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Oncogenic transformation of synchronized C3H 10T1/2 cells was determined after exposure to 4.3 MeV alpha particles (LET = 101 keV/microns). Two synchronization techniques were tested using basic and modified protocols: one based on the release of cells from contact inhibition and the second on the mitotic shake-off method. Progression of cells through the cycle was followed as a function of time by flow cytometric analysis, DNA labeling for passage through S phase, the growth curve for the cell number and mitotic index measurements. The conclusion is that, although the release of cells from confluence provides higher yields of synchronized cells, mitotic shake-off proved to be the best way of collecting a synchronized population of minimally perturbed cells. Cells synchronized by mitotic shake-off were irradiated with 0.30 Gy in the interval between 2 and 10 h corresponding to G1 and early S phases. For comparison asynchronous populations were irradiated in parallel. Oncogenic transformation frequency, corrected for background, in mid-G1 phase was (18 +/- 4) x 10(-5) (average values of frequencies at 4 and 6 h) compared with the value of (8 +/- 4) x 10(-5) for the asynchronous population. While these data are suggestive of a trend toward a slightly increased sensitivity in mid-G1 phase, it is not statistically significant. The surviving fraction is constant in G1 phase.

Oncogenic transformation of C3H 10T1/2 cells was determined after exposure to graded doses of 4.3-MeV α particles LET = 101 keV/μm. The source of α particles was ${}^{244}{\rm Cm}$ and the irradiation was done in an irradiation chamber built for the purpose. Graded doses in the range of 0.2 to 300 cGy were studied with special emphasis on the low-dose region, with as many as seven points in the interval up to 10 cGy. The dose-effect relationship was a complex function. Transformation frequency increased with dose up to 2 cGy; it seemed to flatten at doses between 2 and 20 cGy but increased again at higher doses. A total of 21 cGy was delivered in a single dose or in 3 or 10 equal fractions at an interval of 1.5 h. An inverse dose-protraction effect of 1.4 was found with both fractionation schemes. Measurements of the mitotic index of the population immediately before the various fractions revealed a strong effect on the rate of cell division even after very low doses of radiation. Mit...

Cells synchronized by mitotic selection were irradiated at various times during the first generation cycle with 1.75 and 3.5 Gy of 31-MeV protons and then analyzed both for lethality and for chromosome aberrations. The results showed a maximum of radiosensitivity in late G/sub 1/-early S followed by an increase in resistance to a maximum in S in both endpoints of damage. For both types of effect, early to mid G/sub 1/ is definitely less sensitive than mid to late G/sub 1/. The ratios between the maximum and minimum sensitivity were approximately 1.4 and 2 for 1.75 and 3.5 Gy, respectively. These values agree with data reported to the literature on different mammalian cell lines exposed to X rays at comparable survival levels. For both dose values, the logarithm of the surviving reaction at various times of the generation cycle is linearly related to the corresponding chromosomal aberration frequency, with 37% survival corresponding to about one aberration per cell.

Experimental data are presented on the frequencies of transformations in C3H10T1/2 cells exposed to 31 MeV protons (LET = 1.83 +/- 0.02 keV/mum in tissue) in a dose interval between 0.25 and 7.0 Gy. The transformation frequency per surviving cell curve showed a marked change in slope over the dose range used. At higher doses, above about 2 Gy, it steepened very sharply in comparison with the lower dose range. If fitted to a power of the dose, the power in the higher range was about five times that in the lower range.

The relative biological effectiveness (RBE) was determined for proton beams of 31, 12, and 8 MeV produced at the Milano University Cyclotron. Survival curves for human cells grown in monolayer at different proton energies and for γ rays from60 Co were determined. The minimum clone size to be chosen for definition of true survivors was examined. RBE values of 1.0 ± 0.1, 1.4 ± 0.2, and 1.5 ± 0.2, respectively, were found and compared with the results of other experiments in this energy range.