Quantum Physics

29.7 Medical Applications of Radiation 95929.7 MEDICAL APPLICATIONS OF RADIATIONRadiation Damage in MatterRadiation absorbed by matter can cause severe damage. The degree and kind ofdamage depend on several factors, including the type and energy of the radiationand the properties of the absorbing material. Radiation damage in biologicalorganisms is due primarily to ionization effects in cells. The normal function of acell may be disrupted when highly reactive ions or radicals are formed as the resultof ionizing radiation. For example, hydrogen and hydroxyl radicals producedfrom water molecules can induce chemical reactions that may break bonds inproteins and other vital molecules. Large acute doses of radiation are especiallydangerous because damage to a great number of molecules in a cell may cause thecell to die. Also, cells that do survive the radiation may become defective, whichcan lead to cancer.In biological systems, it is common to separate radiation damage into two categories:somatic damage and genetic damage. Somatic damage is radiation damageto any cells except the reproductive cells. Such damage can lead to cancer at highradiation levels or seriously alter the characteristics of specific organisms. Geneticdamage affects only reproductive cells. Damage to the genes in reproductive cellscan lead to defective offspring. Clearly, we must be concerned about the effect ofdiagnostic treatments, such as x-rays and other forms of exposure to radiation.Several units are used to quantify radiation exposure and dose. The roentgen(R) is defined as that amount of ionizing radiation which will produce 2.08 10 9ion pairs in 1 cm 3 of air under standard conditions. Equivalently, the roentgen isthat amount of radiation which deposits 8.76 10 3 J of energy into 1 kg of air.For most applications, the roentgen has been replaced by the rad (an acronymfor radiation absorbed dose), defined as follows: One rad is that amount of radiationwhich deposits 10 2 J of energy into 1 kg of absorbing material.Although the rad is a perfectly good physical unit, it’s not the best unit formeasuring the degree of biological damage produced by radiation, because thedegree of damage depends not only on the dose, but also on the type of radiation.For example, a given dose of alpha particles causes about 10 times more biologicaldamage than an equal dose of x-rays. The RBE (relative biological effectiveness)factor is defined as the number of rads of x-radiation or gamma radiation that producesthe same biological damage as 1 rad of the radiation being used. The RBEfactors for different types of radiation are given in Table 29.3. Note that the valuesare only approximate because they vary with particle energy and the form ofdamage.Finally, the rem (roentgen e quivalent in man) is defined as the product of thedose in rads and the RBE factor:Dose in rem dose in rads RBEAccording to this definition, 1 rem of any two kinds of radiation will produce thesame amount of biological damage. From Table 29.3, we see that a dose of 1 rad offast neutrons represents an effective dose of 10 rem and that 1 rad of x-radiation isequivalent to a dose of 1 rem.TABLE 29.3RBE Factors for Several Types of RadiationRadiationRBE FactorX-rays and gamma rays 1.0Beta particles 1.0–1.7Alpha particles 10–20Slow neutrons 4–5Fast neutrons and protons 10Heavy ions 20