The Search for a Better Description of Small Radiation Exposures

by Leslie A. BRABY (Texas A&M University, College Station, Texas)

Thursday 27 May 2010, 11:00 → 12:30 Europe/Rome

VILLI Meeting Room (INFN LNL)

Ionizing radiation deposits energy, and initiates biochemical processes, along the tracks of high-energy charged particles. The random nature of these charged particle tracks is a major factor influencing the understanding of the biological effects of small radiation exposures. Microbeam irradiation techniques have been developed to eliminate some of the randomness inherent in conventional irradiation. The difference in the number of charged particle tracks through individual biological cells, resulting from low dose or from microbeam irradiation, causes the energy deposited in the individual cell to differ dramatically from the traditional measure of radiation exposure, the absorbed dose. Many random processes (such as motion of molecules, interaction of excited and stable molecules, enzyme binding to specific structures, and recombination of radiation induced ions) contribute to the consequences of irradiation. These processes generally depend on the local concentration of radiation products. Consequently, the average energy deposited in a large volume of tissue, the basis of the current measure of radiation exposure, is not likely to predict the biological consequences. The absorbed dose, defined as ratio of the average energy imparted in a volume on the mass in the volume, is a very useful description of radiation exposure when the number of events is large enough that the energy deposition is essentially homogeneous, but gives the false impression that the amount of damage done in a small object such as a cell is a continuous variable. In order to avoid erroneous assumptions about the probability of radiation induced changes, more information is needed. The most precise way to communicate the additional information is to specify the number of charged particles per area as a function of particle charge, velocity, and time. An alternative, which is much easier to measure but provides slightly less information, is to specify the probability density of energy imparted in a small volume, that means the ratio of the energy imparted on the mean chord length of the volume.