CALIFORNIUM 252 undergoes spontaneous fission with a half-life of two and six-tenths years. The fission-spectrum neutrons are densely ionizing and therefore may be more efficient than x rays or gamma rays in eradicating malignant tumors, particularly those containing anoxic foci. The use of this nuclide for interstitial and intracavitary radiotherapy was suggested by Schlea and Stoddard (1), and sources have been fabricated for this purpose at the Savannah River Laboratory of the USAEC (2). Ten of the above sources, each originally about 4.5 µg, were obtained by us for the purpose of performing pretherapeutic radiobiological experiments. The determinations of relative effectiveness of 252Cf vis-à-vis conventional forms of protracted irradiation is necessary prior to clinical application. Materials And Methods HeLa S3 cells were cultured in Puck's N16 medium containing 10 per cent fetal calf serum, and 5,000 units of penicillin and 5,000 µg of streptomycin per 100 ml, Six-to seven-day-old stock cultures with confluent growth were used for preparation of clone cultures; the medium was discarded, cells washed once with isotonically buffered Versene 1: 5,000 and treated for five to ten minutes at 37°C with GIBCO EDTA-Trypsin solution (IX). The cell suspension was transferred to a 10 ml screw-cap centrifuge tube and centrifuged five minutes at 700 r.p.m.; the medium was carefully removed and cells resuspended in 2 ml of N 16 medium and cell count made in a counting chamber. If necessary, a further dilution of cells in medium was made and approximately 3,000 cells added to 2.7 ml of medium in Falcon plastic, 25 cc tissue culture flasks. The cultures were gassed with 5 per cent CO2-air mixture, shaken gently to distribute the cells, and placed on a level surface in a 37°C incubator. The preirradiation growth period was about seventeen hours in most instances. Ten needles containing ~4.5µg 252Cf each with a linear intensity of 2.25 µg/cm active length were arranged in a planar array to provide a homogeneous irradiation field over an area of 16 cm2 at a distance of 9.2 mm corresponding to the inner surface of the floor of the tissue culture flask. The dose rate at this distance was 16.1 rads/hr. initially and decreased slightly by completion of the experiments. A similar arrangement of radium needles (total of 40 mg) provided a dose rate of 31 rads/hr. to the floor of the tissue culture flask. Cells were also irradiated acutely with 250 kVp x rays (h.v.l, = 1.2 mm Cu, dose rate = 111 rads/min.). Results The survival curves for HeLa S3 cells irradiated with 252Cf at ~16 rads per hour, radium at 31 rads per hour, and acute 250-kVp x rays are illustrated in Figure 1.
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Fairchild et al. (1969) studied this question.