132 RADIATION GENETICS 



tube and turning the machine on and off, contemporary standards require recognition 

 of a few principles of radiological physics and of problems of variation in machine 

 output. It is standard practice to place the animals on a rotating board during X 

 irradiation. This will randomize variations in the dose field due to slight misalign- 

 ment of the tungsten target of the tube and to unequal absorption of some of the X rays 

 by the target itself. The board, usually a one-half-inch slab of masonite, serves as a 

 back-scattering device to help assure the attainment of electron equilibrium and the 

 elimination of variations in tissue depth dose. Exposure of mice and rats from a single 

 plane, either dorsal or ventral, will usually give a uniform tissue dose, provided that 

 X rays of 120 kev or greater are employed with filtration adequate to remove the lowest 

 energy components of the total spectrum. 



When photon energies of 1 Mev and above are used (Co 60 is an example), the 

 animals should be in a chamber-like device that provides both forward and backward 

 scattering of the secondary electrons. The forward scattering is necessary since high- 

 energy gamma and X rays must penetrate about 3 or 4 millimeters of tissue or tissue- 

 equivalent material to reach secondary electron equilibrium. In the absence of such 

 scattering material, the tissue dose will build up in the animal and lead to an irregular 

 depth-dose curve. Four millimeters of lucite are sufficient to insure equilibrium for 

 the 1 .25 Mev, Co 60 gamma photon. 



When animals the size of guinea pigs, rabbits, monkeys, dogs, and larger are 

 irradiated, it generally becomes necessary to use a bilateral exposure technique. The 

 total dose is delivered in two equal parts, one part to each lateral surface. This 

 prevents sharp changes in depth dosage and accompanying inequalities of exposure of 

 internal organs. Unilateral irradiation of female dogs, for example, could lead to 

 some discrepancy in dose delivered to the two ovaries. Detailed discussions of the 

 pattern of depth dose for different energy radiations in mice, rats, and rabbits are 

 given by Grahn et a/. 476 and for the domestic animals and man by Bond et a/. 115 



A common procedure for genetic studies involves the use of partial body exposure. 

 When doses above the midlethal level are required, local irradiation of the gonads 

 must be done. Simple lead hemispheres can be devised for small mammals which 

 permit full exposure of the gonads with only a limited exposure of surrounding tissue. 

 For obvious reasons, the procedure is simplest for males. Sheet lead one-eighth inch 

 thick provides excellent shielding for 250 kev radiations and below. Partial body 

 exposure with high-energy X and gamma radiation and neutrons cannot be accom- 

 plished through the use of shields alone. Beam collimation is required, although this 

 is not always feasible with all radiation sources. Thus, for some situations, the total 

 dose will be controlled by the survival of the animal rather than by technical 

 manipulations. 



RADIATION GENETIC ANALYSIS IN MAMMALS 



Apparently, the first effort to study the genetic effects of radiation in mice was 

 reported by Little and Bagg in 1923 and 1924. 801 Although several new mutations 



