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Fast Neutrons in Radiotherapy

Neutrons, like Xand gamma-rays, are uncharged and cause ionization via secondary particles. However, most of the ionization in tissue irradiated with neutrons (of energies in the region of a few MeV) results from secondary protons, in contrast with the electrons which arise from Xor gamma-irradiation. The density of ionization is generally much greater along the tracks of these heavier particles, so that neutrons dissipate more energy at a higher linear energy transfer (LET) than Xor gamma-rays. The radiobiological properties of high and low LET radiations are rather different, and may be illustrated in terms of survival curves. Fig 1 shows typical survival curves for cells irradiated with X-rays or neutrons, and illustrates two general features: