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Influence of a magnetic field on the biological effectiveness of ionising radiation

02.01.2012

Studies performed at the Centre for Medical Radiation Physics (CMRP), Australia, have shown a reduction in cell survival of leukaemia Nalm-6 cells when irradiated with 75 kV and 300 kV x-rays in the presence of a 1.3 Tesla magnetic field [1,2]. The fact that fewer cells survive suggests a possible increase in RBE which may be therapeutically beneficial. As this effect cannot be explained by physics, owing to secondary electrons having a mean free path and range orders of magnitude smaller than their radii of curvature, one should turn to biology for an answer. This has been the motivation for the ongoing collaboration between the PTB, CMRP and BfS (Bundesamt für Strahlenschutz) to investigate the potential influence of a magnetic field on the biological effectiveness of ionising radiation.

Experiments were conducted at the PTB with whole blood exposed to "28 kV" mammography x-rays (produced with a molybdenum anode and filter and 28 kV accelerator voltage) of 0.25 Gy and 1.5 Gy average absorbed dose in the presence and absence of a 1 T magnetic field. This x-ray spectrum was chosen as the RBE of photon irradiation between 3 and 30 keV is known to be greater than one [3, 4].) In order to obtain corresponding RBE estimates, additional irradiations were performed with 60Co γ-rays in the absence of a magnetic field for the same absorbed doses. Radiation damage in the form of chromosomal aberrations to the DNA of blood cells (human lymphocytes) was analysed using the biological technique, mFISH (multicolor fluorescence in situ hybridisation). This technique uses staining of the chromosome pairs in DNA to provide information about the number of unrepaired or misrepaired double strand breaks (DSBs), and thus, the late effects of ionising radiation.

As shown in the Figure, the cells irradiated with "28 kV" x-rays incurred more chromosomal damage than those exposed to 60Co γ-rays. Cells irradiated with x-rays of 1.5 Gy average absorbed dose showed a higher rate of chromosomal damage in the presence of magnetic field than in its absence, where the corresponding RBE estimates were 1.94±0.18 and 1.53±0.46, respectively. When the cells were exposed to an average absorbed dose of 0.25 Gy, however, the reverse was observed - those irradiated without a magnetic field exhibited more damage than those irradiated with magnetic field, corresponding to RBE estimates of 2.93±0.63 and 1.53±0.16, respectively.

Figure : Radiation damage in the form of chromosomal aberrations to blood cells (human lymphocytes) by 60Co γ-rays and x-rays with average absorbed doses of (A) 0.25 Gy and (B) 1.5 Gy. Different types of aberrations include: non-exchange aberration (truncated chromosome); simple exchange aberration (2 breaks in 2 chromosomes); and complex aberration (at least 3 breaks in 2 or more chromosomes).

These results show no clear evidence to suggest that a 1 T magnetic field influences the biological effects of "28 kV" x-rays. Experiments are also in progress to irradiate leukaemia Nalm-6 cells with x-rays in the presence and absence of a magnetic field in order to confirm the cell survival results obtained in earlier studies [2]. Additional research is planned to irradiate individual cells using the PTB microbeam facility and live-cell imaging (a technique that uses microscopy of selected proteins tagged with fluorophores) to study potential differences in radiation response, namely DNA damage and repair, in the presence and absence of magnetic field.

Literature:

  1. D. Tyrrell:
    Characterisation of radiation and magnetic field effects on biological systems.
    Bachelor thesis, University of Wollongong, Australia (2006).
  2. C. Abdipranoto:
    Influence of permanent magnetic field on relative biological effectiveness of x-ray radiation.
    Bachelor thesis, University of Wollongong, Australia (2007).
  3. E. Schmid, M. Krumrey, G. Ulm, H. Roos, D. Regulla:
    The maximum low-dose RBE of 17.4 and 40 keV monochromatic X rays for the induction of dicentric chromosomes in human peripheral lymphocytes.
    Radiat Res. 160(5), 499-504 (2003).
  4. M. Krumrey: PTB-Mitteilungen 115, 48-49 (2005).