Cancer radiobiology

The work i have done in this dissertation, was mainly aimed at the literature review of radiotherapy radiobiology discussing the cure of tumours with ionizing radiation, from both the biological and physical point of view. The first chapter an introduction about the radiotherapy and includes: definition, working dose, benefit of radiotherapy, risk of radiotherapy, external and internal radiotherapy and treatment planing. In chapter two the theories of radiobiology and main effects caused by the radiation in the interaction with the biological matter were explained, the damages caused by the use of low and high LET (linear energy transfer) particles to mammalian cells were discussed. And discuss a therapeutic advantage may be gained by one of four hypothetical mechanism: repair the damage of DAN, so when sublethal injury can be repaired if no further hits are sustained. Also the reoxygenation of tumor is important for its effects on stabilization of free radicals produced by ionizing radiation. Hypoxic cells generally require an increased dose of radiation for lethal effect, redistribution, within the cell cycle depends on location of cells and their radiosensitivity also cells undergoing DNA synthesis, the S phase, are much more radioresistant than cells in other phase of the cell cycle, and repopulation of tumor cells is indicator of the surviving cells respond by increased regeneration or repopulation. Repopulation is a greater problem with rapidly proliferating tumors than slower growing neoplasms. These mechanisms are known as the classical four R's of radiation biology. One of the important applications of radiobiology is the radiotherapy and cancer treatment, experimental and theoretical studies in radiation biology contribute to the development of radiotherapy, in this dissertation we discussed the dose response relation so as the size of the tumor increases, and the dose needed for local control like wise increases, the risk of injury to normal tissue becomes greater because the idea of radiotherapy is to destroy the tumor cells without affect the normal cells, there the dose quantity and the time of rate is important factors for radiotherapy. But to safe the normal cells the dose fractionated, conventional fractionation schedules are typically in increments of 1.8 to 2.0 Gy give times per week for 6 to 8 weeks. Altered fractionation schedules have been developed in attempt to optimize treatment results under various clinical circumstances, hyperfractionation is possible to increase the total dose, thereby increasing the probability of tumor control without increasing late complications. In biological system the free radicals produced in water may react with essential macromolecules. A vas range of reaction takes place, most of which are unimportant for the survival and functioning of the cell. The most important reactions are those with DNA, because of the uniqueness of many parts of this molecule. Damage of DNA by free radicals produced in water is called the indirect effect of radiation, ionization of atoms that are part of the DNA molecule is the direct effect. The response of cells of ionizing radiation is strongly dependent upon oxygen, the enhancement of radiation damage by oxygen is dose modifying, i.e. the radiation dose that gives particular level of survival is reduced by the same factor at levels of survival. This allows us to calculate an oxygen enhancement ratio (OER), for the same level biological effect. For most cells the OER for x-rays is around 3.0. However, some studies suggest that at radiation dose of 3 Gy or less the OER is actually reduced (palcic and skarsgard, 1984). This is an important finding because this is the dose range for clinical fractionation treatment. The type of radiation used in radiotherapy depend on (LET), as LET increases, radiation produces more cell killing per Gy, these large differences in energy distribution, at the microscopic level and at equal absorbed dose, result in different biological effects dependant on the radiation quality. Some can be predicted: a higher RBE (ranging between 3 and >>5 in the conditions of fast neutron therapy), different shapes of the dose-effect relationships for tumours and normal tissues and hence the possibility to obtain a differential effect and a therapeutic benefit. So to know how cancer treated with radiation, we must calculate and know the OER, RBE, repairing of DNA and how to fractionate the dose and what type of radiation must be used, and what a relation between all these variables.(Author)