High Energy Radiation in the Treatment of Cancer

The energy of ionizing radiation, directly charged particles or indirectly uncharged particles, when it is transferred or absorbed in the body, following a local interaction, can produce the most unfavorable biological effects on its health. These effects can be grouped into stochastic effects, at the level of a cell, such as cancer, and hereditary effects, without a threshold, in which the probability of occurrence increases linearly with the equivalent dose in tissues and organs, at the frequency of events in the cell nucleus below 1 event per 100 mGy of radiation with LET smaller than 10 keV/µm, and deterministic effects (meaning “causally determined by preceding events” i.e. harmful tissue reactions, early or late, at the level of a population of cells, due to the high values of absorbed energy in tissue or organ. The range of the radiation under the skin at all depths of the tumor volume is determined by the kinetic energy of the particle, and the intensity of the beam ensures the absorbed energy necessary for sterilization of the tumor. These energies are: (4 – 25) MeV for photons and electrons, (50 – 250) MeV for protons and (1200 – 5400) MeV for carbon ions. The biological effect of ionizing radiation increases with increasing the linear energy transfer (LET in units kV/µm), starting from storage of 0.1 -10 for X rays to storage of 100–200, for heavy charged particles (see the Bragg curve). Another important radiobiological factor, the relative biological effectiveness (RBE), is recommended 1.1 for clinical protons and increases depending on the LET to a maximum RBE of 2.5 to 3.5, for carbon ions. The last factor, the oxygen enhancement ratio (OER), is about 3 at high doses and falls to about 2 for doses of 1 to 2 Gy, for x-rays and electrons. The response of the environment (cell, tumor, organism) to the energy of ionizing radiation through the dose-effect relationship, which can be of linear, linear quadratic or sigmoid type, uses an incidence or a probability as an effect, in order to have a sigmoid-shaped increase (i.e. "S"). The sigmoid curve in radiotherapy can be described by the probit and logistic functions, with the inflection point at 50% and by the Poisson function with the inflection point at 37% (about 6 out of 10 tumors will recur). Also, for a good radiotherapy treatment, the tumor cure probability (TCP), i.e. the probability of zero surviving clonogens in a tumor, TCP ≥ 0.5, and the normal tissue complication probability (NTCP) as a function of the absorbed dose, NTCP ≥ 0.05. Maximizing the energy absorbed in the tumor and minimizing the energy absorbed outside the tumor is done using high homogeneity, characterizing the uniformity of dose distribution within the target volume and high conformity, characterizing the degree to which the high dose region conforms to the target volume, usually by the planning target volume (PTV). The distribution of tumor-specific doses is achieved by using new methods and associated devices, entitled: 3D conformal radiotherapy (or 3DCRT), the Intensity modulated radiation therapy (IMRT) and Helical tomotherapy (HT) in the case of electromagnetic radiation (photons), and Intensity modulated particle therapy (IMPT), based on the scattering and/or scanning method in the case of hadron therapy. These methods use particle beams with constant or variable radiation intensities, to cover the entire volume of the tumor, in 3D, and in 4D when the patient's breathing time and heartbeat are taken into account. The Standard Reference Conditions from external photon therapy of 2 Gy per fraction, 5 fractions per week, are recommended in hadron therapy. The aspects listed above are presented in the work.