Introduction to Health Physics Chapter 6 Radiation Dosimetry

1. Introduction to Health PhysicsChapter 6Radiation Dosimetry

2. UNITS • During the early days of radiological experience, there was no precise unit of radiation dose that was suitable either for radiation protection or for radiation therapy • Furthermore, since the fraction of the energy in a radiation field that is absorbed by the body is energy-dependent, it is necessary to distinguish between radiation exposure and radiation absorbed dose

3. Absorbed Dose • Gray • Radiation damage depends on the absorption of energy from the radiation and is approximately proportional to the concentration of absorbed energy in tissue 1 Gy = 1 J/kg • Rad 1 rad = 100 ergs/g 1 Gy = 100 rads

4. Exposure • The exposure unit is a measure of the photon flux and is related to the amount of energy transferred from the X-ray field to a unit mass of air. One exposure unit is defined as that quantity of X- or gamma radiation that produces, in air 1 X unit = 1 C/kg air

5. The Roentgen • The roentgen is an unit of exposure ( X ). The ICRU defines X as the quotient of dQ by dm where dQ is the absolute value of the total charge of the ions of one sign produced in air when all the electrons ( + or - ) liberated by photons in air of mass dm are completely stopped in air. X = dQ / dm • The SI unit is C/kg but the special unit is roentgen ( R ) 1R = 2.58 × 10-4 C/kg

6. Measurement: The Free Air Chamber • Charged Particle Equilibrium (CPE ) : Electron produced outside the collection region, which enter the ion-collecting region, is equal to the electron produced inside the collection region , which deposit their energy outside the region. • Example 6.2

7. Radiation Absorbed Dose • Exposure: photon beam, in air, E＜3MeV • Absorbed dose: for all types of ionizing radiation • Absorbed dose is a measure of the biologically significant effects produced by ionizing radiation Absorbed dose = dE/dm • dE is the mean energy imparted by ionizing radiation to material of dm • SI unit : gray (Gy) ; 1Gy = 1 J/kg ( 1 rad=100ergs/g=10-2J/kg, 1cGy=1rad )

8. Relationship Between Kerma, Exposure, and Absorbed Dose • Kerma ( K ): Kinetic energy released in the medium. K = dEtr / dm • dEtris the sum of the initial kinetic energies of all the charged particles liberated by uncharged particles ( photons) in a material of mass dm • The unit for kerma is the same as for dose, that is, J/kg. The name of its SI unit is gray (Gy)

9. Relationship Between Kerma, Exposure, and Absorbed Dose • Kerma ( K ): Kcoland Krad are the collision and the radiation parts of kerma K = Kcol + Krad ( J / m2 ) × ( m2 / kg ) • the photon energy fluence, Ψ • averaged mass energy absorption coefficient, men / r

10. Relationship Between Kerma, Exposure, and Absorbed Dose • Exposure and Kerma : • Exposure is the ionization equivalent of the collision kerma in air (Kcol)air = X · ( w/e ) , X = dQ/dm • w/e = 33.97 J/C

11. Relationship Between Kerma, Exposure, and Absorbed Dose • Absorbed Dose and Kerma :

12. 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 D1 D2

13. D1 D2 D1 D2 maximum electron range maximum electron range

14. Calculation of Absorbed Dose from Exposure • Absorbed Dose to Air : • In the presence of charged particle equilibrium (CPE), dose at a point in any medium is equal to the collision part of kerma. Dair = ( Kcol )air = X · ( w/e ) Dair(rad) = 0.876 ( rad/R) · X (R)

15. Calculation of Absorbed Dose from Exposure • Absorbed Dose to Any Medium : • Under CPE Dmed / Dair = (men/r)med / (men/r )air · A • A = med / air Dmed(rad) = fmed · X (R) · A • fmed : roentgen-to-rad conversion factor

16. Calculation of Absorbed Dose from Exposure • Absorbed Dose to Any Medium :

17. Calculation of Absorbed Dose from Exposure • Dose calculation with Ion Chamber In Air • For low-energy radiations, chamber wall are thick enough to provide CPE. • For high-energy radiation, Co-60, build-up cap + chamber wall to provide CPE.

18. Relationship Between Kerma, Exposure, and Absorbed Dose • Example 6.4 • Consider a gamma-ray beam of quantum energy 0.3 MeV. If the photon flux is 1000 quanta/cm2/s, and the air temperature is 20℃, what is the exposure rate at a point in this beam and what is the absorbed dose rate for soft tissue at this point?

19. The Bragg-Gray Cavity Theory • Limitations when calculate absorbed dose from exposure: • Photon only • In air only • Photon energy ＜3MeV • The Bragg-Gray cavity theory, on the other hand, may be used without such restrictions to calculate dose directly from ion chamber measurements in a medium

20. The Bragg-Gray Cavity Theory • Bragg-Gray theory • The ionization produced in a gas-filled cavity placed in a medium is related to the energy absorbed in the surrounding medium. • When the cavity is sufficiently small, electron fluence does not change. Dmed / Dgas = ( S / r )med / ( S / r )gas • (S / r)med / (S / r)gas = mass stopping power ratio for the electron crossing the cavity

21. The Bragg-Gray Cavity Theory • Bragg-Gray theory Dmed / Dgas = ( S / r )med / ( S / r )gas Jgas: the ionization charge of one sign produced per unit mass of the cavity gas

22. The Bragg-Gray Cavity Theory • The Spencer-Attix formulation of the Bragg-Gray cavity theory • Φ(E) is the distribution of electron fluence in energy • L/ris the restricted mass collision stopping power with Δ as the cutoff energy

23. INTERNALLY DEPOSITED RADIOISOTOPES • Corpuscular Radiation • The calculation of the absorbed dose from internally deposited radioisotopes • specific effective energy (SEE) • The energy absorbed per unit mass per transformation • For practical health physics purposes, "infinitely large" may be approximated by a tissue mass whose dimensions exceed the range of the radiation from the distributed isotope. For the case of alpha and most beta radiation, this condition is easily met

24. INTERNALLY DEPOSITED RADIOISOTOPES • Example 6.11 • Calculate the daily dose rate to a testis that weighs 18g and has 6660 Bq of 35S uniformly distributed throughout the organ

25. INTERNALLY DEPOSITED RADIOISOTOPES • Effective Half-Life • The total dose absorbed during any given time interval after the deposition of the isotope in the organ may be calculated by integrating the dose rate over the required time interval • In situ radioactive decay of the isotope • Biological elimination of the isotope

26. INTERNALLY DEPOSITED RADIOISOTOPES • Total Dose: Dose Commitment

27. INTERNALLY DEPOSITED RADIOISOTOPES • Total Dose: Dose Commitment • For practical purposes, an "infinitely long time" corresponds to about six effective half-lives

28. INTERNALLY DEPOSITED RADIOISOTOPES • Total Dose: Dose Commitment • Compartment theory • In many cases, an organ or tissue behaves as if the radioisotope were stored in more than one compartment • Each compartment follows first order kinetics and is emptied at its own clearance rate

29. INTERNALLY DEPOSITED RADIOISOTOPES • Total Dose: Dose Commitment • Compartment theory • Since the activity in each compartment contributes to the dose to that organ or tissue

30. INTERNALLY DEPOSITED RADIOISOTOPES • Gamma Emitters • cannot simply calculate the absorbed dose by assuming the organ to be infinitely large because gammas, being penetrating radiations, may travel great distances within tissue and leave the tissue without interacting

31. INTERNALLY DEPOSITED RADIOISOTOPES • Gamma Emitters • C is the concentration of the isotope • G is the specific gamma-ray emission • m is the linear energy absorption coefficient

32. INTERNALLY DEPOSITED RADIOISOTOPES • Gamma Emitters

33. INTERNALLY DEPOSITED RADIOISOTOPES • Gamma Emitters • geometry factor, g

34. INTERNALLY DEPOSITED RADIOISOTOPES • Gamma Emitters • Average geometry factor, g • For a cylinder

35. Gamma Emitters • Example 6.12 • A spherical tank, capacity 1 m3 and radius 0.62 m, is filled with aqueous 137Cs waste containing a total activity of 37,000 MBq (1Ci). What is the dose rate at the tank surface if we neglect absorption by the tank wall? Surface=0.5center

36. MIRD Method • To account for the partial absorption of gamma-ray energy in organs and tissues, the Medical Internal Radiation Dose Committee of the Society of Nuclear Medicine (MIRD) developed a formal system for calculating the dose to a "target" organ or tissue (T) from a "source" organ (S) containing a uniformly distributed radioisotope

37. MIRD Method • based on the concept of absorbed fraction, that is, the fraction of the energy radiated by the source organ which is absorbed by the target organ. S and T may be either the same organ or two different organs bearing any of the possible relationships to each other • These absorbed fractions are calculated by the application of Monte Carlo methods to the interactions and fate of photons or electrons following their emission from the deposited radionuclide

38. MIRD Method

39. MIRD Method • Monte Carlo methods • events such as the interaction of photons with matter are governed by probabilistic rather than deterministic laws • individual simulated photons (or other corpuscular radiation) are "followed" in a computer from one interaction to the next • we know the energy of the emitted radiation, its starting point, and its initial direction. The probability of each possible type of interaction within the organ and the energy transferred during each interaction are also known

40. MIRD Method • Monte Carlo methods • A situation is simulated by starting with a very large number of such nuclear transformations, following the history of each particle as it traverses the target tissue, and summing the total amount of energy that the particles dissipate within the target tissue

41. MIRD Method • Monte Carlo methods • absorbed fraction usually is less than 1 • For non-penetrating radiation, the absorbed fraction usually is either 1 or 0, depending on whether the source and target organs are the same or different =

42. MIRD Method • Example 6.13 • calculations of the dose rate to a 0.6-kg sphere made of tissue-equivalent material in which 1 MBq of 131I is uniformly distributed • The total energy absorbed from the 131I is simply the sum of the emitted beta-ray energy plus the fraction of the emitted gamma-ray energy that is absorbed by the sphere

43. MIRD Method • Example 6.13

44. MIRD Method • Example 6.13

45. MIRD Method • Example 6.13, return to the MIRD method • Let us consider two organs in the body • The rate of energy emission by the radionuclide in the source at any time that is carried by the ith particle is given by

46. MIRD Method • Example 6.13, return to the MIRD method

47. MIRD Method • Furthermore, since the radioactivity is usually widespread within the body, a target organ may be irradiated by several different source organs. The dose to the target, therefore is • where rk represents the target organ and rh represents the source organ

48. NEUTRONS • The absorbed dose from a beam of neutrons may be computed by considering the energy absorbed by each of the tissue elements that react with the neutrons • For fast neutrons up to about 20 MeV, the main mechanism of energy transfer is elastic collision • Thermal neutrons may be captured and initiate nuclear reactions

49. NEUTRONS • For fast neutrons • For isotropic scattering, the average fraction of the neutron energy transferred in an elastic collision with a nucleus of atomic mass number M is

50. NEUTRONS • For thermal neutrons • Exp. 14N( n, p )14C reaction • Exp. 1H( n, g )2H reaction