Radiation Doses: Acute vs. Chronic PDF

Acute vs. Chronic Dose Potential biological effects depend on how much and how fast a radiation dose is received. Radiation doses are grouped into: 1- Acute dose: - high dose of radiation received in a short period of time (seconds to days). The body’s cell repair mechanisms are not as eff...

Acute vs. Chronic Dose Potential biological effects depend on how much and how fast a radiation dose is received. Radiation doses are grouped into: 1- Acute dose: - high dose of radiation received in a short period of time (seconds to days). The body’s cell repair mechanisms are not as effective for repairing damage caused by an acute dose. Damaged cells will be replaced by new cells and the body will repair itself, although this may take a number of months. In extreme cases the dose may be high enough that recovery would be unlikely. 2- Chronic dose: - a small dose of radiation received over a long period of time (months to years).  Typical examples are: The dose we receive from natural background The dose we receive from occupational exposure Body is better equipped to tolerate chronic doses. IONIZATION  Ionization is the removal of an orbital electron from an atom. Since the electron has a negative charge, the atom is consequently left positively charged. The atom and the electron, so separated, are known as an ion pair, that is, a positive ion (the atom minus one electron) and a negative ion (the electron). Directly and Indirectly ionizing radiation ▶ For charged particles, most of the energy loss is directly absorbed: (( Energy Absorption )) ▶ For uncharged particles, energy is transferred in a first step to (secondary) charged particles: (( Energy Transfer )) ▶ In a second step, the secondary charged particles lose their energy according to the general behavior of charged particles (again Energy Absorption). ▶ So the energy of uncharged particles (like photons or neutrons) is imparted to matter in a two steps (Energy Transfer then Energy Absorption). Basic Concepts in Radiation Dosimetry 1- External Exposure ( χ ) The X or γ radiation interaction with matter leads to the production of ion pairs. The simplest way to measure the quantitative effects of these radiations is to measure the number of ion pairs produced in air by using oppositely charged surfaces. This approach is called exposure. External Exposure is defined as the total charge dQ of either sign produced by X or γ radiation in air of mass dm. Mathematically it can be presented as follows:  The unit of exposure is coulomb per kilogram (C/kg). The unit used for exposure is the roentgen R, where 1 R = 2.58 × C/kg, but in the SI system of units, roentgen is no longer used. 2. Absorbed Dose ( D ) Radiation damage depends on the absorption of energy from the radiation and is approximately proportional to the mean concentration of absorbed energy in irradiated tissue. The energy transferred to the primary charged particles per unit mass from incident radiation beam may not be necessarily absorbed in the volume of interest. Absorbed dose is a measure of energy deposition ( ) in any medium containing a finite mass ( ) by any type of ionizing radiation: The SI unit of absorbed dose is the joule per kilogram (J/kg), termed the gray (Gy). The original unit of absorbed dose was the rad (Radiation Absorbed Dose) , which was defined as an energy deposition of 0.01 joule per kilogram (J/kg). The unit of absorbed dose in SI units is the gray (Gy), and is defined as an energy deposition of 1 J/kg. Thus:  1 Gy = 1 J/kg  Equivalent Dose ( H ) -3  We are interested in the effect of radiation exposure on human tissue. Not all radiation has the same biological effect, even for the same amount of absorbed dose of different types of radiation.  It is found, for example, that 0.05 Gy of fast neutrons can do as much biological damage as 1 Gy of Gamma radiation.  This difference in the radiobiological effectiveness must be taken into account if we wish to add doses of different radiations to obtain the total biologically effective dose. To do this, we must multiply the absorbed dose of each type of radiation by a radiation weighting factor (wR), which reflects the ability of the particular type of radiation to cause damage.  The quantity obtained when the absorbed dose D is multiplied by the radiation weighting factor wR is known as the equivalent dose,.  The unit of equivalent dose in SI units is the sievert, which is related to the gray as follows: (Sv) = D (Gy) × wR Total The radiation weighting factor (WR): The radiation weighting factor (WR) takes into account that some kinds of radiation are more dangerous to biological tissue, even if their "energy deposition" levels are the same.  4- Effective Dose (E) The probability of a harmful effect from radiation exposure depends on what part or parts of the body are exposed. Some organs are more sensitive to radiation than others. A tissue weighting factor (WT) is used to take this into account. When an equivalent dose to an organ is multiplied by the tissue weighting factor for that organ the result is the effective dose ( E ) to that organ:  The unit of effective dose is the sievert (Sv). If more than one organ is exposed then the effective dose, E, is the sum of the effective doses to all exposed organs. Tissue Weighting Factors