Worm Breeder's Gazette 9(2): 105

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More Fun with Heavy Ions

G. Nelson and T. Marshall

Figure 1

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We are continuing our investigations of cell survival/disfunction 
and mutagenesis as functions of accelerated ion irradiation.  One 
assay for cell function is to irradiate L1's with a 4-cell gonad (the 
target) and to score the worms as adults for gonad morphology and 
output, i.e.  brood size.  The velocity and charge of the accelerated 
ions is varied to manipulate the structure of the track of energy 
deposited in the cells and the fluence is varied to control the number 
of tracks per cell from 1 to > 1000.  A standard measure of energy 
deposition by ions is the Linear energy transfer (LET) which is the 
total energy deposited per unit track length usually in Kev/micron.  A 
typical X-ray has a value of 1 in these units.  In order to compare 
the effectiveness of different ions one normalizes to a reference 
standard - often Cobalt-60 gamma rays: to calculate the Relative 
Biological Effectiveness or RBE which is the ratio of absorbed dose of 
a test radiation to the dose of the reference standard required to 
produce the same biological effect.  Shown below are 'inactivation 
curves' for some accelerated ions by using N2 larvae.  Notice the 
strong dependence on LET at any given fluence.  50% and 37% normal 
brood size RBE versus LET curves have been constructed from this and 
other data and are shown below.  The curves indicate that the most 
effective ions for gonad cell inactivation have LETs of about 200 
KeV/micron (mammalian cells typically display maximum values of from 
100 to 200).  Above this LET energy is wasted (you can't kill a cell 
twice).  Below 50 KeV/ m the particles may sometimes pass through 
cells without inactivating them as effectively as gamma rays supplying 
the same absorbed dose in a uniformly distributed fashion.
Preliminary calculations of RBE vs LET using the eT1(III;V) 
mutagenesis assay reported previously (WBG 9(1):38) indicate a broad 
peak from LET = 25 to 200 KeV/micron and a high RBE value of 6 to 7.
In examining DAPI stained animals irradiated as L1's we found that 
intestinal cells can be easily scored for inhibition of mitosis and 
formation of cytoplasmic bridges between daughter nuclei.  The effect 
is does and LET dependent and follows about the same inactivation 
kinetics as brood size.  We'll report more on this I-cell assay in the 
Nov.  1 issue.
[See Figure 1]
[See Figure 2]

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Figure 2