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# 輻射劑量學之品質保證 講者：蕭安成 物理師 - PowerPoint PPT Presentation

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• Daily : Sun Nuclear Daily QA 2

• Output calibration
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

The Roentgen
• 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.
• 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
• The SI unit for absorbed dose is the gray (Gy)

1Gy = 1 J/kg

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)
Relationship Between Kerma, Exposure, and Absorbed Dose
• Kerma ( K ): Kcoland Krad are the collision and the radiation parts of kerma

( J / m2 ) × ( m3 / kg )

• the photon energy fluence, Ψ
• averaged mass energy absorption coefficient, men / r
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
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

D1

D2

D1

D1

maximum electron range

maximum electron range

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 )

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
Calculation of Absorbed Dose from Exposure
• Absorbed Dose to Any Medium :
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.
Calculation of Absorbed Dose from Exposure
• Dose calculation with Ion Chamber In Air
• X = M · Nx ; D f.s. = ftissue· X · Aeq
• Nxis the exposure calibration factor for the given chamber
Calculation of Absorbed Dose from Exposure
• Dose Measurement from Exposure with Ion Chamber in a Medium

Dmed = M · Nx ·W/e ·[(men/r)med / (men/r)air] ·Am

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
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
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

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
Effective Point of Measurement
• Plane Parallel Chambers
• at the inner surface of the proximal collecting plate
• Cylindrical Chambers
• Shift proximal to the chamber axis by
• 0.75r for an electron beam (TG-21)
• 0.5r for an electron beam (TG-25)
• 0.6r for photon beams, 0.5r for electron beams(TG-51)
Cavity-Gas Calibration Factor (Ngas)
• The AAPM TG-21 protocol for absorbed dose calibration introduced a factor (Ngas ) to represent calibration of the cavity gas in terms of absorbed dose to the gas in the chamber per unit charge or electrometer reading.
• For an ionization chamber containing air in the cavity and exposed to a Co-60 gray
Cavity-Gas Calibration Factor (Ngas)
• Ngasis derived from Nx and
• other chamber-related parameters, all determined for the calibration energy, e.g., Co-60
Cavity-Gas Calibration Factor (Ngas)
• Once Ngas, is determined, the chamber can be used as a calibrated Bragg-Gray cavity to determine absorbed dose from photon and electron beams of any energy and in phantoms of any composition
• Ngas, is unique to each ionization chamber, because it is related to the volume of the chamber
Cavity-Gas Calibration Factor (Ngas)
• Nx = XM-1
• Dgas = Jgas ( W/e )
• Ngas = D gas Aion M-1
• Assume Aion =1
• Ngas = D gas M-1
• D gas = M × ( W/e ) / (rair× Vc )
• Ngas = ( W/e ) / (rair× Vc )
• if the volume of the chamber is 0.6 cm3, its Ngaswill be 4.73 ×107Gy/C
Chamber as a Bragg-Gray Cavity
• Photon Beams
• Suppose the chamber, with its build-up cap removed (it is recommended not to use buildup cap for in-phantom dosimetry), is placed in a medium and irradiated by a photon beam of given energy
Chamber as a Bragg-Gray Cavity
• Photon Beams
• Dose to medium at point P corresponding to the center of the chamber will then be
• P’ corresponding to the chamber's effective point of measurement
Chamber as a Bragg-Gray Cavity
• Photon Beams
• Pion
• correction factor for ion recombination losses
• Prepl
• corrects for perturbation in the electron and photon fluences at point P as a result of insertion of the cavity in the medium
• Pwall
• accounts for perturbation caused by the wall being different from the medium
Chamber as a Bragg-Gray Cavity
• Photon Beams
• The AAPM values for Prepl and Pwall have been derived with the chamber irradiated under the conditions of transient electronic equilibrium (on the descending exponential part of the depth dose curve )
Chamber as a Bragg-Gray Cavity
• Electron Beams
• When a chamber, with its build-up cap removed, is placed in a medium and irradiated by an electron beam
• usually assumed that the chamber wall does not introduce any perturbation of the electron fluence
• thin-walled (≦0.5 mm) chambers composed of low atomic number materials (e.g., graphite, acrylic)
• Pwall = 1
Chamber as a Bragg-Gray Cavity
• Electron Beams
• For an electron beam of mean energy Ez , at depth Z of measurement
Chamber as a Bragg-Gray Cavity
• Electron Beams
• Prepl
• fluence correction
• increases the fluence in the cavity since electron scattering out of the cavity is less than that expected in the intact medium
• Displacement in the effective point of measurement, which gives rise to a correction if the point of measurement is on the sloping part of the depth dose curve
Chamber as a Bragg-Gray Cavity
• Electron Beams
• Recommends that the electron beam calibration be made at the point of depth dose maximum
• Because there is no dose gradient at that depth, the gradient correction is ignored
• Prepl , then, constitutes only a fluence correction
• for cylindrical chambers as a function of mean electron energy at the depth of measurement and the inner diameter of ion chamber
Chamber as a Bragg-Gray Cavity
• Electron Beams
• a depth ionization curve can be converted into a depth dose curve using

A

A

A

B

B

B

Chamber as a Bragg-Gray Cavity
• Electron Beams
• The gradient correction, however, is best handled by shifting the point of measurement toward the surface through a distance of 0.5r
• For well designed plane-parallel chambers with adequate guard rings, both fluence and gradient corrections are ignored, i.e., Prep, = 1; the point of measurement is at the front surface of the cavity
Calibration Phantom
• The TG-21 protocol recommends that calibrations be expressed in terms of dose to water
• polystyrene, or acrylic phantoms may be used, but, requires that the dose calibration be reference to water
• Scaling factors
• SF = d plastic / d water = mwater / mplastic
Calibration Phantom
• Acalibration phantom must provide at least 5 cm margin laterally beyond field borders
• and at least 10 cm margin in depth beyond the point of measurement
• Calibration depths for a megavoltage photon beams are recommended to be between 5- and 10-cm depth, depending on energy
• For electron beams, the calibration depth recommended by TG-21 is the depth of dose maximum for the reference cone

• Monthly :
• Keithley 35-040 electrometer + NE 2571 Farmer chamber
• Victoreen 530 electrometer + PTW N30001 Farmer chamber
• Solid phantom : Acrylic, Polystyrene, and solid water
• Sun Nuclear Daily QA 2
• et al.

• Annual :
• Keithley 35-040 electrometer + NE 2571 Farmer chamber
• Victoreen 530 electrometer + PTW N30001 Farmer chamber
• Solid phantom : Acrylic, Polystyrene, and solid water
• Sun Nuclear Daily QA 2
• WellHoffer water phantom + IC 10 chamber
• et al.

• 醫用直線加速器
• 每月及年度品質保證作業
• 物理師執行
INTRODUCTION

AAPM TG-51 has recently developed a new protocol for the calibration of high-energy photon and electron beams used in radiation therapy. The formalism and the dosimetry procedures recommended in this protocol are based on the used of an ionization chamber calibrated in terms of absorbed dose-to-water in a standards laboratory’s Co-60 gamma ray beam. This is different from the recommendations given in the AAPM TG-21 protocol, which are based on an exposure calibration factor of an ionization chamber in a Co-60 beam. The purpose of this work is to compare the determination of absorbed dose-to-water in reference conditions in high-energy photon and electron beams following the recommendations given in the two protocols.

METHODS AND MATERIALS
• Calibrations of photon beams ( nominal energy of 6 and 10 MV ) and electron beams ( nominal energy of 6, 8, 10, 12, 15 and 18 MeV ), generated by a Siemens KDS-2 linac, are performed.
• Farmer-type ( NE 2571 ) ionization chamber was used for photon beam dosimetry and plane parallel ( PTW Markus ) chambers was used for electron beam dosimetry.
• Absorbed-dose-to-water calibration factor, ND,w , and exposure calibration factor, Nx , for Farmer-type chamber were provided by NATIONAL RADIATION STANDARD LABORATORY INER.
• Plane-parallel chamber was calibrated against calibrated cylindrical chamber in a 18 MeV electron beam, as recommended in TG-21, TG-39 and TG-51.

60Co

METHODS AND MATERIALS

60Co

• ND,w = 4.5394 cGy/nC , expanded uncertainty = 1% ( k=2 ). Date of report : 2001/12/24, report No. : NRSL – 90084.
• Nx = 4.7179 R/nC , expanded uncertainty = 1% ( k=2 ).

Date of report : 2001/10/17, report No. : NRSL – 90073.

• Keithley electrometer and Nucleartron water phantom were used in this study.
• The depth of clinical dosimetry for electron beams was performed at dref , dref = 0.6R50 – 0.1 ( cm ), as recommended in TG-51.
• Depth ionization measurements along the central axis were made by using the Markus chamber and referenced to that of a 0.12 cm3 RK chamber mounted on the head of the machine.
RESULTS

60Co

pp

60Co

Discussion and Conclusion
• The doses at 10 cm in water for 6 MV and 10 MV photon beams and the doses at dref in water for 6 to 18 MeV electron beams determined with TG-51 and TG-21 are within 0.3% and 1.4% .
• According to TG-51, P-P chambers must be used for reference dosimetry in electron beams of energies 6 MeV or less. In the meantime, NRSL provided ND,W factor for Farmer-type chamber only. So, the ND,W factor of a P-P chamber should be determined by using the cross calibrating method.
• Measurements at the IAEA Dosimetry Lab. have shown that at Co-60, the absorbed dose to water determined by using the ND,W is about 1% higher than that by using the Nx. But, it is different in this study. Detailed analysis should be done including the data given in the two protocols and the calibration factors provided from air-kerma and absorbed dose to water.

Co-60

Co-60

Co-60