LINEAR ENERGY TRANSFER • The amount of energy a radiation deposits per unit of path length (ie. Kiloelectron volts/micron-KeV/um) is defined as linear energy transfer, LET, of that radiation. • Mass charge and velocity of a particle all affect the rate at which ionization occurs and is related to range.
High and Low LET • Heavy, highly charged particles lose energy rapidly with distance and do not penetrate deeply. • Radiation with a long range such as x-ray and high energy B particles usually have low LET. • Large particulate radiations with short range (ie. Alpha particles, neutrons, protons have a high LET.
IONIZING RADIATION • The higher the charge of the particle and the lower the velocity, the greater the likelihood to produce ionizing radiation. • The biologic effect of a radiation depends upon the amount of energy transferred to the tissue volume or critical target and is therefore a function of LET.
RELATIVE BIOLOGIC EFFECTIVENESS • Many types and energies are used in radiation biology. • RBE was developed to compare results of experiments not just on LET, but in more general terms. • RBE= • Biological efficiency of radiation under investigation • Biological efficiency of 250 kV x-rays • Dose (gy or rad) to produce effect with 250 kV x-rays • Dose (gy or rad) to produce effect with radiation under investigation
RBE VS LET • RBE is initially proportional to LET and as the LET for ionizing radiation increases, so does RBE. • However beyond 100 keV/um of tissue, the RBE decreases with increasing LET.
QUALITY FACTOR • As more ionizations are produced in the biological system, part of the energy deposited in the system is wasted due to and overkill effect. • The quality factor is a more conservative upper limit of RBE used in radiation safety.
CONCLUSION • There is still some elements of doubt about the specific cell structures which must be damaged to kill or injure the cell. • Research has identified two major radiation processes which may lead to cell impairment or death, indirect and direct action.
QUESTIONS • Name a low LET radiation. • Name a High LET radiation. • Does ionizing radiation deposit ionizing energy in the cell. T/F • What is the standard reference of radiation used in the RBE formula. • Dose measurements are in Gy or rad.T/F