PHYSICAL AND RADIOBIOLOGICAL CHARACTERIZATION OF EXTREMELY HIGH DOSE PER PULSE XRAY BEAMS PRODUCED BY A PLASMA FOCUS DEVICE FOR IORT

Introduction: To investigate the main physical properties of low-energy X-ray beams produced by a Plasma Focus (PF) device and to carry out dosimetric and radiobiological characterization, in view of a possible clinical application to IORT treatments. Materials and Methods: PF is a device designed to generate a plasma sheet between two co-axial electrodes by means of a high voltage difference. The energy of a bank of capacitors (typically between a few and a few tens of kJ) is instantly transferred to the electrodes producing a plasma sheet, which is pushed towards the open end of the electrodes by the self-generated IxB force. The sheet implodes into a very dense magnetized plasma pinch, in a region called focus. Under such a condition, thermo-nuclear reactions may take place and charged particles are emitted. Apart from neutron production, ions and electrons are accelerated as well by the strong electromagnetic field following the focus decay. The charged particles emission can be described as follows: a) ion beam forward peaked; b) relativistic electron beam emitted with a narrow cone of aperture. When the filling gas is hydrogen or some other pure inert gas, as in our case, the reactions are neutron-free, while an ion beam plus an electron one are generated in opposite directions. A dedicated PF device has been designed and is currently under advanced status of construction and testing by the research groups of the Universities of Bologna and Ferrara, coordinated by the Alma Mater s.r.l. of the University of Bologna and with the financial support of ABO Project (Biotechnology Applications for Oncology). It can be used to produce an electron beam to be extracted from the PF vacuum chamber through a dedicated channel. The electron beam can be directly used for the irradiation of small targets or to generate X-rays by interaction with appropriate targets (Bremsstrahlung and/or characteristic emission). The system can deliver up to 5 Gy/pulse and each pulse lasts about 10-20 ns. Absorbed dose to water determination, as well as relative dosimetry (PDD curves and transversal dose profiles), will be performed mainly using calibrated radiochromic films (ISP MD-55) in a water-equivalent phantom. TLD-100 detectors, in form of micro-rods, will also be investigated. The biological endpoint for RBE calculation will be also evaluated, through an in-situ approach of apoptosis induction and gamma-H2AX foci expression. Results: With the present asymmetric PF configuration, an electron beam of about 1 kA was extracted by means of an electron pipe 25 cm long and converted into X-rays in the interval 10–50 keV from impact with a Cu or W target. The X-ray spectra were assessed through an original spectrometer made of LiF dosimeters staked together. The preliminary dosimetric results using radiochromic films revealed doses higher than 1 Gy/shot, uniformly distributed over a surface of 7 cm2, 5 mm away from the target. One order of magnitude higher results are expected with an appropriate symmetric assembly. Conclusion: Preliminary results on the prototype device seem encouraging, although deeper investigation is warranted in the perspective of a potential clinical application.