The Relative Effects of Fast Neutrons and X Rays on Tumour and Normal Tissue in the Rat 1. Single doses

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Abstract
1. Malignant and normal tissues in rats have been irradiated with single does of X rays and fast neutrons. The malignant tissue was a transplanted sarcoma known to contain hypoxic cells, and the normal tissue was one hind foot. 2. The response of the tumour to each radiation is similar to that expected from cell killing alone. 3. For large doses the RBE of the neutrons is greater for the tumour tissue than for the normal tissue. 4. For equal damage to normal tissue the therapeutic effect of radiation, in terms of delaying the growth of the tumour, is greater with neutrons than with X rays. 5. The long-term damage to normal tissue after irradiation with neutrons is not greater than after irradiation with X rays. 6. The therapeutic gain factor, denned as the ratio of the fast neutron RBE for neoplastic tissue to that for normal tissue was found to be about 1·8. This value has been shown to represent a large therapeutic gain in the test system used.