Dosimetry and radiobiology with laser-driven relativistic electron beams
Electron accelerators based upon the so-called Laser Wake-Field Acceleration (LWFA) mechanism in a plasma are deserving a growing attention, mainly due to their intrinsic reduced footprint when compared to conventional LINACs. This feature is of a particular importance in view of possible applications in medicine, and in particular for radiotherapy.
As it is well known, the LWFA process basically relies on the excitation, due to the ponderomotive force, of a plasma wave in the wake of an ultrashort and ultraintense laser pulse propagating in an underdense plasma. Such a plasma wave exhibits the right features to sustain the acceleration of electrons up to relativistic energies, such as a very intense longitudinal electric field (many orders of magnitude higher than in a typical radiofrequency cavity LINAC) and a phase velocity close to the speed of light.
Over the past few years, laser-driven electron accelerators have been greatly evolving, in terms, for instance, of operation stability and reliability, so that their possible use for radiotherapy, such as, for instance, Intra-Operative Electron Radiation Therapy (IOERT), can be foreseen within the next decade. Indeed, the stable production of electron bunches with energies up to tens or even hundreds of MeV have been demonstrated to be easily achievable, thus representing a new option for radiotherapy applications. From a practical point of view, laser-driven electron accelerators would exhibit a wealth of advantages over conventional ones in terms of radioprotection requirements and flexibility. For instance, the usage of a laser-driven accelerator for IOERT would allow a much smaller device to be introduced in the operating room, as the most bulky component, the laser system, may be placed and monitored outside, leaving only the“accelerator stage”, a few centimeters in size, close to the patient.
A laser-driven electron accelerator features an electron bunch duration much smaller than a conventional accelerator. Indeed, while durations of a few up to a few tens of femtoseconds have been reported for the bunches on leaving the plasma, a bunch duration of a few picosecond can be safely estimated/calculated at the position of the biological sample or patient (that is, after a few tens of centimeters propagation and possibly a vacuum-air interface); this value is still about six orders of magnitude higher than the one of a typical LINAC used in radiotherapy. By taking into account the typical bunch charge in the two cases (which is more or less comparable), one can easily realize that a much higher instantaneous dose rate is actually obtained, whose biological consequences have to be investigated in depth yet.
Further differences of a laser-driven accelerator when compared to a conventional one rely in the broader energy spectrum (when no advanced injection schemes are implemented, such as in the typical case of a tentatively “easy-to-use” accelerator for medicine) and a higher divergence. All of these issues demand for accurate studies related to both the dosimetric and the biological issues of a laser-driven accelerator.
A research activity, with fundings from the italian Ministry of Health, is going on, based at the Istituto Nazionale di Ottica – CNR in Pisa and involving a large collaboration from CNR (INO, IFC, IBFM, NANO) and AOUP-Pisa, aimed at:
– optimizing a laser-driven accelerator (LDA) for different radiobiology/radiotherapy applications
– characterizing, in terms of electron energy spectrum, total charge, electron bunch divergence, available dose and so on, LDAs
– establishing and achieving the technical prerequisites for effective applications in medicine
– investigating the effects of LDA electron bunches on biological cells and comparing the results with the effects from bunches produced by conventional, RF based LINACS