Experimental procedures for the simultaneous exposure of large numbers of animals to total body x-radiation.

Abstract

IN experimental radiology it is often desirable to irradiate simultaneously large numbers of animals in an incident field of identical characteristics. In attempting such experiments the following difficulties are encountered: With conventional clinical x-ray units or small industrial units, the fields of uniform radiation are small; only a limited number of small animals can be exposed at one time, and for the exposure of large animals, unless distance and intensity are altered, numerous ports are needed. With multiple ports there is much overlapping of the fields of irradiation and the ultimate dosage may be difficult to estimate. In addition, the slightest inconsistency in tube output or any discrepancy in duplicating geometrical relationships makes it difficult to repeat the lethality of a given exposure with the smaller x-ray units. Biological variations are evidenced by the wide range of individual responses observed following a given exposure. Lorenz (1) has shown that there is less variability in the lethality of total body irradiation when highly inbred strains of laboratory animals are used. Inbred rabbits, guinea-pigs, and dogs are not readily available, however, and the availability of inbred mice and rats is limited because of the great demand for them in other experimental fields. On the other hand, when a heterogeneous strain of laboratory animal is used, the basic genetic pattern of the strain is constantly changing and it becomes desirable to employ large, paired experimental and control groups of simultaneously irradiated animals in order to obtain statistically reliable data. Animals caged in groups may exhibit varying degrees of pugnacity and cannibalism. While these factors are not always in evidence, they are unknown potentialities which may directly or indirectly affect the outcome of any experiment. By housing and irradiating animals in individual cages we have attempted to eliminate these factors and at the same time provide a greater degree of uniformity in experimental irradiation procedures. In previous work at this Institute the radial beam of a G.E. 1,OOO-kv.p., 3-ma., industrial x-ray unit was used to irradiate swine and goats (2). The present report is concerned with the use of this same x-ray unit and, in addition, a 2,000-kv.p. unit of similar design, for simultaneously irradiating large numbers of different laboratory animals under rigidly controlled conditions. Procedure In order to eliminate as many of the above mentioned difficulties as possible, the following procedures have been adopted for handling and irradiating mice, rats, guinea-pigs, rabbits, and dogs. The methods of handling mice will be described in detail. Other species are handled similarly inasmuch as they are housed and irradiated in individual cages. When mice are obtained from the Institute's stock colony at weaning, they are divided so that litter mates are equally distributed between male and female experimental and control groups.