Authors: Scarlat F, Stancu E, Scarisoreanu A.
Starting from the concept of technology (applied knowledge) defined as the crossing of two multitudes, science and associated technique, this paper provides an overview of the technology of charged particle accelerators (leptons and hadrons) for radiation therapy. High energy lepton beams, 4-25 MeV, refer to electrons and photons, and hadron beams to protons of 50 - 250 MeV and carbon ions of 100 - 450 MeV/u. Achieving clinical values for medical therapy is achieved using: the induced electric field for the electron acceleration (betatron), the microwave electric field for electrons (linac), the magnetic field for steering accelerated electrons on a metal target to obtain braking radiation (betatron, and linac), the high-frequency electric field for heavy particles (cyclotron and its variants), wake electric field generated by laser radiation in electron plasma and the electric field generated by laser radiation in a metallic target for protons and ions. Resonant acceleration (or energy transfer from the external acceleration field to the corpuscular radiation), on a rectilinear trajectory, occurs when the particle velocity is equal to the wave phase velocity associated with the electric field. Cyclic resonance, in a magnetic field perpendicular to the plane of the equilibrium orbit, occurs when the rotation frequency of the particle is equal to the frequency of the radio frequency electric voltage. Conversely, the transfer of energy from the particle to the wave, for example, Cerenkov radiation generated in a dielectric medium, occurs when the speed of the beam is greater than the speed of light in that medium. In turn, radiation sources (Cerenkov, Synchrotron, and Free Electron Laser) generated by ionizing particles can be used to accelerate charged particles. In addition to using the principle of resonance during acceleration, there are other principles to be fulfilled, such as, for example, weak focus, intense focus, transversal stability and longitudinal stability. Based on these principles, a large number of resonant cyclic accelerators have been developed, which differ from each other by one of the quantities that varies synchronously with the particle's energy. These quantities are the magnetic field, B(T), the radius of the orbit, ρ(m), and the frequency – f (MHz). In this way, we have: the betatron for electrons, 40 MeV, ρ = 0.25 m, 5 tons, the synchrotron for protons, 250 MeV, ρ = (6 - 9) m, and the synchrotron for heavy ions, 450 MeV/u, ρ = (18 – 25) m. All have constant radius and the field B(t) varies synchronously with the energy. For these, the variable size is fRF - decreases in proton synchrotron, B(ρ) - varies in isochronous proton cyclotron, isochronous carbon ion cyclotron and in non-scaling FFAG of hadrons. The transition, from conventional magnets of 1.8 T, to superconducting magnets of (4-10) T, led to the reduction of the size and weight of the accelerators, resulting in the following: synchrocyclotron (250 MeV, 9T, 0.25 m, 20 tons), the proton isochronous cyclotron (250 MeV, 4 T, 3.1 m, 90 tons) and the isochronous cyclotron for protons and ions therapy (5400 MeV, 4.5/2.45 T, 6.6 m, 700 tons). The development of relativistic laser technology (a0 > 1), allows the transition from conventional technology for electric fields, (1-50) MV/m in vacuum, to laser technology with fields of ~ 100 GV/m in plasma for electron acceleration (Bubble regime) and intensities of 1024 W / cm2 for the acceleration of protons and ions (TNSA and RPA). The therapeutic energy of heavy particles determines the depth of penetration in tumors located at depths of (1-33) cm and the intensity of the beam (1 nA = 6.24 x 109 p/s in proton therapy and 0.1nA in carbon therapy) ensures the fractional absorbed dose value in tumor of 2 Gy in 1L. In summary, this paper presents the principles of obtaining therapeutic energies for the treatment of malignant tumors by the particle acceleration method and the body's response through the energy absorbed in tumor from the radiation, as energy imparted by the photon radiation or as energy lost by the charged particle, both based on the characteristic of the deeply absorbed dose, for each type of ionizing radiation.
View/Download pdf