Measurement of absorbed dose

measurement of absorbed dose in radiation physics

1. Measurement of absorbed dose Dr. Purvi Rathod

2. Radiation measurements / dosimetry deals with the methods for quantitative determination of energy deposited in a given medium by the directly and indirectly ionizing radiation.

3. fluence It is used to describe a monoenergetic ionizing radiation beam. Consist of particle fluence , energy fluence , particle fluence rate and energy fluence rate. Particle fluence - it is the number of particles dN incident on a sphere of cross sectional area dA. ie , dN / dA.

4. Particle fluence rate- it is the quotient of increment of the fluence in a fixed time interval. Energy fluence - amount of radiant energy dE incident on a sphere of cross sectional area dA. Its unit is J/ . Energy fluence rate- it is the quotient of increment of the energy fluence in a fixed time interval.  

5. KERMA Acronym for kinetic energy released per unit mass. It is the average amount of energy transferred from indirectly ionizing radiation to directly ionizing radiation. The energy of photons is imparted to matter in two stages- 1) photon radiation transfers energy to the secondary charged particles (electrons) through different photon interactions. 2) the charged particles transfer energy to the medium through atomic excitation and ionizations. Unit of KERMA- J/Kg= Gy Kerma is made up of collision Kerma and radiative Kerma ( bremsstrahlung)

6. Exposure and KERMA Exposure is defined as dQ / dM where dQ is the total charge of the ions of one sign produced in air when all electrons liberated by photons in air of mass dM are completely stopped in air. Exposure is the ionization equivalent of the collision KERMA ( ) in air. It is calculated from by knowing the ionization charge produced per unit of energy deposited by photons.  

7. KERMA pertains to photon beams only. It is proportional to the photon energy fluence . It is maximum at the surface and decreases exponentially with depth.

8. Absorbed dose Absorbed dose is applicable to both directly and indirectly ionizing radiations. It is defined as the mean energy E imparted by ionizing radiation to matter of mass M in a finite volume V. Energy E- sum of all energy entering the volume of interest minus the energy leaving the volume. Its unit is j/Kg= Gy .

9. Absorbed dose and KERMA KERMA is maximum at surface and reduces with depth. Dose increases upto a maximum value at surface and then decreases as the same rate as KERMA. β is the ratio of absorbed dose to collision KERMA. β = 1, when absorbed dose is equal to KERMA at state of electronic equilibrium. In built up region β < 1 because KERMA is maximum at surface but dose initially builds up to a maximum then falls with KERMA.

10. β > 1, in regions of transient electronic equilibrium , at depths greater than the maximum range of electrons, the combined effect of attenuation of the photon beam and forward motion of electrons. A fixed value of β = 1.005 is used for cobalt 60 in conjunction with ion chamber dosimetry.

11. Absorbed dose in Air Determination of absorbed dose from exposure is readily accomplished under electronic equilibrium. Low energy beams are calibrated in air in terms of exposure in roentgen. Exposure is then converted into dose in free space by multiplying dose in free space with backscatter factor. We get Dmax ( dose at the depth of maximum dose in a water phantom.) A conversion factor of 0.876 is used from roentgen to cGy for air under electronic equilibrium.

12. Absorbed dose in any medium A conversion factor f is used to convert roentgen to rad . It depends on the mass energy absorption coefficient of the medium relative to air . It is a function of the medium of composition as well as the photon energy. At higher photon energies where the Compton effect is predominant, f factor is almost same for all materials.

13. Stopping power linear stopping power and mass stopping power. Linear stopping power is the expected value of the rate of energy loss per unit path length of the charged particle. Mass stopping power is linear stopping power divided by the density of the absorbing medium. Stopping power are of two types- 1) collision (ionization) stopping power resulting from the interaction of charged particles with atomic orbital electrons. 2) radiative stopping power resulting from interaction of charged particles with atomic nuclei.

14. Mass collision stopping power- the average rate of energy loss by a charged particle in all hard and soft collisions. Soft collision- when a charged particle passes an atom at a considerable distance, then a very small amount of energy is transferred to an atom of the absorbing medium in a single collision. Hard collision- when a charged particle passes an atom at a distance same as the atomic radius, a secondary electron with considerable energy is ejected and forms a separate track.

15. Bragg Gray cavity theory In order to measure the absorbed dose in a medium it is necessary to introduce a radiation sensitive device/ dosimeter into the medium. The cavity sizes are small intermediate or large in comparison with the range of secondary charged particles produced in the medium. This theory relates the absorbed dose in the dosimeter sensitive medium to the absorbed dose in surrounding medium containing the cavity.

16. Conditions for application of this theory- 1) cavity must be small as compared to the range of the charged particles incident on it, so that its presence does not perturb the fluence of charged particles in the medium. 2) the absorbed dose in the cavity is deposited solely by charged particles crossing it. According to (1) electron fluence are same and equal to the equilibrium fluence in the surrounding medium. This condition can be valid only in regions of charged particle equilibrium ( cpe ). The presence of cavity will always cause some amount of fluence perturbation that requires correction factor.

17. According to (2) all electrons depositing the dose inside the cavity are produced outside the cavity and completely cross the cavity. No secondary electrons are hence produced inside the cavity and no electrons stop within the cavity. Under the two conditions according to Bragg Gray Theory, the dose to medium is equal to product of dose to cavity and ratio of average unrestricted mass collision stopping powers of the medium and the cavity. = (  

18. Fulfilment of the Bragg Gray conditions depends on the Cavity size Range of electrons in the cavity medium The cavity medium Electron energy This theory dose not take into account the creation of secondary electrons generated as a result of hard collision.

19. Effective point of measurement Plane parallel chambers- If the chamber has a small plate separation and the electron fluence is mostly forward directional then it can be assumed that the point of measurement is the front surface of the cavity. Cylindrical chambers- The point of measurement in a unidirectional beam from the center and towards the source is displaced by 0.85r (r= the internal radius of the chamber).

20. Calibration phantom The calibration of photon and electron beams are performed in a water phantom. Its recommended dimensions are atleast 30x30x30 . If the beam is entering the phantom from the side walls of plastic then a scaling factor of 1cm acrylic= 1.12cm of water is used. Chamber water proofing- cylindrical ion chamber can be waterproofed with a thin acrylic sleeve (<1mm thick). The chamber should slip into the sleeve with little resistance and minimal air gaps  

21. Water phantom

22. Charge measurement Fully corrected charge reading M is given by M= M raw= raw chamber reading in Coulombs. P ion= ion recombination correction( readings with full voltage and half voltage are taken. There ratio is the P ion reading.) P t.p = air temperature and pressure correction. ( standard temp= 22*C, pressure= 760mm hg)  

23. P elec = electrometer calibration factor(if the electrometer is separate then a calibration factor for charge measurement should be applied .) P pol= polarity correction( depends on the chamber design, cable position and beam quality. Reading from both polarities should be taken and final value should be calculated accordingly.)

24. Units and conversion 1 Rad= 0.01 Gray 1 Rem= 0.01 Sievert 1 Roentgen=2.58x C/Kg 1 Curie= 3.7x Becquerel Roentgen to Rad conversion factor = f ( different for different medium and energy) Roentgen to Rad conversion factor for air= 0.876  

25. Methods of absorbed dose measurement Calorimetry Chemical dosimetry Solid state methods Silicon diodes Radiographic film Radiochromic film

26. Calorimetry Energy absorbed in a medium from radiation appears as heat energy. This causes small increase in temperature of the absorbing medium and can be related to the absorbed dose. The increase in temperature produced by 1Gy radiation is 2.39x ‘C. This small temperature rise is measured by using thermistors. Thermistors are semiconductors and show large change in electrical resistance with small change in temperature (5%/1’C). The Domen’s calorimeter measures dose rates in water of about 4Gy/min with a precision of 0.5%.  

27. Chemical dosimeters Energy absorbed from the ionizing radiation may produce chemical change. This chemical change can be used to measure the absorbed dose. Fricke’s dosimeter is considered the most developed system for the measurement of absorbed dose. It contains ferrous sulfate, NaCl and sulphuric acid. When the solution is irradiated the ferrous ions get oxidized to ferric ions. Concentration of ferric ions is determined by spectrophotometry of the dosimeter solution.

28. G value The radiation chemical yield is expressed in terms of G value that is, number of molecules produced per 100eV of energy absorbed. G value of 15.7 +/- 0.6/100 eV for Frickes dosimeter is recommended for electrons in the range 1 to 30 MeV for a 0.4mol/L sulphuric acid dosimeter solution.

29. Solid state methods Integrated type dosimeters Thermoluminescent crystals. Radiophotoluminescent glasses. Optical density type dosimeters. Most widely used system is the thermoluminescent dosimeters. Electrical conductivity dosimeters Semiconductor junction detectors. Induced conductivity in insulating materials.

30. Thermoluminescent dosimetry Crystalline materials exhibit thermoluminescence . When such a crystal is irradiated, minute fraction of the absorbed dose is stored in the crystal lattice. If this material is heated later then this store energy is reposed as light. This phenomenon is known as thermoluminescence .

31. Measurement of TL The irradiated material is placed on a heating cup. Heat produced will cause the material to emit light. This light is measured by a photomultiplier tube ( pmt ) and converts light energy to electrical energy. Current in amplified by an amplifier and recorded. There are many phosphors available – lithium fluoride ( LiF ), Lithium borate (Li2B4O7), calcium fluoride (CaF2), calcium sulphate (CaSO4). Lithium fluoride is most widely used. It contains trace impurities which gives rise to radiation induced thermoluminescence . In I ndia we use calcium sulphate phosphor as it is more economical.

32. Fluorescence In the crystal lattice there is mutual interaction between the atoms giving rise to energy bands. When the material is irradiated some electrons in ground state are raised to conduction state due to gain of energy. The valency thus created in the valence band is called positive hole. There are multiple transitions taking place until the material reaches a metastable state. These multiple transitions cause a continuous emission of light called Fluorescence.

33. Glow curve On plotting the thermoluminescence against temperature we get a glow curve. As the temperature increases the release of trapped electrons increases. The light emission increases initially to maximum then falls to minimal. Most of the phosphors contain traps at different energy levels causing a number of glow peaks at those energy levels.

34. Practical use of TLD They should be calibrated before use for measuring the unknown dose. Material should be annealed to remove the residual effects of the previous radiation history. 1 hr heating at 400’C  24hrs at 80’C ( removes peak 1 and 2 of glow curve by reducing the trapping efficiency.) By removing these peaks the glow curve becomes more stable and predictable.

35. Calibration should be done with the same TLD reader with approximately same quality beam and same absorbed dose. When considerable care is used a precision of about 3% is obtained using the TLD. It is not as precise as the ion chamber but it is easy to use in places where an ion chamber cannot be used. Egs - personal dose monitoring, monitor around brachytherapy source.

36. Silicon diodes Silicon p-n junction diodes are used for relative dosimetry. There advantages over ionization chambers- higher sensitivity, instantaneous response, small size. Uses- relative measurement of electron beams, output constancy checks, in vivo patient dose monitoring. Limitations- energy dependence in photon beams, direction dependence, thermal effects, radiation induced damage.

37. Operation of silicon diodes The diode consist of silicon crystals mixed with impurities to make a p and n type silicon. P type silicon is electron receptor. N type silicon is the electron donor. At the interface of p and n a depletion zone is created due to diffusion of electrons from n to p. This depletion zone creates a electric field which opposes further diffusion until equilibrium is achieved.

38. Radiation diode detector- silicon p-n junction diode connected to a coaxial cable in such a way that the radiation beam will be incident perpendicular to the detector. The diode is connected to a amplifier to measure radiation induced current. No voltage is applied. Silicon diodes are more sensitive then ion chambers. Density of silicon is 1,800 times of air hence current produced per unit volume is 1,800 times larger in a diode than an ion chamber. Hence diode even with a small collecting volume can provide an adequate signal.

39. Angular dependence- diodes show angular dependence, hence angle of incidence of beam must be taken into account. Temperature dependence- in diodes the temperature dependence is smaller compared to that of ion chambers. The response is independent of pressure and humidity. Radiation damage- diode can get permanently damage if irradiated with ultrahigh dose of ionization. Due to displacement of silicon atoms from their lattice. It can also happen after prolonged use.

40. Uses of silicon diodes Electron beam dosimetry and in some situations in photon beam measurement. Patient dose monitoring. Calibration factors are applied to convert the diode reading into expected dose at the reference point, taking into account source to detector distance , field size, etc.

41. Radiographic film Transparent film base coated with emulsion containing very small crystals of silver bromide. When the film is exposed to ionizing radiation/ visible light, a chemical change takes place with the exposed crystals to form a latent image. When the film is developed the affected crystals are reduced to small grains of metallic silver . This film is fixed then. The metallic silver that is not affected by the fixer causes darkening of the film.

42. It is a method of measuring electron beam distribution , but its use in photon beam dosimetry measurement is limited. The film suffers changes in processing conditions and artifacts caused by air pockets adjacent to the film. Hence absolute dosimetry with film is not possible. It is usefull in checking radiation fields, field flatness and symmetry. In megavoltage range of photon energies the film has been used to measure the isodose curves.

43. Radiochromic film Advantages of these films- Show Tissue equivalence. Have High spatial resolution. Have Large dynamic range( to Gy ). Low spectral sensitivity variation. Insensitive to visible light. No need for any chemical processing (self developing). It is well suited for brachytherapy dosimetry.  

44. The film consist of ultrathin 7 to 23micrometer thick, colorless, radiosensitive leucodye that is bonded on a 100 micrometer thick Mylar base. The unexposed film is colorless and changes to shades of blue due to polymerization induced by ionizing radiation. The degree of coloring is measured with a spectrophotometer using a narrow spectral wavelength. Radiochromic films are almost tissue equivalent with effective Z of 6 to 6.25.

45. They are insensitive to visible light, they exhibit some sensitivity to ultraviolet light and temperature . They should be stored in dry dark environment almost similar to that at which it will be used for dosimetry . They must be calibrated before use . Most commonly used films that are commercially available are GafChromic EBT film, double layer GafChromic MD-55-2 film.

46. MOSFET Metal oxide semiconductor field effect transistor. Very little attenuation to the beam as it has small size. Usefull for vivo dosimetry. The integrated dose can be measured during or after irradiation. Ionizing radiations penetrate the oxide and generate charge that gets permanently trapped. This charge causes change in voltage that can be measured and from that the absorbed dose. Uses- in vivo and phantom dose measurement, brachytherapy, IMRT, intraoperative radiotherapy, radiosurgery.

47. Diamond dosimeters Diamonds change there resistance upon radiation exposure. The commercially available dosimeters have a diamond crystal sealed in a polystyrene case and a bias voltage is applied through the thin golden contacts. They are tissue equivalent and require almost no energy correction. They have a small size, negligible directional dependence. They are ideal for use in high dose gradient regions like stereotactic radiosurgery. They should be irradiated prior to every use to reduce the polarization effect. They are waterproof and can be used for measurement in the water phantom.

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