of tissue is referred to as the linear energy transfer (LET). X-rays produce low LET radiation, and protons and neutrons produce high LET radiation. Low LET radiation damages cells predominantly through the generation of reactive oxygen species, see free radicals. The neutron is uncharged and damages cells by direct effect on nuclear structures. Malignant tumors tend to have low oxygen levels and thus can be resistant to low LET radiation. This gives an advantage to neutrons in certain situations. One advantage is a generally shorter treatment cycle. To kill the same number of cancerous cells, neutrons require one third the effective dose as protons. Another advantage is the established ability of neutrons to better treat some cancers, such as salivary gland, adenoid cystic carcinomas and certain types of brain tumors, especially high-grade gliomas

When therapeutic energy X-rays (1 to 25 MeV) interact with cells in human tissue, they do so mainly by Compton interactions, and produce relatively high energy secondary electrons. These high energy electrons deposit their energy at about 1 keV/µm. By comparison, the charged particles produced at a site of a neutron interaction may deliver their energy at a rate of 30-80 keV/µm. The amount of energy deposited as the particles traverse a section of tissue is referred to as the linear energy transfer (LET). X-rays produce low LET radiation, and neutrons produce high LET radiation.

Because the electrons produced from X-rays have high energy and low LET, when they interact with a cell typically only a few ionizations will occur. It is likely then that the low LET radiation will cause only single strand breaks of the DNA helix. Single strand breaks of DNA molecules can be readily repaired, and so the effect on the target cell is not necessarily lethal. By contrast, the high LET charged particles produced from neutron irradiation cause many ionizations as they traverse a cell, and so double-strand breaks of the DNA molecule are possible. DNA repair of double-strand breaks are much more difficult for a cell to repair, and more likely to lead to cell death.