The Relative Biological Effectiveness of Thermal Neutrons and of the Heavy Particles from the B¹⁰(n, α) Li⁷Reaction for Acute Effects in the Mouse

Abstract

Although the relative biological effectiveness (RBE) of alpha or of heavier ionizing particles has been extensively studied in microorganisms, only a few attempts have been made to determine the RBE of such particles in the mammal. This is due principally to the technical difficulties inherent in obtaining total-body or large-volume irradiation of the intact animal with heavy particles with a range in tissue of the order of a few microns. The most direct approach, i.e., injection of alpha emitters into the intact animal, is not completely satisfactory. The dose received by the tissues cannot be determined accurately because of the uncertain biological half-life of the injected material, the non-uniform and changing body distribution of dose as it is being administered, and the protracted and changing rate of dose administration to the various tissues. It is possible, by delivering total-body thermal neutron radiation to mammals previously injected with a capture element such as boron or lithium, to obtain irradiation of the tissues with the heavy recoil particles liberated instantaneously in the capture process. It was the purpose of the investigation to be described here to determine the apparent RBE of the heavy particles thus liberated for acute effects in the mouse. Although there are many well recognized objections to the use of the term RBE (1, 2), it remains a useful expression, provided its limitations and necessary qualifications are recognized. In the present study, the RBE of a given radiation is taken as the ratio of the physically measured dose in rep of a standard radiation (Co60 gamma, or in this report 250-kvp x-radiation) to produce a given biological endpoint, to the dose of the radiation in question, delivered over approximately the same time interval, required to produce the same biological effect. The RBE of different radiations has recently been reviewed (1, 2, 3), and the difficulties of obtaining meaningful values in the mammal are well recognized. Extensive studies have been made in the mouse and, despite the differences and uncertainties incident to the varying technics employed, fair agreement has been obtained in recent investigations. (Wide discrepancies in the earlier literature may be due in large part to the inadequacies of the radiation sources then available.) The RBE of high-energy x-rays (200 or 250 kvp) for acute mortality and spleen-thymus weight loss in the mouse (1) is approximately 1.5 compared to Co60 gamma radiation. The RBE for fast neutrons, which liberate energy in tissues chiefly by means of the recoil protons from elastic collisions with the hydrogen in the tissue, have an RBE of between 1 and 3, and possibly higher. It appears likely that the figure may decrease as neutron energy increases over the range of approximately 1 to 20 MEV or higher.