Shortly after their discovery, the possibility of using neutrons in radiotherapy was recognized, but Stone's extensive trial of cyclotron-produced neutrons in the treatment of cancer between 1938 and 1943 (1) discouraged further exploration. New understanding of the physical and biological aspects of neutron interaction with tissue has rekindled interest in the potentialities of neutron therapy (2). The principal reason for the recent interest is that neutrons with their high linear energy transfer, unlike lightly ionizing x- and gamma-radiation, may overcome the radioresistance of anoxic foci within the tumor. One should not depreciate therapeutic advances afforded by new radioactive isotopes; none of these, however, has reduced the difference in the effective doses received by anoxic and well oxygenated cells. We are developing californium-252 sources for possible use in neutron therapy. This radionuclide with a half-life of 2.6 years emits 2.4 × 1012 neutrons per second per gram by spontaneous fission. The Cf252 sources could either be inserted into tissue and body cavities, or used as surface applicators. Other proposed methods of neutron therapy (3, 4) require nuclear reactors or cyclotrons found only at a few large universities and medical centers because of their cost and complexity of operation. In contrast, Cf252 sources will be much less expensive; they can be used in the same familiar manner as radium needles and the attendant problems of radiation protection are comparatively simple. Although only microgram quantities of Cf252 are now available for experimental use, quantities up to several grams will be available in the next few years. The energy spectrum of Cf252 neutrons is shown in Figure 1. Gamma radiation from this radionuclide is primarily from the prompt gamma rays associated with spontaneous fission and from the resulting fission products; gamma rays associated with alpha decay contribute less than 0.1 per cent of the total gamma emission. The gamma spectrum of a Cf252 source is shown in Figure 1. A source resembling a cell-loaded radium needle was prepared, containing 1.46 µg of Cf252 (Fig. 2). The design of the source and its testing during and after construction were aimed at eliminating the possibility of leakage and minimizing the consequences of capsule rupture. Californium hydroxide was uniformly electrodeposited on a 2-cm length of thin platinum-iridium rod previously sandblasted to increase the surface area. The rod was then heated to form Cf2O3 which is relatively insoluble in warm saline solution. The californium-bearing rod was inserted into a close-fitting cell that was sealed with a silver-soldered cap. Integrity of the cell was verified with a helium leak detector after the cell was exposed in 30-psi helium for thirty minutes. The leak rate was less than 2.4 × 10−9 cc of helium per second, the lower detection level of the instrument.
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Wright et al. (1967) studied this question.