Radiation biology: dosimetry, target and. action of radiation, radiation sensitivity

Transcription

1 Radiation biology: dosimetry, target and molecular theories, direct and indirect action of radiation, radiation sensitivity Dose concepts (II/4.1) Dose dependence of radiation effects, target theory (Poisson distribution), molecular theory (II/ ) Factors influencing radiation sensitivity (II/4.6) Indirect effect ofradiation, theory ofactivation ofwater water, dilution effect (p lecture material) Radiation sickness (II/ ) 1/20

2 Physical dose concepts 1 Only the absorbed fraction of radiation leads to physical, chemical or biological effects. This is characterized by dose: the energy absorbed by the material during the interaction with radiation divided by the mass of the absorbing material. 1. Absorbed dose: the energy absorbed by a body of unit mass: D a E m unit: J/kg=gray (Gy) principally the easiest way to measure D a is to detect the temperature increase induced by the absorbed energy (E), but: the absorption of 8 J/kg of energy is lethal in humans E 8 J cm kj 4 1 kg kg K 3 E c m T T 210 K it is difficult to measure such a low temperature increase an alternative dose concept is required since a dose of 8 J/kg leads to severe biological lconsequences, the damage has to be generated by molecular events, and not by heat transfer 2/20

3 Physical dose concepts 2 2. Exposure: the amount ofpositive ornegative charges generated by X ray or gamma radiation in a body ofunit mass during electron equilibrium: X Q, unit: m electron equilibrium: the number of electrons entering and leaving the detected volume. C kg detected volume chamber wall environment 3/20

4 Physical dose concepts 3 3. Kinetic energy released in material (KERMA): in the case of indirectly ionizing, high energy radiation a fraction of the evoked electrons loses its energy in the environment, outside the absorbing mass of m ionization radiation damage absorbent a fraction ofthe primary absorbed dose results in heating of the environment of the absorbing mass of m electrons giving off their energy in the environment of mass m also contribute to the radiation damage of mass m a concept characterizing the amount of the primarily absorbed energy is required secondary radiation electrons, some of which gives off its energy in the environment definition of KERMA: the ratio of the total initial kinetic energy of all charges particles and the mass of the absorbing material. In the case of high energy radiation: KERMA > absorbed dose unit of KERMA: gray 4/20

5 Biological dose concepts 1 1. Equivalent dose: the physical properties of radiation (type (electromagnetic, corpuscular, what kind of particle), energy, LET) influence the extent of biological damage this is taken into account by a weighing factor which used to be called quality factor (Q R ), but recently its name is radiation weighing gf factor (w R R) definition of equivalent dose (H T ) : unit of equivalent dose: sievert=j/kg (Sv) H wd T R T, R R, where w R radiation weighing factor D T,R the dose absorbed by a given tissue from a given type of radiation Radiation and energy range Photons 1 Electrons 1 Neutrons (E N <10 kev) 5 w R Neutrons (10 kev<e N <10 kev) 10 Neutrons (100 kev<e N <2 MeV) 20 Neutrons (2 Mev<E N <20 MeV) 10 N Neutrons (E N >20 MeV) 5 Protons, E P >2 MeV 5 a particles, heavy nuclei 20 5/20

6 2. Effective dose: Biological dose concepts different tissues and organs exhibit different radiation sensitivity and contribute differently to the overall radiation damage of the organism this is taken into account by a tissue specific weighing factor (w T ) definition of effective dose (E):, E w H w w D T T T T R T R T, R equivalent dose 6/20

7 Dose dependence of radiation effects, dose effect (dose response) curves Dose effect curve: the fraction of surviving (i.e. non inactivated, not damaged) d) individuals d (objects) as a function of dose. It is often called survival curve. 1 N surviving objects N 0 number of all objects N/N 0 often plotted on a logarithmic scale dose Two models have been created to explain the shape of the curves: target theory: generation of radiation damage is stochastic the interaction between the radiation and the biological object is not described molecularly but it appropriately described the dose effect curves of molecules molecular theory: generation of radiation damage is stochastic radiation damage is described molecularly; the major determinant of radiation damage is DNA double strand break it is appropriate to describe radiation damage of cells 7/20

8 Inactivation of molecules according to the target theory in the case of one target 1 There is one target in the molecules. The volume ofthe target is V. The distribution of hits in volume V follows a Poisson distribution with parameter (mean value) of Vi (i number of hits in unit volume) The probability that the target with a volume ofv V receives n hits: VD n n n Vi Vi P n e e n! n! Since i D, with correct choice of the unit of D the equation can be written in the following form: If k hits are required to inactive the target: P n n! e VD.. 0 hit, P 0 VD 0 0! VD e 1 hit, P 1 VD 1 1! VD e k1 VD k1 hits, Pk 1 e k 1! VD k or more hits inactivated molecules the fraction of non inactivated molecules: N N k 1 0 VD n 0 n! n e VD 8/20

9 Inactivation of molecules according to the target theory in the case of one target 2 N/N 0 N/N 0 dose The width of the shoulder of the curve increases with the number of hits required for inactivation: at low doses no molecules are inactivated (because the probability that a single target is hit k times is negligible (if k >> 1). dose The simplest case: one target which is inactivated by a single hit Only those molecules are not inactivated which are not hit by any radiation. Therefore, the fraction of surviving targets is N N 0 0! 0 VD VD e e If VD=1 (the number of expected hits in the radiosensitive volume is one), then N N N 0 e VD This dose is called D 37, because 37% of the objects survive. VD D V In the case of the one target single hit model D 37 is the reciprocal of the radiosensitive volume. 9/20

10 Molecular theory of radiation damage 1 n, N/N 0 viving fractio surv (A) HeLa, (B) CHO, (C) T1 cells dose (Gy) The curves cannot be interpreted using the target theory. A new model was required which h interprets the radiation i damage of mammalian cells by DNA damage. Evidence for the key role of DNA damage in radiation damage: in simple organisms there is a quantitative relationship between DNA damage and radiation damage in eukaryotic cells the loss of biological function correlates with single and double strand breaks in DNA DNA repair is correlated with radiation sensitivity: cells lacking DNA repair mechanisms exhibit extreme radiation sensitivity agents inhibiting DNA repair increase radiation sensitivity 10/20

11 Molecular theory of radiation damage 2: The model The key event leading to radiation damage is DNA double strand break. radiation radiation generated free radicals (see indirect effect of radiation) formation of DSB induced by a single particle joint effect of two independent events n, N/N 0 Surv viving fractio Molecular or linear quadratic model: 2 N D D a and empirical constants (a and S e characterize DSB generated in one and N two steps, respectively. 0 Explanation of the term D 2 : the probability of the joint occurrence of two independent events. The probability of the independent events is proportional to dose (D):, P SSB D P SSB SSB D 2 Dose 11/20

12 Direct effect of radiation: Direct and indirect effects of radiation the biological molecule is directly hit and inactivated by the radiation it is the only mechanism taking place when irradiating dried samples its probability bilit is much smaller than that t of hitting a solvent molecule l when irradiating solutions. Indirect effect of radiation: water radical In dilute aqueous solutions the probability that the radiation hits a water molecule is much larger than the probability of hitting a target (e.g. enzyme molecule). Radiation leads to the generation of free radicals from water which reach and inactivate the target. 12/20

13 Generation of radicals from water (radiolysis of water) Radical: anatom atom or molecule possessing anunpaired electron. ionization of water: HOHO +e hydrated electron (e water) H0 2 H+OH + + e+h0 2 HOH * excitation of water: H2OHO 2 H +OH The most important radicals generated: H, OH, e water Reactions of radicals: R H + H R + H R H + H R H 2 2 R H + OH R + HO R H + OH R HOH 2 H + OH HO H + H H 2 2 OH + OH HO 2 2 damage of biological molecules (R) these processes compete with each other recombination: the reactive radicals react with each other leading to harmless (or less harmful) molecules. 13/20

14 Enzymes can be inactivated with a lower dose in aqueous solutions Dried: the molecule is only inactivated if the target is directly hit. ity (%) enzyme activi dried (water free) ribonuclease Aqueous solution: radicals generated from water molecules surrounding the enzyme reach and inactivate the target. Thetarget gets bigger. 5 mg/ml solution dose (kgy) 14/20

15 Factors influencing radiation sensitivity 1 A. Quality of radiation 1. Ionization density (LET) 2. Penetrability B. Biological variation 1. Cell cycle 2. Differentiation C. Time factor 1. Fractionation, the role of DNA repair D. Metabolism and temperature E. The effect of oxygen 15/20

16 Factors influencing radiation sensitivity 2 A. The quality of radiation the extent of radiation damage depends on ionization density (LET). This is characterized by relative biological effectiveness (RBE), a constant similar to quality factor (Q R ) and radiation weighing factor (w R ). penetrability: alpha and beta radiation cannot penetrate t the skin they can only generate systemic effects if they can into the organism Relative biological effectiveness (RBE) The ratio of a dose of X ray with 250 kev energy (D R ) to the dose of the test radiation (D X ) required to cause the same biological effect : RBE RBE D D R X RBE is similar, but not identical to quality factor (Q R ) and radiation weighing factor (w R ) LET (kev/m) 16/20

17 B. Biological variability Factors influencing radiation sensitivity 3 1. cells display different radiation sensitivity in different parts ofthe cell cycle (implications for radiation therapy of cancer: in cancer a higher fraction of cells is in the M phase than in normal tissue). G2: preparation for mitosis M:mitosis cstart of the cycle G1: cell growth Largest radiation sensitivity: M and G2 phases Lowest radiation sensitivity: S phase S: replication of DNA 2. the less differentiated the cells are, the higher their radiation sensitivity is (implications for radiation therapy of cancer: cancer cells are less differentiated than normal ones) The radiation sensitivity of tissues based on the dependence of radiation sensitivity on cell cycle and differentiation: tissue tissue 1 lymphatic tissue 6 blood vessels 2 white blood cells, immature erythrocytes in bone 7 glands, liver marrow 3 mucous membrane of stomach and intestine 8 connective tissue 4 gametes 9 muscle tissue 5 proliferating cell layer of the skin 10 nervous tissue 17/20

18 Factors influencing radiation sensitivity 4 C. Time factor If a certain dose is given in fractions, a part of the radiation damage can be repaired between fractions the extent of radiation damage is reduced. Repair: primarily DNA repair, repair of double strand breaks. 1 ng fraction survivi dose given in two fractions surviving fraction if the dose would have been given in a single fraction D. Metabolism and temperature dose (Gy) Cells with a higher metabolic rate usually have higher radiation sensitivity. Since the rate of metabolism increases with temperature, a temperature increase usually leads to higher radiation sensitivity. 18/20

19 Factors influencing radiation sensitivity 5 E. The effect of oxygen in the presence of O 2 the amount of radiation generated radicals increases higher radiation sensitivity OER (oxygen enhancement ratio): the ratio of doses generating an arbitrary, equal surviving fraction under hypoxic and normoxic conditions. survivin ng fraction OER normoxic hypoxic dose (cgy) cancer therapy: malignant tumors are often hypovascularized hypoxia radiation therapy ofa hypoxic tumor is not efficient hypoxic tumors were cured less efficiently more patients died normoxia hypoxia anoxia Source: The Oncologist, 9(Suppl. 5), 31 40; Medscape 19/20

20 Radiation sickness (radiation poisoning) Ionizing radiation Symptoms of radiation exposure: radiation sickness accumulation of mutations development of tumors damage of offsprings (in the case of damage of gametes) 1 2 Gy 2 6 Gy 6 8 Gy 8 30 Gy >30 Gy dominant hematopoietic i hematopoietic i gastrointestinal i gastrointestinal i central nervous affected organ system system latency days 7 28 days <7 days none none leading symptoms mortality without medical care mortality with medical care Source: Merck Manual white blood cell count (leukopenia), leukopenia, bleedings, infections, severe leukopenia, fever, nausea, fatigue epilation vomiting, diarrhea, electrolyte disturbance, hypotension high fever, nausea, vomiting diarrhea, electrolyte disturbance, shock 0 5% 5 100% % 100% 100% 0 5% 5 50% % 100% 100% seizures, ataxia tremor 20/20