Chapter 1 Quantities and Fundamental Units of External Dosimetry

In this chapter, the radiation field is characterized in terms of energy and flux at a point of space. These physical datas than make possible to obtain the radiometric and dosimetric quantities of reference for external exposure, such as: fluence, kerma for the indirectly ionizing radiations and the dose. In addition, other quantities such as linear energy transfer and relative biological effectiveness are detailed; they relate to the effect of radiation on biological tissues and to the damage that can occur at the cellular level. Initially, we will attempt to define the characteristic quantities of dosimetry and radiation fields, called radiometric quantities. The fundamental radiometric and dosimetric quantities are essentially threefold: • Fluence Φ; • Absorbed dose D; • Kerma K. We will note that these quantities were defined by the ICRU (International Commission on Radiation Units and measurements) as transition variables in obtaining protection and operational quantities (detailed in Chaps. 2 and 3), they are measurable and above all they correspond to punctual concepts. Add to that two other intermediate variables that will be particularly suitable in the dose-biological effects relationship: The linear energy transfer (LET) and the relative biological effectiveness (RBE). All these quantities are detailed in the following. 1.1 Dosimetric Quantities 1.1.1 Total Absorbed Dose and Linear Energy Transfer Biological dosimetry will consist initially to characterize the physical dose deposited in human tissue. This approach is immediately responsible for a dual problem that © Springer International Publishing AG 2017 R. Antoni and L. Bourgois, Applied Physics of External Radiation Exposure, Biological and Medical Physics, Biomedical Engineering, DOI 10.1007/978-3-319-48660-4_1 1 we will discuss: “typology” of energy deposition as a function of radiation type and size of the volume of tissue in which is characterized this deposition. In the first approach, for a macroscopic context, i.e. a volume V surrounding a mass m of material sufficiently large (tissue level), subject to radiation responsible for a deposited energy E, the dose is defined by the expression (1.1). D 1⁄4 E m ð1:1Þ In the international system, this quantity is consistent with J kg but his unit, the gray (Gy), comes from the name of one of the founding fathers of dosimetry (1 Gy = 1 J kg). The bar on the variable D indicates that the result of this calculation is an average: no matter the “topology” of deposited dose. In other words, no matter how the mass m has been irradiated, in homogeneous way or not, the final result is averaged over the total mass. This macroscopic quantity will be called later in the book: “total deposited dose. ” Without details, we will say that energy depositions are the result of excitations, ionization and generally the setting in motion of any secondary charged particle (e.g. electrons for photons, recoil nuclei for neutrons). The total number of ionizations and excitations can be extremely important and can reach the order of 10 for a dose of 1 Gy in a volume the size of the cell nucleus. Once the dose determined, the “typology” of the deposited dose along the path of the incident particle is then decisive because it greatly influences the response of the medium crossed in this case of human tissues. Indeed, one of the major limitations of the approach leading to such a dose as defined in (1.1) is the failure to take into account the geometry of the energy deposition in the target volume. In particular, it is clear that understanding of the action of radiation on the biological material requires information on the scale of the DNA, that is to say to lower target dimensions less than micron. It has been found indeed that for equal amount of energy absorbed, the effect depends on the nature of the incident particle. For a certain type of radiation such as photons, electrons and protons to high energies, the location of ionization and excitations are separated in space. However, for low-energy protons, neutrons and ions, ionization are relatively clustered in close proximity of the passage of the particle into tissue. Figure 1.1 shows a schematic view of the difference in density of ionization and excitation to the same sensitive volume and for photons and ions. The physical quantity to characterise the ionization density disparity depending on the type of radiation and therefore the effects on biological tissues, is the ratio of the deposited energy per unit of path length in the material. For a macroscopic approach, this means, in a given environment, to determine the amount of energy ΔE lost by the particle on a portion of its path Δl. L 1⁄4 DE Dl ð1:2Þ 2 1 Quantities and Fundamental Units of External Dosimetry