Radiation Dosimetry and Interaction of Nuclear Radiation with Biological Specimen
| Nuclear radiation though hazardous to living organisms could be of immense use in medicine both for diagnostic and therapy. Anyone handling radioactive material or working in a radiation environment should have at least elementary ideas concerning the effects of exposure to radiation, the permissible limits and safety precautions. In order to use nuclear radiation in medicine, knowledge of radiation doses and limits is must. In this lecture, I am going to discuss about radiation dose and dose limits and possible effects of nuclear radiation with biological specimen. | ||||||||||||||||||||||
| Dosimetric Units | ||||||||||||||||||||||
| The amount of nuclear radiation that is absorbed by anybody can be determined based on two processes, the ionization produced by the radiation and the energy deposited by it. The unit, which was used initially, is Roentgen, which is a measure of the exposure and is defined as 1 Roentgen (R) = the quantity of x-rays producing an ionization of 1 esu/cm3 | ||||||||||||||||||||||
= (2.58 x 10-4 Coul/kg) in air at STP —————- (m4.43) | ||||||||||||||||||||||
| This definition refers specifically to x-rays and γ-rays in air and can be measured easily with the help of a gas ionization chamber. However, it is inconvenient to measure in case of living tissue or some other material. A more relevant quantity to measure radiation dose is Gray (Gy), which is defined as | ||||||||||||||||||||||
1 Gray (Gy) = 1 Joule/kg ———— (m4.44) | ||||||||||||||||||||||
| This is also known as absorbed dose. It has been studied that the effect produced by any nuclear radiation on biological samples depends strongly on the type and energy of the incident radiation. An absorbed dose of α-particles produces more damage than an equal dose of protons. The difference lies in the linear energy transfer (LET) of the different particles, i.e. the energy locally deposited per unit path length of any specimen under study. Therefore, the radiation, which produces more ionization, causes higher damage. In order to account this effect, a radiation weighting factor, wR is associated with each radiation, the value of which is listed below in the table: | ||||||||||||||||||||||
TABLE m4.1 Radiation weighting factors4 | ||||||||||||||||||||||
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| * Excluding Auger electrons emitted from nuclei bound to DNA | ||||||||||||||||||||||
| If DR is the average absorbed dose received by any organ exposed to radiation, then equivalent dose is to be considered, which is defined as | ||||||||||||||||||||||
/equ1.png){kind=small} | ||||||||||||||||||||||
| where, DT,R is the average absorbed dose received by organ T from the radiation type R. | ||||||||||||||||||||||
| The damage produced by any radiation of any energy is different for different tissues and organs. To obtain a normalized measure of the biological effect suffered by a tissue or organ due to irradiation, tissue-weighting factor (wT) is to be considered, which is then multiplied with equivalent dose to get the effective dose absorbed by any tissue. The tissue-weighting factor for different organs is tabulated below: | ||||||||||||||||||||||
TABLE m4.2 Tissue weighting factore4 | ||||||||||||||||||||||
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| It is to be noted that the tissue-weighting factor is totally independent of the radiation type and energy, as the radiation-weighting factor is independent of tissue type. The effective dose is defined as | ||||||||||||||||||||||
| where, sum is over different tissues and organs exposed. The unit of equivalent dose is the Sievert (Sv), which has the same dimension as the Gray (J/kg). However, this dose is normalized by the radiobilogical effectiveness (RBE), so that 1 Sv of α-particles produces approximately same effect as 1 Sv of γ-rays. | ||||||||||||||||||||||
| Let us now discuss the the sequence of events on interaction of radiation with biological specimen. This can be divided in three phases5 | ||||||||||||||||||||||
| I. Physical phase | ||||||||||||||||||||||
| It is the first event for radiation damage of any biological sample. The basic physical phenomena involved in this phase are ionization, excitation absorption etc., which have already been discussed elaborately in module 2. | ||||||||||||||||||||||
| II. Chemical phase | ||||||||||||||||||||||
| Absorption of radiation producing ions, electrons and excited atoms induce chemical changes in the cell. The chemical changes occur by two ways: (i) direct action, where molecules directly absorbs energy from radiation and (ii) indirect action, where molecules absorb energy from other molecules, which acts as an intermediary for the transfer of energy. The intermediary in the cell is H2O, which by ionization, produces highly reactive ions and free radicals like H+, OH– and H2O* and their products like H2O2, which act as powerful oxidizing agents, react with important biomolecules like DNA,RNA and enzymes for alteration of their function. The chemical phase occurs within milliseconds. | ||||||||||||||||||||||
| III. Biological phase | ||||||||||||||||||||||
| The important biomolecules are intimately related to the cellular function. If they are damaged, the cellular function will be seriously hampered or there may be cellular death. Time involved in this process is from seconds to hours.2 Different changes in this phase are shown in table 8.3. | ||||||||||||||||||||||
TABLE m4.3 Changes in some important biomolecules by radiation5 | ||||||||||||||||||||||
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| Some important changes that occur in the cells due to radiation are: | ||||||||||||||||||||||
| 1. Aberration (breakage) of chromosomes 2. Onset of mitosis is delayed followed by normal mitosis 3. Death of cells after few subdivisions 4. Death of cells without division 5. Transformation to malignancy. | ||||||||||||||||||||||