Logo of the Physikalisch-Technische Bundesanstalt

Erhöhen magnetische Felder die biologischen Wirkung von Sekundärelektronen in der Strahlentherapie?

05.10.2009

The goal of radiation therapy is to produce lethal damage of cells within the tumor while sparing the surrounding tissue as much as possible. It has been shown in many studies that a magnetic field applied during irradiation can improve the absorbed dose distribution of electrons in the tumor region. However, effects on the micro-distribution of energy depositions at DNA level have not been investigated. The increasing application of MRI-guided (Magnetic Resonance Imaging) radiation therapy evokes the necessity to investigate microscopic and nanoscopic magnetic field effects which could have an impact on the biological effectiveness of therapeutic radiation beams.

Preliminary cell survival experiments (unpublished to this date) performed at the Australian Centre for Medical Radiation Physics (CMRP) in collaboration with the Westmead Institute for Cancer Research (WMI) showed an enhancement of the biological effectiveness by about 10 % when leukemia cells were irradiated with the same doses of low energetic X-rays within a magnetic field (1 T to 1.5 T) as compared to cells irradiated without magnetic field.

Dose delivered in radiotherapeutical treatment is mainly due to energy deposition by so-called δ-electrons, having energies of a few eV up to about 10 keV and a range of a few nanometers. A change of the δ-electrons’ track structure would lead to a modification of the pattern of ionizations within sub-cellular volumes. A higher number of ionizations within a volume equal to a DNA segment of a few nanometers length is related to an increase in DNA damage and therefore would enhance the biological effectiveness of the radiation.

In order to study the effect of a magnetic field on the track structure of low energy secondary electrons at the DNA level, Monte Carlo simulations were carried out using the recently released Geant4-DNA extensions for very low energy processes in comparison with the wel-established PTB track structure code for nanodosimetric studies. DNA segments of 10 base pairs length and nucleosomes were approximated by simplified models in form of water-filled cylinders (diameter 2.3 nm and height 3.4 nm, diameter 6 nm and height 10 nm). The simulations were performed for a uniform static magnetic field with field strength ranging from 1 T to 14 T.

The two Monte Carlo codes produced compatible results, demonstrating that the distribution of ionization events derived from low energy electrons in a DNA segment or a nucleosome does not change significantly due to the presence of a magnetic field between 1 T and 14 T. Therefore, an enhancement of the biological effectiveness, when irradiation is performed within a magnetic field, is not likely to be related to the modification of the δ-electrons’ track structure. In the future, the Monte Carlo simulation will be refined and extended to achieve more realistic DNA interaction models and take into account also the radiochemical transport processes.The Geant4 simulation study will be complemented by experimental research activity mainly in the form of experimental biological research at the WMI.