The effect of radiation on the growth of vertebrae in the tails of rats. 3. The response to cyclotron neutrons

Experimental studies on the response of growing bones to X ray and neutron irradiation

The effects of X ray and 14 MeV neutron irradiation on growing tibia of mice were studied. As reported in the literature, the age of the animal at irradiation was an important factor if the endpoint in analyzing the radiation effect is the absolute shortening, difference in length between unirradiated and irradiated tibia, achieved by the treatment. However, if the growth remaining at the moment of irradiation is taken into account no significant differences in effect were observed with aging, and the dose given appeared to be the overruling factor. This was valid for both X ray and neutron irradiation. Absorbed dose-response curves after X ray irradiation showed a shoulder in the lower range of absorbed doses, which was not the case for neutron irradiation. RBE values for 14 MeV neutrons as compared to 250 kV X rays varied from 3.6 to 2.0 depending on the single absorbed dose level. No differences in RBE were found with aging. A mathematical formula was derived which can predict the absolute shortening when the initial and final length of the tibia are known as well as the absorbed dose of irradiation to be administered.

Can late effects of neutron irradiation in man be estimated from early responses?

Slowly-developing tissue changes after neutron irradiation should be more easily predicted from acutely-developing injury than is the case with X rays. The difference between tissue responses to neutrons and X rays is that cell survival in both rapidly and slowly responding tissues is a direct logarithmic function of neutron dose, at least up to about 3 Gy, whereas the X-ray dose-survival relationship differs between the two types of tissue: the target cells for late injury are more susceptible to killing from accumulation of sub-lethal X-ray injury and hence the survival curve diverges from its initial essentially linear region more rapidly than does that for the target cells for acute injury.

Biological bases for high RBE values for late effects of neutron irradiation

The late effects of fractionated irradiation with neutrons have been relatively more severe than after x-irradiation. Reasons for the RBEn/x being higher for late than for acute effects may include: (1) Late effects are reduced more by fractionation of X ray doses than are acute effects, whereas, with neutrons, the fractionation response is the same in rapidly-and slowly-responding tissues; (2) Late-responding tissues are less "sensitized" (and are, therefore, relatively protected) by redistribution throughout the division cycle during a fractionated regimen than are acutely-responding tissues: since neutron responses are less affected by cell-cycle distribution than are X ray responses, the relative protection of slowly-responding tissues is less; (3) The target cells for late, but not acute injury, may repair potentially lethal damage after X ray, but not after neutron exposure. Thus, the dissociation of RBE values for acute and late injury reflects mainly the dissociation between acute and late responses to conventional fractionated X ray regimens and, from the point of view of complications of radiotherapy, we should not condemn neutrons but praise X rays. Since fractionation of neutron doses into increments equivalent to those used in X ray therapy does not provide a preferential sparing of slowly-responding tissues, it is reasonable to shorten the overall duration of neutron treatment to deliver the total dose tolerated by the relevant "late-effects" tissue(s) in the shortest time consistent with acceptable acute responses.