SPACE RADIATION SHIELDING: BIOLOGICAL EFFECTS OF ACCELERATED IRON
IONS AND THEIR MODIFICATION BY ALUMINUM OR LUCITE SHIELDS
M. Durante1*, F. Antonelli2, F. Ballarini3, M. Belli2, D. Bettega3, M. Biaggi3, P. Calzolari3, A.
Ferrari4, G. Gialanella1, A. Giussani3, G. Grossi1, P. Massariello3, A. Ottolenghi3, M. Pugliese1, P.
Scampoli1, G. Simone2, E. Sorrentino2, M.A. Tabocchini2, L. Tallone3
1. Università "Federico II", Napoli, Italy; 2. Istituto Superiore di Sanità, Roma, Italy; 3. Università
di Milano, Italy; 4. CERN, Geneva, Switzerland
ABSTRACT
The research program described here is focused on the effect of the shielding on the biological
effects of heavy ions. Both experiments and models are included in the program. Experiments aim
to determine genetic effects of heavy ions with or without shielding. Mathematical models, based
on Monte Carlo codes, will be used to interpret the biological results. The final goal is to get a
feasible model able to predict the radiation-induced biological damage in space, given the freespace radiation field and the spacecraft shielding. Preliminary results about chromosomal
aberrations in human peripheral blood lymphocytes induced by accelerated 56Fe ions are presented
here.
INTRODUCTION
For terrestrial radiation workers, protection against radiation exposure can be provided
through shielding of the radiation source. In extra-terrestrial space, shielding is effective against
trapped protons, but its efficiency is poor against galactic cosmic rays (GCR) penetration. Indeed,
high-energy particle radiation in space is very penetrating and GCR produce a large number of
secondary particles, including neutrons, generated by nuclear interactions with the nuclei in the
shield. These particles have generally lower energy, but can have higher quality factors than
incident primary cosmic particles [1].
Considering the current uncertainties in space radiation physics and biology, NASA [2] has
pointed out that major improvements are urgently needed in a) models of biological response in
monochromatic and mixed charged particle fields, and b) experiments on biological effects of heavy
ions with shielding. For this very reason, we have established a large International collaboration to
measure the biological effects of heavy ions and their modification by shielding [3]. The goal of the
project is to gather experimental data on the RBE of accelerated heavy ions for genetic effects in
human cells, namely DNA double-strand breaks, chromosomal aberrations, and lethal mutations.
These data will be used to improve and validate Monte Carlo computer models of shield
performance. In this paper, we present preliminary results obtained at the HIMAC accelerator
(Chiba, Japan) with 56Fe-ions.
MATERIALS AND METHODS
Accelerated 56Fe beams were obtained at the HIMAC accelerator in Chiba, Japan.
Characteristics of the beams are described in Table 1. Monitoring and dosimetry of the heavy-ion
beams at the HIMAC accelerator are described elsewhere [4]. We used two different shields:
PMMA (lucite), a low-Z plastic material (= 1.16 g/cm3), and aluminium ( = 2.7 g/cm3), the usual
spacecraft shield. Thickness of the two shields were chosen to reduce the residual range of the 500
MeV/n beam to the same value of the 200 MeV/n unattenuated iron beam.
*
Author to whom correspondence should be addressed at: Dipartimento di Scienze Fisiche, Università "Federico II",
Montre S. Angelo, Via Cintia, 80126 Napoli, Italy. E-mail: durante@na.infn.it
1
Biological samples were human peripheral blood lymphocytes obtained by a healthy donor
and isolated by centrifugation in Ficoll gradients. Isolated lymphocytes were resuspended in
RPMI1640 growth medium and exposed at room temperature at a dose rate around 1 Gy/min.
Following exposure, samples were incubated at 37 °C in medium supplemented with 1%
phytohemagglutinin. After 48 h, chromosomes were prematurely condensed by calyculin A (50 nM)
for 1 h at 37 °C following the original protocol described by Durante et al. [5]. Slides were
hybridized in situ with DNA fluorescent probes specific for human chromosomes 1 and 2. All kinds
of chromosome aberrations were scored in prematurely condensed chromosomes 1 and 2, including
translocations, dicentrics, excess fragments, and complex-type exchanges.
Energy in
vacuum
(MeV/n)
Energy on
sample
(MeV/n)
Shielding
500
500
500
200
414
PMMA (56 mm)
Al (30 mm)
-
114
Residual range
(mm) in H2O
LET
(keV/m)
71.6
8
8
8.03
184.9
Fraction of
ions in the
beam at
sample
position
1
0.40
0.46
1
56Fe
302.2
Table 1. Characteristics of the 56Fe beams used at the HIMAC accelerator in Chiba (Japan).
RESULTS AND DISCUSSION
The fraction of aberrant lymphocytes (i.e., the fraction of scored cells displaying any type of
visible structural aberration involving painted chromosomes 1 and/or 2) is plotted vs. the radiation
dose at the sample position in Figure 1. It can be noted that iron beams are more efficient than Xrays in the induction of chromosomal aberrations, and the 500 MeV/n beam is more efficient then
the 200 MeV/n beam. As a function of dose at the sample position, no significant differences are
Figure 1. Dose-response curve for the
induction of chromosomal aberrations in
human lymphocytes exposed to X-rays or
56Fe accelerated beams. Y-axis: fraction
of lymphocytes with aberrations in
prematurely condensed chromosomes 1
or 2. X-axis: dose measured in the
sample position.
Figure 2. Fluence-response curve for
the induction of chromosomal
aberrations in human lymphocytes
exposed to 56Fe accelerated beams.
Y-axis: fraction of lymphocytes with
aberrations
in
prematurely
condensed chromosomes 1 or 2. Xaxis: fluence of iron ions incident on
the shielding.
2
observed for the 500 MeV/n beams unshielded or shielded with Al or PMMA.
The same data are plotted in Figure 2 as a function of the 56Fe ions fluence incident on the shield.
The 500 MeV/n ions induce chromosomal aberrations more efficiently than the 200 MeV/n ions.
The shield increases the effectiveness per ion of the 500 MeV/n beam, and the cytogenetic damage
behind the 56 mm lucite shield seems to be slightly higher than behind a 30 mm Al shield.
Data in Figure 1 basically show that the radiation spectrum produced by the shield does not
significantly change the quality factor. In fact, cytogenetic damage is similar at the same radiation
dose absorbed by the sample. However, when plotted as a function of the number of ions hitting the
shield, the curves are separated and the shield increases the effectiveness per unit ion. The
difference is caused by nuclear fragmentation of the beam in the target. A lower number of Fe ions
are required to produce a certain dose at the sample when the sample is shielded either with Al or
PMMA.
Wilson et al. [6] showed that the shield performance is dependent upon shield material and
thickness, as well as incident beam energy and charge. Their calculations suggest that the biological
effectiveness of GCR can be increased behind thin shields made by high atomic number materials.
Results in Figure 2 give the first experimental evidence of this possible deleterious effect of space
radiation shielding. Preliminary data obtained at the HIMAC accelerator and at the AGS accelerator
in Brookhaven, Upton (USA) on human cellular DNA fragmentation induced by 56Fe beams of
various energies, without and with shielding, support this hypothesis.
ACKNOWLEDGEMENTS
This research project is generously supported by the Italian Space Agency (ASI), and involves
Italian radiation biophysics groups (Universities of Milan and Naples, National Institute of Health
in Rome), in collaboration with NASA (USA), NIRS (Japan), CERN (Switzerland), Brookhaven
National Laboratories (USA), and TERA (Italy).
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