902 
GENETIC AND BIOLOGICAL EFFECTS OF RADIATION 
If the events causing the breakage are inde- 
pendent, the frequency of these two break aber- 
rations typically approaches proportionality to 
the square of the dose. 
Dose and Dose Rate 
Neary et al.^^ report that breakage can be 
induced by as little as 60 eV, while earlier work 
indicated a somewhat higher energy require- 
ment.^'^ Sixty electron volts is roughly equivalent 
to the energy involved in producing two ion 
pairs and this amount of energy, or slightly 
more, is found in the ion clusters along the 
tracks of y, x, or energetic (3 rays. Thus one 
typically observes a linear response of terminal 
deletions (caused by a single break) to increas- 
ing absorbed dose.^^ In the data reported, fre- 
quencies of both swine and human chromosome 
deletions were approximately proportional to 
the X-ray dose. There was no effect of X-ray 
dose on chromatid deletions since the leuko- 
cytes were in the Gi (interdivisional) stage of 
the cell cycle at the time of irradiation. The 
expected proportionality of rings and dicentrics, 
i.e., two break aberrations, with the square of 
the dose was observed with our human data but 
the response was linear with the swine chromo- 
somes. Presumably this is merely the result of 
the small sample involved and we presume that 
with more data the frequencies of swine rings 
and dicentrics will relate more closely to the 
dosage squared. 
Not all breaks result in a change in the chro- 
mosomes. Following breakage the two broken 
ends are in close proximity and a vast majority 
of breaks rejoin with no residual damage or 
prejudice to cell survival. The importance of the 
proximity of the broken ends is well estab- 
lished ^^ ^o as is also the factor of time involved 
in restitution. 21 Since breaks remain open for 
limited times, it is to be expected that at lower 
dose rates or with fractionated doses a break 
may restitute before a second break occurs 
within the volume critical for exchanges, i.e., 
within 0.4-0.6 fim of each other.^^^i This is a 
factor of considerable importance with metabo- 
lized isotopes such as 90Sr-^°Y which may de- 
liver large cumulative doses but at relatively 
low dose rates. 
Quality of Radiation, LET 
The rate at which energy is imparted to the 
biological system linearly along the track of a 
penetrating radiation is referred to as linear 
energy transfer (LET) and is expressed in 
terms of kilovolts/jum.^^ Generally X, gamma, 
and hard beta radiations are considered to be 
low LET radiations while protons, alphas, ac- 
celerated nuclei, etc., are considered high LET 
radiations. While two break aberrations typi- 
cally are proportional to the square of the dose 
with low LET radiations, they are directly 
proportional to dose when high LET radiations 
are involved. In the region of the high LET 
track the energy released is so great that multi- 
ple chromosome breakage is highly probable 
for chromosomes lying within the zone in which 
recombinations can occur, i.e., a volume having 
a diameter of 0.4-0.6 yam. Since the simultane- 
ous production of two breaks with four broken 
ends in close juxtaposition encourages the pro- 
duction of two-hit aberrations and tends to 
minimize restitution this accounts for some of 
the higher cell killing efficiency of the high LET 
radiations. 
Oxygen 
As with most biological damage, oxygen in- 
creases the frequency of chromosome breakage 
induced by low LET radiations. Typical data are 
reported by Deschner and Gray^^ who eval- 
uated chromosome abnormalities in ascites cells 
exposed both in vivo and in vitro. Apparently 
the presence of oxygen during the exposure 
period encourages production of free radicals in 
the suspending solution, radicals themselves 
capable of producing chromosome breaks. Aber- 
rations induced by high LET radiations are not 
influenced or are less influenced by the presence 
or absence of oxygen. Seemingly there is ample 
energy available along each high LET particle 
track to break any chromosomes within the re- 
joinable distance and additional free radicals 
are of no consequence. In any event, high LET 
radiations typically show neither an oxygen 
effect nor a dose rate effect. 
The role of oxygen in affecting aberration 
frequency is not a simple one since, in addition 
to enhancing the frequency of break induction 
