輻射劑量學之品質保證輻射劑量學之品質保證

講者：蕭安成 物理師

參考資料：1. The Physics of Radiation Therapy

Faiz M. Khan2 Introduction to Radiological Physics and Radiation Dosimetry2. Introduction to Radiological Physics and Radiation Dosimetry

Frank H. Attix,

醫用直線加速器品質保證作業項目

項 次 校 驗 項 目結 果 或 誤 差 容 許 值

A1-5 光子輸出劑量 小於百分之三

A2-1 光子輸出劑量 小於百分之二

A2-4 電子輸出劑量 小於百分之二

A2-5 光子射束中心軸於治療深度之劑量參數 每一點小於百分之二

A2 6 電子射束中心軸於治療深度之劑量參數每一點小於百分之二或於治

A2-6 電子射束中心軸於治療深度之劑量參數療深度內小於二毫米。

A2-7 光子平坦性 小於基準直百分之二

A2-8 電子平坦性 小於基準直百分之三

A2-9 光子對稱性 小於百分之三

A2-10 電子對稱性 小於百分之三

品保作業儀器設備• 品保作業儀器設備– Daily : Sun Nuclear Daily QA 2y y Q

品保作業儀器設備• 品保作業儀器設備– Beam uniformity

品保作業儀器設備• 品保作業儀器設備– Output calibrationp

Measurement of Absorbed Dose

Th RThe 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 )The SI unit is C/kg but the special unit is roentgen ( R )

1R = 2.58 × 10-4 C/kg

Th RThe Roentgen• Charged Particle Equilibrium (CPE ) : Electron• Charged Particle Equilibrium (CPE ) : Electron

produced outside the collection region, which enter the ion collecting region is equal to theenter the ion-collecting region, is equal to the electron produced inside the collection region , which deposit their energy outside the regionwhich deposit their energy outside the region.

R di i Ab b d DRadiation 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 radiationAbsorbed dose = dE/dm

– dE is the mean energy imparted by ionizing d s t e ea e e gy pa ted by o gradiation to material of dm

– The SI unit for absorbed dose is the gray (Gy)S g y (Gy)1Gy = 1 J/kg

( 1 rad=100ergs/g=10-2J/kg 1cGy=1rad )( 1 rad 100ergs/g 10 J/kg, 1cGy 1rad )

Relationship Between Kerma, Exposure, and Absorbed Dose

• Kerma ( K ): Kinetic energy released in the medium.medium.

K = dEtr / dmdE i th f th i iti l ki ti i f– dEtr is the sum of the initial kinetic energies of all the charged particles liberated by uncharged

ti l ( h t ) i t i l f dparticles ( 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)

R l ti hi B t K ERelationship Between Kerma, Exposure, and Absorbed Dose

• Kerma ( K ): Kcol and Krad are the collision• Kerma ( K ): Kcol and Krad are the collision and the radiation parts of kerma

K = Kcol + Krad

– the photon energy fluence, Ψ– averaged mass energy absorption coefficient, μen / ρ

R l ti hi B t K ERelationship Between Kerma, Exposure, and Absorbed Dose

• Exposure and Kerma :– Exposure is the ionization equivalent of

the collision kerma in airthe collision kerma in air.

X = (Kcol)air · ( e/w )( )air ( )• w/e = 33.97 J/C

R l ti hi B t K ERelationship Between Kerma, Exposure, and Absorbed Dose

• Absorbed Dose and Kerma :

R l ti hi B t K ERelationship Between Kerma, Exposure, and Absorbed Dose

• Absorbed Dose and Kerma :– Suppose D1 is the dose at a point in some

material in a photon beam and another material pis substituted of a thickness of at least one maximum electron range in all directions from gthe point, then D2 , the dose in the second material, is related to D1 by1 y

Calculation of Absorbed Dose from ExposureCalculation of Absorbed Dose from Exposure

• Absorbed Dose to Air :• 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.

D i = ( Kcol ) i = X · ( w/e )Dair ( K )air X ( w/e )

Dair(rad) = 0.876 ( rad/R) · X (R)

Calculation of Absorbed Dose from ExposureCalculation of Absorbed Dose from Exposure

• Absorbed Dose to Any Medium :

Under CPE– Under CPE Dmed / Dair = (μen/ρ)med / (μen/ρ )air · A

• A = Ψmed / Ψair( ) f ( ) ADmed(rad) = fmed · X (R) · A

• fmed : roentgen-to-rad conversion factorfmed g

Calculation of Absorbed Dose from ExposureCalculation of Absorbed Dose from Exposure

• Absorbed Dose to Any Medium :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.

F hi h di ti C 60 b ild– For high-energy radiation, Co-60, build-up cap + chamber wall to provide CPE.

F Ch bFarmer Chamber

P ll l Pl Ch bParallel-Plate Chamber

ElElectrometer

Calculation of Absorbed Dose from Exposure

• Dose calculation with Ion Chamber In Air

– X = M · N ; D f = fti · X · AX M Nx ; D f.s. ftissue X Aeq– Nx is the exposure calibration factor for the

given chambergiven chamber

Calculation of Absorbed Dose from Exposure

• Dose Measurement from Exposure with Ion h b i diChamber in a Medium

D = M · N ·W/e ·[(μ /ρ) / (μ /ρ) ] ·ADmed M Nx W/e [(μen/ρ)med / (μen/ρ)air] Am

The Bragg-Gray Cavity Theorygg y y y

• Limitations when calculate absorbed dose from exposure:

Ph t l– Photon only

– In air onlyy

– Photon energy ＜3MeV

• The Bragg-Gray cavity theory, on the other hand, may be used without such restrictions d, y be used w ou suc es c o sto calculate dose directly from ion chamber measurements in a mediummeasurements in a medium

The Bragg-Gray Cavity Theorygg y y y

• Bragg-Gray theoryBragg Gray theory– The ionization produced in a gas-filled cavity

placed in a medium is related to the energyplaced in a medium is related to the energy absorbed in the surrounding medium.

h h i i ffi i l ll l– When the cavity is sufficiently small, electron fluence does not change.

Dmed / Dgas = ( S / ρ )med / ( S / ρ )gas• (S / ρ)med / (S / ρ)gas = mass stopping power ratio for

the electron crossing the cavity

Th B G C it ThThe Bragg-Gray Cavity Theory

• Bragg-Gray theoryD d / D = ( S / ρ ) d / ( S / ρ )Dmed / Dgas ( S / ρ )med / ( S / ρ )gas

Jgas : the ionization charge of one sign produced per unit f th itmass of the cavity gas

The Bragg-Gray Cavity Theory

• The Spencer-Attix formulation of the Bragg-Gray i hcavity theory

Φ(E) i th di t ib ti f l t fl i– Φ(E) is the distribution of electron fluence in energy – L/ρ is the restricted mass collision stopping power with ⊗

as the cutoff energyas the cutoff energy

Effective Point of Measurement

• Plane Parallel Chambers– at the inner surface of the proximal collecting plate

• Cylindrical ChambersCylindrical Chambers– Shift proximal to the chamber axis by

• 0 75r for an electron beam (TG 21)• 0.75r for an electron beam (TG-21)• 0.5r for an electron beam (TG-25)

0 6 f h t b 0 5 f l t b (TG 51)• 0.6r for photon beams, 0.5r for electron beams(TG-51)

CALIBRATION OFCALIBRATION OF MEGAVOLTAGE BEAMS:

TG-21 PROTOCOL

Cavity-Gas Calibration Factor (Ngas)

• The AAPM TG-21 protocol for absorbed dose lib ti i t d d f t (N ) t tcalibration introduced a factor (Ngas ) to represent

calibration of the cavity gas in terms of absorbed d h i h h b i hdose 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 Go-60 γ ray

Cavity-Gas Calibration Factor (Ngas)

• Ngas is derived from Nx and • other chamber-related parameters, all determined p ,for the calibration energy, e.g., Co-60

Cavity-Gas Calibration Factor (Ngas)

• Once Ngas, is determined, the chamber can gasbe used as a calibrated Bragg-Gray cavity to determine absorbed dose from photon anddetermine absorbed dose from photon and electron beams of any energy and in phantoms of any compositionof any composition• Ngas, is unique to each ionization chamber, b i i l d h l f hbecause 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-1gas gas ion– Assume Aion =1– N = D M-1 Ngas D gas M– D gas = M × ( W/e ) / (ρair × Vc )

N = ( W/e ) / (ρ × V )– Ngas = ( W/e ) / (ρair × Vc )– if the volume of the chamber is 0.6 cm3, its Ngas will be

4 73 × 107 Gy/C4.73 × 10 Gy/C

Cavity-Gas Calibration Factor (Ngas)

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 fl t i t P lt f i ti f th itfluences at point P as a result of insertion of the cavity in the medium

P– Pwall• accounts for perturbation caused by the wall being

different from the mediumd e e o e ed u

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 g p p pcurve )

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 p yelectron beam

– usually assumed that the chamber wall does notusually assumed that the chamber wall does not introduce any perturbation of the electron fluence

• thin-walled (≦0 5 mm) chambers composed of lowthin walled (≦0.5 mm) chambers composed of low atomic number materials (e.g., graphite, acrylic)

• Pwall = 1wall

Chamber as a Bragg-Gray Cavity

• Electron Beams– For an electron beam of mean energy Ez , at depth

Z of measurementZ of measurement

Ch b B G C iChamber as a Bragg-Gray Cavity

• Electron Beams– Preplrepl

• fluence correction– increases the fluence in the cavity since electronincreases the fluence in the cavity since electron

scattering out of the cavity is less than that expected in the intact medium

• Gradient correction– Displacement in the effective point of

t hi h i i t ti ifmeasurement, which gives rise to a correction if the point of measurement is on the sloping part of the depth dose curvep

Chamber as a Bragg-Gray Cavity

• Electron Beams– Recommends that the electron beam calibration

be made at the point of depth dose maximump p– Because there is no dose gradient at that depth,

the gradient correction is ignoredthe gradient correction is ignored– Prepl , then, constitutes only a fluence correction

• for cylindrical chambers as a function of mean electron• for cylindrical chambers as a function of mean electron energy at the depth of measurement and the inner diameter of ion chamberdiameter of ion chamber

Chamber as a Bragg-Gray Cavity

• Electron Beams– a depth ionization curve can be converted into a

depth dose curve usingp g

A

A A

B

B

B BB

Chamber as a Bragg-Gray Cavity

• Electron Beams– The gradient correction, however, is best

handled by shifting the point of measurementhandled by shifting the point of measurement toward the surface through a distance of 0.5r

– For well designed plane-parallel chambers with– For well designed plane-parallel chambers with adequate guard rings, both fluence and gradient corrections are ignored i e Prep = 1; the pointcorrections are ignored, i.e., Prep, 1; the point of measurement is at the front surface of the cavitycavity

Calibration Phantom

• The TG-21 protocol recommends that calibrations b d i f dbe 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 l ti / d t = μ t / μ l tiSF d plastic / d water μ water / μ plastic

Calibration Phantom

• A calibration phantom must provide at least 5 cm margin laterally beyond field bordersmargin laterally beyond field borders

• and at least 10 cm margin in depth beyond the point of measurementpoint of measurement

• Calibration depths for a megavoltage photon beams are recommended to be between 5 and 10beams are recommended to be between 5- and 10-cm depth, depending on energy

• For electron beams the calibration depth• For electron beams, the calibration depth recommended by TG-21 is the depth of dose maximum for the reference conemaximum for the reference cone

品保作業儀器設備• 品保作業儀器設備Monthly :– Monthly : • Keithley 35-040 electrometer + NE 2571

F h bFarmer chamber• Victoreen 530 electrometer + PTW N30001

Farmer chamber• Solid phantom : Acrylic, Polystyrene, and p y , y y ,

solid water• Sun Nuclear Daily QA 2Sun Nuclear Daily QA 2• et al.

品保作業儀器設備• 品保作業儀器設備Annual :– Annual : • Keithley 35-040 electrometer + NE 2571

F h bFarmer chamber• Victoreen 530 electrometer + PTW N30001

Farmer chamber• Solid phantom : Acrylic, Polystyrene, and p y , y y ,

solid water• Sun Nuclear Daily QA 2Sun Nuclear Daily QA 2• WellHoffer water phantom + IC 10 chamber

t l• et al.

品質保證計畫‧品質保證計畫

– 醫用直線加速器– 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

di i h Th f li d h d i dradiation therapy. The formalism and the dosimetry procedures recommended in this protocol are based on the used of an ioni ation chamber calibrated in terms of absorbed dose to aterionization 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 21different 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 isionization 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 g gy pfollowing the recommendations given in the two protocols.

METHODS AND MATERIALS Calibrations of photon beams ( nominal energy of 6 and 10Calibrations 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 )photon beam dosimetry and plane parallel ( PTW Markus ) chambers was used for electron beam dosimetry. Absorbed-dose-to-water calibration factor, ND,w , and

60Co,

exposure calibration factor, Nx , for Farmer-type chamber were provided by NATIONAL RADIATION STANDARD LABORATORY INER.LABORATORY INER. Plane-parallel chamber was calibrated against calibrated cylindrical chamber in a 18 MeV electron beam, as

d d i TG 21 TG 39 d TG 51recommended in TG-21, TG-39 and TG-51.

METHODS AND MATERIALSN 4 5394 G / C d d t i t 1% ( k 2 )

60CoND,w = 4.5394 cGy/nC , expanded uncertainty = 1% ( k=2 ). Date of report : 2001/12/24, report No. : NRSL – 90084.N = 4 7179 R/nC expanded uncertainty = 1% ( k=2 )

Co

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 wereKeithley 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 ofto that of a 0.12 cm3 RK chamber mounted on the head of the machine.

RESULTS Ch t i ti f th li d i l h bCharacteristics of the cylindrical chamber.

Cavity length WallWall

thickness2

Innerdiameter Nx Ngas ND,W

60Co

Chamber type ( mm ) material ( g cm-2 ) ( mm ) ( R/nC ) ( cGy/nC ) ( cGy/nC )

NE 2571 24.0 graphite 0.065 6.3 4.7179 4.0246 4.5394

Characteristics of the plane-parallel chamber.

Collecting Ngas kecalND,W60Copp

Chamber type

Windowthickness

( mg cm-2 )

Electrodespacing( mm )

Collectingelectrodediameter( mm )

Guard ringwidth( mm )

gas

( from crosscalibration )( cGy/nC )

ecal ,

( from crosscalibration )( cGy/nC )Chamber type ( mg cm ) ( mm ) ( mm ) ( mm ) ( cGy/nC ) ( cGy/nC )

PTW Markus 102 2.0 5.3 0.2 46.447 47.705

RESULTS

Beam characteristics and calibration condition for photon beams.Stated

( IonizatioNominal

accel. Beam QualityQuality

C iPhantom

i l SSDFieldi (

Depth ofenergy (

MV )

o on ratio

cce .Potential( MV )

Q y[ %dd (10)x ]

Conversionfactor ( kQ )

material( med )

SS( cm )

size (cm2 )

measurement ( cm )

6 0 675 5 3 67 42 0 993 water 100 10 x 10 106 0.675 5.3 67.42 0.993 water 100 10 x 10 10

10 0.746 10.0 75.20 0.981 water 100 10 x 10 10

1 mm lead foil was added for the measurements of beam quality for 10 MV photon beam. Th %dd (10) ( i l d d - t i ti ) f 10 MV h t b 74 57The %dd (10)pb ( included e contamination ) for 10 MV photon beam : 74.57 .

RESULTS B h t i ti d lib ti diti f l t bBeam characteristics and calibration conditions for electron beams.

Statedenergy ( d50

( )R50

( )dref

( )k'R50

E0( M V )

Rp( )

Ez(M V)

Phantommaterial SSD( )

Fieldsizegy (

MeV ) (cm) (cm) (cm) ( MeV ) (cm) (MeV) ( med ) (cm) (cm2)

6 2.18 2.183 1.210 1.0385 5.080 2.85 2.920 water 100 15 x 15

8 3.05 3.078 1.747 1.0313 7.107 3.95 3.964 water 101 15 x 15

10 3.82 3.871 2.222 1.0250 8.900 4.89 4.856 water 102 15 x 15

12 4.57 4.643 2.686 1.0190 10.648 5.71 5.639 water 103 15 x 15

15 5.80 5.908 3.445 1.0105 13.510 7.20 7.046 water 104 15 x 15

18 7 01 7 153 4 192 1 0023 16 330 8 55 8 324 t 105 15 1518 7.01 7.153 4.192 1.0023 16.330 8.55 8.324 water 105 15 x 15

RESULTS P t f l l ti th d t t fParameters for calculating the dose to water from photon beams for TG-21.

water waterDwater = M × Ngas × ( L/ρ )air × Pwall × Pion × Prepl × ( μen/ρ )

Stated energy Ngas ( L / ) P P P

water

water

waterwater

gy( MV )

gas

( cGy/nC)( L / ρ )air Pwall Pion Prepl

6 4.0246 1.128 0.998 1.005 0.9924

10 4.0246 1.117 1.0 1.006 0.994

The chamber signal M is normalized to 22℃ and 1 atmosphere.The chamber signal M is normalized to 22℃ and 1 atmosphere.Ngas, cavity-gas calibration factor. ( L / ρ )air , stopping-power ratio.Pwall , chamber wall correction factor. Pion , ionization recombination

water

correction factor. Prepl , replacement ( gradient ) correction factor.

RESULTS Parameters for calculating the dose to water fromParameters for calculating the dose to water from electron beams for TG-21.

Dwater = M × Ngas × ( L/ρ )air × Pion × Prepl × ( S /ρ ) × φpp water water

t

waterppwater gas ( ρ )air ion repl ( ρ ) φ

Stated energy( MeV )

Ngas( cGy/nC)

( L / ρ )air Pion Preplwaterwater

water water

pppp

( y )6 46.447 1.0840 1.004 0.98758 46.447 1.0700 1.006 0.992010 46 44 1 0 91 1 006 0 99310 46.447 1.0591 1.006 0.993712 46.447 1.0510 1.008 0.995315 46.447 1.0381 1.009 0.997015 46.447 1.0381 1.009 0.997018 46.447 1.0283 1.007 0.9982

Ngas, cavity-gas calibration factor for plane-parallel chamber.pp

Prepl , replacement ( gradient ) correction factor for plane-parallelchamber, derived from TG-39.

pp

RESULTS Parameters for calculating the dose to water from photon beams for TG-51.

C 60Dwater = M raw × Pion × PTP × Pelec × Ppol × kQ × ND,w

Stated energy N

Co-60

Co-60Stated energy

( MV )ND,W

( cGy/nC)kQ Pion Pelec Ppol

6 4 5394 0 993 1 006 1 0 9996 4.5394 0.993 1.006 1 0.999

10 4.5394 0.981 1.007 1 1.0007

M raw , uncorrected ion chamber reading. PTP , Temp./Press. Correction.ND,W , absorbed-dose-to-water calibration factor. kQ , quality

Co-60, , Q , q y

conversion factor. Pelec , electrom. corr factor. Ppol , polarity correction.

RESULTS Parameters for calculating the dose to water fromParameters for calculating the dose to water from electron beams for TG-51.

Dwater = M raw × Pion × PTP × Pelec × Ppol × k'R50 × kecalND,wCo-60

water raw ion TP elec pol R50 ecal D,w

Stated energy( MeV )

kecalND,W( cGy/nC)

Pelec Pion Ppol k'R50Co-60

( ) ( y )6 47.705 1 1.004 0.990 1.03858 47.705 1 1.006 0.992 1.031310 47.705 1 1.006 0.994 1.025012 47.705 1 1.008 0.995 1.019015 47 705 1 1 009 0 997 1 010515 47.705 1 1.009 0.997 1.010518 47.705 1 1.007 0.997 1.0023

kecalND,W , photon-electron conversion factor for p-p chamberCo-60

by using the cross-calibration method. k'R50 , electron qualityconversion factor.

RESULTS Ratios of absorbed dose to water for TG-51 and TG-21 protocols.

Dwater = M raw × PTP × CFwater raw TP C

Stated energy CF ( cGy/nC) variation ( % )

TG-51 TG-21TG 51 TG 216 MV photon beam 4.530 4.519 0.25

10 MV 4.487 4.495 -0.186 l b 49 243 49 918 1 36 MeV electron beam 49.243 49.918 -1.35

8 MeV 49.098 49.597 -1.0110 MeV 48.896 49.175 -0.5712 MeV 48.755 48.975 -0.4515 MeV 48.493 48.505 -0.0218 M V 48 005 48 009 0 0118 MeV 48.005 48.009 -0.01

Discussion and ConclusionThe doses at 10 cm in water for 6 MV and 10 MV photon beamsThe 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 chamberCo-60meantime, 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.

Co 60

Co-60

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 N But it is different in this

Co-60

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.