Dose-survival curves, alpha/beta ratios, RBE values, and equal effect per fraction for neutron irradiation of jejunal crypt cells

Jejunal crypt cell survival after regimens of one, two, three and five fractions was used to reconstruct composite single-dose survival curves for neutrons of five different energies. There is no obvious shoulder, but there is a gradual divergence from linearity that is most evident at low levels of cell survival (high doses). The effect of dose fractionation is relatively small, especially at the low doses characteristic of each treatment session in neutron radiotherapy. The α/β values for the linear-quadratic survival curves range from 27 to 40 Gy. The curves for neutrons are different from the curves for γ rays mainly in their α coefficients, as predicted by Kellerer and Rossi's theory of dual radiation action, but both α and β values are higher the lower the mean neutron energy, which is not consistent with the theory. The ratio of α coefficients reported here for various neutron beams to those for γ rays reported elsewhere ranges between 3.2 and 4.6. This range of values represents the maximum limits for RBE(n/γ) values (at very low doses), and is sometimes referred to as RBE(m). These values increase with decreasing neutron energy. The ratios of β coefficients for neutron and γ-ray survival curves were lower than the α ratios, ranging between 0.9 and 1.9, although not reliably distinguished from 1.0. Each of a series of equal dose fractions given at 3-h intervals produced a constant (logarithmic) decrease in cell survival as evidenced by the consistency of the estimate of cell survival from a certain single dose fraction, regardless of the level of cell survival (number of dose fractions) from which the estimate was made. Even more significant than the overlap of individual data points is the excellent fit of all the data to survival curves reconstructed on the assumption of an equal effect per fraction. An implication of these results is that, with neutrons, too little unrepaired injury persists at 3 h to influence the response to a subsequent exposure measurably.