Measurement of Absorbed Dose

1. Measurement of Absorbed Dose 參考資料：1. The Physics of Radiation Therapy.Faiz M. Khan2. Introduction to Radiological Physics and Radiation Dosimetry. Frank H. Attix

2. RADIATION ABSORBED DOSE • Exposure and its unit, the roentgen (C/kg), • Applies only to x and g radiations, • Measure of ionization in air only, • cannot be used for photon energies above about 3 MeV • Absorbed dose • all types of ionizing radiation, • all materials • all energies

3. RADIATION ABSORBED DOSE • Absorbed dose is a measure of the biologically significant effects produced by ionizing radiation • Definition of absorbed dose, dEavg/dm • dEavg : the mean energy imparted by ionizing radiation to material of mass dm • Unit • The old unit of dose is rad, • 1 rad = 100 ergs/g = 10-2J/kg • The SI unit of dose is the gray (Gy), • 1 Gy = 1J/Kg

4. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Kerma (K) = kinetic energy released in the medium • "the quotient of dEtr, by dm, where dEtr, is the sum of the initial kinetic energies of all the charged ionizing particles (electrons and positrons) liberated by uncharged particles (photons) in a material of mass dm"

5. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Unit for kerma = J/kg, SI unit is gray (Gy) • For a photon beam traversing a medium, kerma at a point • Ψ : photon energy fluence • mtr: the mass energy transfer coefficient for • the medium averaged over the energy • fluence spectrum of photons

6. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Energy Transfer Coefficient (mtr) • When a photon interacts with the electrons in the material, a part or all of its energy is • converted into kinetic energy of electrons • The fraction of photon energy transferred into kinetic energy of charged particles per unit thickness of absorber :mtr

7. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Energy Absorption Coefficient (men) • Most of the electrons set in motion by the photons will lose their energy by inelastic collisions (ionization and excitation) with atomic electrons of the material. A few, depending on the atomic number of the material, will lose energy by bremsstrahlung interactions with the nuclei. The bremsstrahlung energy is radiated out of the local volume as x-rays and is not included in the calculation of locally absorbed energy

8. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Energy Absorption Coefficient (men) • where g is the fraction of the energy of secondary charged particles that is lost to bremsstrahlung in the material

9. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Energy Absorption Coefficient (men) • For most interactions involving soft tissues or other low Z material in which electrons • lose energy almost entirely by ionization collisions, the bremsstrahlung component is • negligible. Thus men = mtr, under those conditions. • allows the evaluation of energy absorbed in the tissue • interest in predicting the biologic effects of radiation

10. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Energy Absorption Coefficient (men) • g is the average fraction of an electron energy lost to radiative processes.

11. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Electrons lose their energy by • inelastic collisions (ionization and excitation) with atomic electrons of the material • bremsstrahlung interactions with the nuclei • Kerma can thus be divided into two parts • Kcol : the collision parts • Krad : the radiation parts of kerma

12. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • K = Ψ ( mtr / r) = Kcol + Krad • Krad =Ψ ( mtr / r)．g • Kcol =Ψ ( mtr / r)．( 1 – g ) • ( mtr / r ) = ( men / r ) / ( 1 – g )

13. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Exposure and Kerma • Exposure was defined as dQ/dm where dQ is the total charge of the ions of one sign produced in air when all the electrons (negatrons and positrons) liberated by photonsin (dry) air of mass dm arecompletely stopped in air • Exposure is the ionization ( dQ )equivalent of the collision kerma in air

14. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Exposure and Kerma • The mean energy required to produce an ion pair in dry air is almost constant for all electron energies and has a value of • W/e = 33.97 eV/ion pair • = 33.97 CV/C • = 33.97 J/C, J = C．V

15. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Exposure (x) : • X = dQ/dm , ( Kcol )air=dEcol/dm • dEcol =dQ · ( w/e ) • ( Kcol )air=dQ · ( w/e )/dm • =dQ/dm · ( w/e ) • =X · ( w/e ) • X = ( Kcol )air / ( w/e )

16. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Exposure (x) : • ( Kcol )air = Ψair ( men / r)air • X = ( Kcol )air / ( w/e )air • X = Ψair ( men / r)air/ ( w/e )air • The SI unit for exposure is C/kg and the special unit is roentgen (1 R = 2.58 x 10-4 C/kg)

17. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Absorbed Dose and Kerma • kerma is maximum at the surface and decreases with depth • The dose initially builds up to a maximum value and then decreases at the same rate as kerma.

18. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Absorbed Dose and Kerma • complete electronic equilibrium does • not exist within megavoltage photon beams • conceptually electronic equilibrium • would exist if it were assumed that photon attenuation is negligible throughout the region of interest

19. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Absorbed Dose and Kerma • Transient electron equilibrium, at depths greater than the maximum range of electrons

20. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Absorbed Dose and Kerma • the dose is being deposited by electrons originating upstream, one can think of a • point somewhere upstream at a distance less than the maximum electron range from where the energy is effectively transported by secondary electrons • This point has been called the "center of electron production" ( c. e. p. )

21. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Absorbed Dose and Kerma • Since the effective center of electron production is located upstream relative to the point of interest, the dose is greater than kerma in the region of transient electronic equilibrium • at a point where a transient electron equilibrium exists

22. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Absorbed Dose and Kerma • Suppose D1is the dose at a point in some material in a photon beam and another material is substituted of a thickness of at least one maximum electron range in all directions from the point, then D2, the dose in the second material, is related to D1 by

23. D1 D2 D1 D1 maximum electron range maximum electron range

24. RELATIONSHIP BETWEEN KERMA, EXPOSURE,AND ABSORBED DOSE • Absorbed Dose and Kerma • b varies with energy, not medium • A fixed value of b= 1.005 has been used for 60Co in conjunction with ion chamber dosimetry

25. Relation of Kerma to Energy Fluence for Photons • Kerma • the Kerma is the expectation value of the energy transferred to charged particles per unit mass at a point of interest, including radiative-loss energy but excluding energy passed from one charged particle to another

26. Relation of Kerma to Energy Fluence for Photons • Kerma • For monoenergetic photons • If a spectrum of photon energy fluence

27. Relation of Kerma to Energy Fluence for Photons • Kerma

28. Relation of Kerma to Energy Fluence for Photons • Collision Kerma, Kc

29. Relation of Kerma to Energy Fluence for Photons • ABSORBED DOSE

30. COMPARATIVE EXAMPLES OF ENERGY IMPARTED, ENERGY TRANSFERRED, AND NET ENERGY TRANSFERRED

31. COMPARATIVE EXAMPLES OF ENERGY IMPARTED, ENERGY TRANSFERRED, AND NET ENERGY TRANSFERRED

32. COMPARATIVE EXAMPLES OF ENERGY IMPARTED, ENERGY TRANSFERRED, AND NET ENERGY TRANSFERRED

33. Homework

34. Homework

35. Homework

36. Homework a) b)

37. Homework c)