Dosimetry - Measurement of Ionising Radiation

Transcription

1 Dosimetry - Measurement of Ionising Radiation Assoc. Prof. Katarína Kozlíková, RN., PhD. IMPhBPhITM FM CU in Bratislava katarina.kozlikova@fmed.uniba.sk

2 Contents Dosimetry Dose Radiation dose Absorbed dose Absorbed dose rate Integral dose Equivalent dose Effective dose Kerma Kerma rate Exposure Exposure rate 007 ISO radioactivity danger logo [Cit ] Available at: ková,, 2012 Dosimetry 2

3 Why Dosimetry? Ionising radiation initiates destructive processes in organism The damage is manifested either At once Later (even after years) Dosimetry deals with quantities characterising the radiation from the aspect of their biological effects To infer them in advance Dosimetry involves measurement of radiation with different types of detectors ková,, 2012 Dosimetry 3

4 Basic Aim of Dosimetry Radiation dosimetry Measurement and calculation of the absorbed dose in matter and tissue resulting from the exposure to indirect an direct ionising radiation Physical radiodosimetry Determination of the energy absorbed by the tissues in a certain region Biological dosimetry To establish the unknown dose of exposure from wellmeasurable, statistically evaluable biological changes Medical dosimetry Calculation of the absorbed dose and optimisation of dose delivery in radiation therapy ková,, 2012 Dosimetry 4

5 Dose In Pharmacology The total drug quantity administered into the organism in various ways Regardless of the effective quantity taken by the organism Regardless of the quantity taken by the organism but secreted without exerting any effect Expressed as the administered drug quantity per weight or mass unit ková,, 2012 Dosimetry 5

6 Radiation Dose Biologically effective quantity of radiation taken by the organism and related to mass (or volume) units Only the energy absorbed by the body or organism is considered Radiation passing through the organism without any interactions is not considered This lecture deals with dosimetric quantities characterising the effects of radiation on matter, and especially the human body ková,, 2012 Dosimetry 6

7 Absorbing Region of The Organism Is located in various depths May contain different tissues Surroundings of the region have to be considered because of scattered radiation Therapeutic application The dose must be known from area to area (from point to point) Radiation protection It is sufficient to know the average dose for a given organ or tissue Development of the biological effects The knowledge of the absorption of biologically effective radiation is of basic importance but not sufficient Other physical, chemical and biological factors are important as well ková,, 2012 Dosimetry 7

8 Absorbed Dose (1) The numerical value of the absorbed dose D equals the mean energy E absorbed by some mass of the body dm (dm=ρ dv, ρ is density, dv is volume element) SI unit D = de dm or gray (Gy): 1 Gy = 1 J kg -1 Older international unit (not used in SI) rad (radiation absorbed dose): D = 1 ρ de dv 1 rad = 0.01 Gy 1 Gy = 100 rad ková,, 2012 Dosimetry 8

9 Absorbed Dose (2) Can be used for any type of radiation Is used to calculate the deterministic effects of radiation The dose characterises the absorbed energy (in the mass element), however, it does not take into account immediate interaction of primary radiation with the matter If indirectly ionising radiation is concerned, this may interact elsewhere, and only secondary charged particles contribute to the dose when ionising the medium Therefore, kerma was introduced ková,, 2012 Dosimetry 9

10 Absorbed Dose Rate A given dose of some radiation may be obtained by the body during different times In this case, the absorbed dose rate (dose power ) - the time increase in dose - is important The absorbed dose rate D & is the quotient of the absorbed dose D and the duration of irradiation SI units D& D = t gray per second (Gy s -1 ): 1 Gy s -1 = 1 W kg -1 ková,, 2012 Dosimetry 10

11 Biological Effects of Radiation Depend on Absorbed energy Represented by absorbed dose Radiation type Represented by radiation weighting factor in dose equivalent Irradiated target (organ, tissue) Represented by organ weighting factor in effective dose Distance of the radiating source from the target Duration of irradiation ková,, 2012 Dosimetry 11

12 Involves the biological effect of the absorbed dose The dose equivalent H T is obtained by multiplying the absorbed dose by a weighting factor w R characteristic for the radiation R, if for radiation R the average value of the absorbed dose is D T,R in a given T tissue (organ) SI unit Dose Equivalent (1) H T = wr DT, R sievert (Sv): 1 Sv = 1 J kg -1 Older international unit (not used in SI) rem (röntgen equivalent man): 1 rem = 0.01 Sv 1 Sv = 100 rem ková,, 2012 Dosimetry 12

13 Dose Equivalent (2) If the radiation consists of more components (radiations of different types and different energies), the different radiations are considered separately The averaged absorbed doses D T,R of each type of radiation R are multiplied by the corresponding radiation weighting factors and the sum of these weighted dosed gives the dose equivalent H = T wr DT, R R ková,, 2012 Dosimetry 13

14 Radiation Weighting Factor (1) Shows the effectivity of a given radiation in eliciting the stochastic effect as compared to that of gamma-rays and X-rays As many times smaller is the dose of radiation required for the same effect as that of X-radiation, as many times higher is the effectivity of radiation This factor depends on the value of the linear energy transfer L of the radiation If this dependence is not exactly known, conventional values are used (Table 1) ková,, 2012 Dosimetry 14

15 Radiation Weighting Factor (2) Table1: Weighting factors of different radiations and energies Radiation type Photons Electrons, muons Neutrons Protons Alpha-particles Fission products, heavy nuclei Energy range all energies all energies <10 kev 10 kev 100 kev 100 kev 2 MeV 2 MeV 20 MeV > 20 MeV > 2 Mev Weighting factor w R Values after Rontó and Tarján, 1997 ková,, 2012 Dosimetry 15

16 The relationship between the equivalent dose and the probability of the stochastic effect depends on the irradiated organ The total stochastic effect is characterised by the effective dose E The effective dose E is obtained by multiplying the equivalent dose H T of a given T tissue (organ) by the tissue weighting factor w T characteristic for the tissue SI unit Effective Dose (1) E sievert (Sv): 1 Sv = 1 J kg -1 T = w T H T ková,, 2012 Dosimetry 16

17 Effective Dose (2) The tissue weighting factor w T shows the proportion of the given tissue or organ in the whole damage caused by the homogeneous irradiation of the total body The effective dose E for the whole body is obtained by the summation of weighted doses for all tissues (organs) E T = wt T H T or w T wr E T = DT, T R R ková,, 2012 Dosimetry 17

18 Tissue Weighting Factors Table 2: Weighting factors of different tissues and organs Measurement of radiation of hands. [Cit ] Available at: Organ or tissue Breasts Bone marrow Colon Lungs Stomach Gonads Bladder Liver Oesophagus Thyroid gland Bone surfaces Brain Salivary glands Skin Remainder Weighting factor w T Values after Harrison and Day, 2008 ková,, 2012 Dosimetry 18

19 Integral Dose Integral or volume dose is the total energy absorbed in some part of the body or the whole body of mass m If the dose distribution is Homogeneous The dose is the same at every site in some part of the body of mass m Integral dose is the product of the mass and the dose Inhomogeneous The dose is different for different sites of the body Sufficiently small parts have to be considered, in which the dose distribution is homogeneous The integral dose is calculated for each part and the results are summed SI unit Joule (J) ková,, 2012 Dosimetry 19

20 Kerma K is an acronym for kinetic energy released to matter The numerical value of the kerma K equals the sum of initial (kinetic) energies E k released by indirectly ionising radiation in some mass of the body dm SI unit gray: 1 Gy = 1 J kg -1 Kerma (1) K = dek dm ková,, 2012 Dosimetry 20

21 Kerma (2) Kerma Kerma around around X-ray X-ray device device at postero-anterior at postero-anterior projection projection of lungs. of lungs. [Cit. [Cit ] ] Available Available at: at: ková,, 2012 Dosimetry 21

22 The Difference Between The Absorbed Dose And The Kerma (1) Absorbed dose is the energy deposited (as ionisation and excitation) by any type of ionising particles in the element In the case of indirectly ionising radiation, these will be the particles originating in the surrounding material, for example secondary electrons The absorbed dose in the element will depend on the material surrounding this element Kerma is the kinetic energy transferred per unit mass of irradiated material, for example from photons to electrons It is independent from the surrounding material For diagnostic energies they are effectively equal ková,, 2012 Dosimetry 22

23 The Difference Between The Absorbed Dose And The Kerma (2) Kerma is defined only for indirectly ionising radiation Absorbed dose Depth Kerma and absorbed dose at a border of two media. [Cit ] Available at: ková,, 2012 Dosimetry 23

24 Kerma Rate A given kerma of some radiation may be obtained by the body during different times In this case, the kerma rate (kerma power ) the kerma increase in time - is important The kerma rate K & is the quotient of the kerma K and the duration of irradiation SI units K& = As for absorbed dose rate K t gray per second (Gy s -1 ): 1 Gy s -1 = 1 W kg -1 ková,, 2012 Dosimetry 24

25 Exposure (1) Refers only to X-rays and gammaradiation Characterises only the ionising capability of radiation in the air (not the energy absorbed) Gives only indirect information about radiation actually absorbed by the tissue ková,, 2012 Dosimetry 25

26 Exposure (2) The numerical value of the exposure X equals the electric charge dq (positive or negative) produced at electron equilibrium by ionisation in some volume dv or mass dm X = dq dm or X = 1 ρ dq dv SI unit coulomb per kilogram (C kg -1 ) Older international unit (not used in SI) roentgen (R): 1 R = C kg -1 (exact value) ková,, 2012 Dosimetry 26

27 A given exposure of some radiation may be obtained by the body during different times In this case, the exposure rate (exposure power ) is important The exposure rate X & is the quotient of the exposure X and the duration of irradiation SI units Exposure Rate X& = X t coulomb per kilogram and second (or ampere per kilogram): 1 C kg -1 s -1 =1 A kg -1 ková,, 2012 Dosimetry 27

28 Ionisation In Air Is used for calibration purposes Can be measured Fairly exactly With good reproducibility Relatively simply Runs parallel to the biological effects Is independent of the wavelengths If the ionising effects produced by radiation of two different wavelengths are the same in air, the biological effects in a given tissue (under given conditions) will be practically the same The ratio of absorption (scattering) coefficients in the air and in the tissues is practically the same at various wavelengths ková,, 2012 Dosimetry 28

29 Literature ALLISY-ROBERTS, P., WILLIAMS, J. Farr s Physics for Medical Imaging. Edinburgh : Saunders - Elsevier, p. ISBN BIERSACK, H.J., FREEMAN, L.M. Clinical Nuclear Medicine. Berlin : Springer Verlag, p. ISBN CHUDÝ, M. Základy dozimetrie žiarenia. [Cit ] Available at: HARRISON, J., DAY, PH. Radiation doses and risks from internal emitters. In: Journal of Radiological Protection 2008; 28(2): HOLÁ, O., HOLÝ, K. Radiačná ochrana. Ionizujúce žiarenie, jeho účinky a ochrana pred ionizujúcim žiarením. Bratislava : STU, s. ISBN HRAZDIRA, I., MORNSTEIN, V., BOUREK, A., ŠKORPÍKOVÁ, J. Fundamentals of Biophysics and Medical Technology. Brno : Masaryk University, Faculty of Medicine, p. ISBN KOZLÍKOVÁ, K., MARTINKA, J. Theory And Tasks For Practicals On Medical Biophysics. Brno : Librix, p. ISBN RONTÓ, G., TARJÁN, I. (eds.) An Introduction To Biophysics With Medical Orientation. Budapest : Akadémiai Kiadó, p. ISBN STN ISO 31-9: Veličiny a jednotky. 9. časť: Atómová a jadrová fyzika. Bratislava : Slovenský ústav technickej normalizácie, s. STN ISO 31-10: Veličiny a jednotky. 10. časť: Jadrové reakcie a ionizujúce žiarenie. Bratislava : Slovenský ústav technickej normalizácie, s. ková,, 2012 Dosimetry 29