Nuclear Radiation Based Therapy
| Due to high LET and RBE nuclear radiations can be used as a good tool to selectively destroy defective organ or tissues. Different particles with different energies have range of LETs and RBEs, and therefore can be very judiciously use for therapeutic process. Both gamma and charged particles are used for radiation therapy. In the next section I am going to discuss therapeutic process by two important techniques: (i) Boron neutron capture therapy (BNCT) and (ii) heavy ion therapy. | ||
| Boron neutron capture therapy (BNCT) | ||
| (BNCT) is a promising technique, where a stable isotope of 10B is irradiated with low energy or thermal neutrons to yield highly energetic Helium-4 (4He) nuclei (i.e., alpha particles) and recoiling Lithium-7 (7Li) ions. These ions eventually destroy the cancer cells with minimam effect on good cells. G.L. Locher of the Franklin Institute of Pennsylvania introduced the concept of neutron capture therapy10. Clinically, the following procedure is followed in BNCT. An epithermal beam of neutrons is directed towards a patient’s head. During their passage through tissue, these neutrons rapidly lose energy by elastic scattering (a process called thermalization) until they end up as thermal neutrons. The thermal neutrons thus formed, are captured by the 10B atoms, which become 11B atoms in the excited state for a very short time (~ 10-12 seconds). The 11B atoms subsequently produce alpha particles by fission. Tumor cells are killed selectively by the energetic alpha particles12 and 7Li fission products as pictorially represented in Fig. m4.6. | ||
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FIGURE m4.6 Pictorial representation of BNCT of brain tumor cells | ||
| Heavy Ion Cancer Therapy | ||
| The energetic ion beams are best suited to treat tumours that are deep-seated, located close to critical organs and respond poorly to conventional radiotherapy. The basic physics involved here is the linear energy transfer (LET), attenuation, range-dose profiles and multiple scattering of ion beams in matter. The linear energy transfer in tissues is different in case of photons and charged particles. While in the MeV domain the depth profile of photons is dominated by hard collisions absorbing most or all the energy of the incident photon, the energy loss of the ion is achieved by a high number of soft collisions. The variation of LET with depth in the tissues for photons is exponential in nature, and the maximum energy is transferred within initial depth. However, the LET in case of ions is totally different and it peaks up in a much deeper region. The depth versus LET in both the cases are shown in Fig. m4.7 (a and b) | ||
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/m4.9(b).png) | ||
FIGURE m4.7 (a) LET versus depth in tissues for photon, (b) LET versus depth in tissues for ions | ||
| With increase in LET, the relative biological effectiveness (RBE) increases11. | ||
| The increased RBE can be explained on the basis of increased ionization density, which causes a cluster of damages. The decrease in RBE with very high LET could be a result of over production of ‘local damage’. Selection of ionic species for some specific purpose is very much important. For deep-seated therapy, the typical energy requirement is around 70 to 250 MeV of protons and 120 to 400 MeV/nucleon of carbon ions. These high energy can be achieved in syncrotrons, cyclotrons linear accelerators etc. The following design aspects are important: | ||
| (i) In practice, the Bragg peak must be spread out over the full depth of the tumour. This can be achieved either by the interposition of an absorbing material of variable thickness in the beam path (passive spreading) by ‘energy degrading’ or the active modulation of the extracted beam (ii) Beam size control (iii) Beam extraction (iv) Matching the beam path with target volume. | ||
| Briefly, the instrumentation involved in a medical accelerator for therapy and their basic purpose can be listed below: | ||
| Stage 1: Beam production | ||
| Energetic beams are generally produced in an accelerator, like cyclotron or synchrotron. For heavy ion, like C, typical energy ~200 MeV/amu or more is generally required for this purpose. Therefore, it is important to design accelerators which can deliver energy of this range. However, tunability of beam energy as per requirement of therapy process is another important need of accelerator design. | ||
| Stage 2: Beam delivery | ||
| Beam delivery system is designed in such a way to select the optimum accelerated beam direction and angles for the target (patient). | ||
| Stage 3: Beam shaping | ||
| Beam shaping system is designed to tailor the dose of incident beam for the required volume of the target. In this system a beam is tightly focussed like a pencil beam and then deflected laterally for scanning of the beam over entire volume of the target. | ||