DNA DSB induced by iron ions in human fibroblasts: LET dependence and shielding efficiency
Abstract
Galactic Cosmic Rays represent one of the main sources of charged particle radiation outside the magnetic field of the Earth, containing particles of all charges, in a wide energy range. As HZE particles interact with a shield, they fragment and deposit energy at rates depending on the nature and energy of the incident particles, and on the nature and thickness of the shield. We have studied the DNA DSB induction in human fibroblasts by iron ions of different energies (5 GeV/u, 1 GeV/u, 500 MeV/u and 200 MeV) in the absence or presence of different shields (PMMA, Al and Pb). Measure of DNA DSB was performed by gel electrophoresis and .fragmentation analysis. The RBE for unshielded and shielded beams has been plotted as a function of the dose average LET, and the relationship shows a maximum of 1.7 at about 140 keV/μ m (5 GeV/u iron ions). The dose average LET seems to adequately describe the effectiveness of degraded beams since their RBEs fall roughly on the same relationship of the unshielded beams. The cross section for unshielded beams increases with LET. The shielding efficiency has been evaluated by the ratio between the cross sections for unshielded and shielded beams (SPF, shielding protection factor). When this ratio is plotted as a function of the shield thickness (g/cm2), different relationships were found for beams with different energy. For 500 MeV/u iron ions, the presence of the shields gives SPF lower than unity, independently of thickness and material. For 1 GeV/u iron beams, SPF values are always higher than unity. Moreover, PMMA shield gives a higher SPF than Al or Pb shields having the same residual range but different thickness. The higher SPF of PMMA has been confirmed in experiments performed with 5 GeV/u iron beams and different shields having the same thickness. The SPF found for 1 and 5 GeV/u iron ions remain constant also when the fragmentation analysis used for evaluating DNA DSB is extended to fragments of smaller size. The fragment size distributions have been analyzed with a recently introduced theoretical method, that can evaluate the degree of randomness in DSB production. It has been found that DSB induction is non random due to an enhanced production of small fragments.
- Publication:
-
35th COSPAR Scientific Assembly
- Pub Date:
- 2004
- Bibcode:
- 2004cosp...35.3102A