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推動醫療曝露品質保證作業研討會

李 振 弘

國家游離輻射標準實驗室

原子能委員會 核能研究所

http://nrsl.iner.gov.tw/ 台灣地區遠隔治療及192Ir近接治療劑量標準之校正追溯

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Contents

一、游離輻射標準現況說明

二、192Ir近接治療劑量標準

三、校正服務

四、遠隔治療劑量標準

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一、游離輻射標準現況說明

台灣地區游離輻射領域追溯體系

國際度量衡局(BIPM)

國家游離輻射標準實驗室(NRSL)

中華民國實驗室認証(CNLA)認可的實驗室

儀器或射源使用者

其他國家的標準實驗室(NMIs)

追溯 (醫療曝露)

追溯

追溯

追溯 追溯 比對

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1993-2004建立之游離輻射標準及應用領域 應用領域 量測標準 放射診斷 放射治療 核子醫學

實驗室認證

相互認可協定 國際趨勢

10~50 kV X射線空氣克馬 V V V V

100~250 kV X射線空氣克馬 V V V V V

137Cs 加馬射線空氣克馬 V V V

60Co 加馬射線空氣克馬 V V V V

60Co 加馬射線水吸收劑量 V V V

192Ir 參考空氣克馬 V V V

90Sr/90Y 組織吸收劑量 V V

252Cf 周圍/個人等效劑量 V V

241Am-Be 周圍/個人等效劑量 V V

4πβ-γ 符合計測(活度) V V V V

4πγ 游離腔(活度) V V V V

2πα/β 比例計數器(發射率) V V V

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15 MeV

100 keV

200 keV

300 keV

光子標準

1.25 MeV

高能量

中能量

低能量

2 MeV

662 keV

50 keV

10 keV

線性加速器 TG21、TG51

60Co 60Co 空氣克馬 水吸收劑量

137Cs空氣克馬

100-250 kV空氣克馬(BIPM)

10-50 kV空氣克馬(BIPM)

線性加速器(熱卡計)

192Ir參考空氣克馬(近接治療)

20-240 kV空氣克馬(ISO/low)

10-50 kV空氣克馬(BIPM)

23-35 kV空氣克馬(乳房攝影)追溯NIST

(95-96)

(95-97)

(98以後)

(95-97)

已建立 待建立 發展中

工業用高劑量標準(化學劑量計)

電腦斷層掃瞄(CT)X光劑量 (96)

血管X光攝影劑量 (97) 數位式及胸部X光劑量 (98)

192Ir參考空氣克馬(近接治療)追溯PTB

30-300 kV空氣克馬(ISO/narrow) (94-95)

125I, 103Pd參考空氣克馬(近接治療)

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χ Radiotherapy

External radiotherapy

遠隔治療

Brachytherapy

近接治療

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二、192Ir近接治療劑量標準

IAEA-TECDOC-1274：Calibration of photon and beta ray sources used in brachytherapy

Guidelines on standardized procedures at Secondary Standards Dosimetry

Labotatories (SSDLs) and hospitals (March 2002)

http://www-pub.iaea.org/MTCD/publications/PDF/te_1274_prn.pdf

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1.Characterization of brachytherapy source

Gamma ray sources The recommended quantity for the specification of the gamma sources is the reference air kerma rate, defined by the ICRU report 38, 58, 65 as the kerma rate to air, in air, at a reference distance of one meter, corrected for air attenuation and scattering.

Beta ray sources The recommended quantity for specification of beta ray sources is the reference absorbed dose rate in water at a reference distance from the source. The reference distance differs from one type of source to another.

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安裝射源（個） 國內後荷式近接治療

系統數量（台） HDR 192Ir HDR 60Co LDR 137Cs 51 41 7 3

2.Specification of brachytherapy sources used in Taiwan

ICRU 38 notes that conventional dose rate where the prescribed dose at the point of dose prescription

“Low” dose rate (LDR) : 0.40 ∼2.0 Gy h-1

“High” dose rate (HDR) ≥ 12.0 Gy h-1

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3. High dose rate 192Ir Source calibration at PDSL •Primary standards for HDR 192Ir sources are not available. •PTB/Germany comprises the evaluation of the entire calibration function of the ionization chamber between 30 keV and 60Co radiation, and a subsequent interpolation for the 192Ir emission lines weighted with their emission probability.

• The overall relative uncertainty is 2.5% (k=2).

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To obtain the response functions, the chambers were calibrated against primary air kerma standards: •free air chambers for low and medium energy x-ray qualities • graphite cavity chambers for gamma rays for 137Cs and 60Co

ISO narrow-spectrum x-ray qualities used for chamber calibration

Free air chamber at INER

kVp Filtration (mm) Ē (keV) HVL (mm) 40 0.21 Cu 33 0.084 Cu 60 0.6 Cu 48 0.24 Cu 80 2.0 Cu 65 0.58 Cu 100 5.0 Cu 83 1.11 Cu 120 1.0 Sn + 5.0 Cu 100 1.71 Cu 150 2.5 Sn 118 2.36 Cu 200 1.0 Pb +3.0 Sn + 3.0 Cu 164 3.99 Cu 250 3.0 Pb +2.0 Sn 208 5.19 Cu 300 5.0 Pb +3.0 Sn 250 6.12 Cu

Graphite cavity chamber at INER

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The set-up for the air kerma calibration for HDR 192Ir sources at 1 m

(1)Without a shadow-shield：primary + room scattered + environment radiation (2)With a shadow-shield：only room scattered + environment radiation (1)-(2), corrected for attenuation in air ⇒ air kerma rate in vacuum which is identical to the reference air kerma rate in the case of measurements at 1 m dtstance

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4.HDR 192Ir Source calibration at SSDLs and hospitals

4.1 Free in-air measurement The reference air kerma rate, KR, may be determined from measurements made free in-air using the equation:

KR=NK·(Mu/t)·kair·kscatt·kn·(d/dref)2 NK： the air kerma calibration factor of the ionization chamber Mu： the measured charge collected during the time t and corrected for ambient temperature and pressure, recombination losses and transit effects during source transfer in the case of afterloading systems kair ： the correction for attenuation of the primary photons by the air between the source and the chamber kscatt： the correction for scattered radiation from the walls, floor, measurement set-up, air, etc. kn ： the non-uniformity correction factor, accounting for the non-uniform electron fluence within the air cavity d ： the measurement distance dref ： the reference distance of 1 m.

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4.1 Free in-air measurement (con’t) 4.1.1 Ionization chambers to be used

For HDR sources, ionization chambers with volumes greater than 0.5 cm3 can be used (e.g. Farmer 0.6 cm3 chamber). For 192Ir calibrations, it is recommended to use chambers that have a variation of the air kerma calibration factor of less than 5% between 60Co and 60 keV. 4.1.2 Air kerma calibration of ionization chamber (NK)

The air kerma weighted average energy of an 192Ir brachytherapy source is 397 keV, the principle proposed by Goetsch is to calibrate the chamber at 250 kV x-ray quality and at 137Cs, or at 60Co if a 137Cs beam is not available. A typical X ray beam that can be used for calibration at the SSDLs is 250 kV, added filtration of 1.0 mm Al and 1.65 mm Cu, and a HVL of 2.50 mm Cu.

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•137Cs calibration point

The ionization chamber wall (inner wall and cap) must be thick enough (0.36 g/cm2 ) to block all electrons emanating from the source or capsule, and to provide CPE in the 137Cs beam.

Air kerma calibration factors, NK, for both the 137Cs and x-ray beam must be determined with the build up cap (equivalent wall) in place for both beams. The factor Aw for the attenuation of the cap and scattering effects of the chamber wall must be taken into account.

4.1.2 Air kerma calibration of ionization chamber (NK) (con’t)

where x = 0.0037(t/ 9.3×1022) for a wall thickness of t electrons/cm2

2/][ ,,250,250,,, CsKCswkVKkVwIrKIrw NANANA +=

2/])[1( ,250,, CsKkVKIrK NNxN ++=

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4.1.2 Air kerma calibration of ionization chamber (NK) (con’t)

•60Co calibration point

In the event that there is no 137Cs beam energy at the SSDL, a 60Co beam may be used as the high energy point using the appropriate build up cap and wall thickness for 60Co, 0.5g/cm2. The weighted interpolation factors are given by the following equations

where and are the air kerma weighted average energies of 192Ir and 60Co gamma rays, respectively, and represents the effective energy (131 keV) of the 250 kV x-ray beam. This results in the following equation for NK,Ir with the weighted air kerma values

8.0250

250, =−−

=kVCo

CoIr

kVw hhhhfυυυυ

2.0250

250

, =−−

=kVCo

kVIr

Cow hhhhf

υυυυand

IrwCoKCowkVKkVwIrK ANANAN ,,,250,250,, /)2.08.0( ⋅+⋅=

Irhυ CohυkVh 250υ

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Table IV includes Aw factors for different ionization chambers. If the chamber in use is not listed in the table, then Aw can be set to 1.000

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4.1.3 Correction factors for free in-air measurements

•Measurement distances

A practical criteria is that the distance between the chamber center and the center of the source must be at least 10 times the length of the source in order to ensure that the error introduced due to the point source approximation is less than 0.1%. For HDR source calibrations, the measurement distances can be selected around the optimum distance (e.g. between 10 cm and 40 cm).

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4.1.3 Correction factors for free in-air measurements

•The scatter correction factor

Two methods have been used to determine the scatter correction: the multiple distance method and the shadow shield method

In the former method, the air kerma rate due to scattered radiation is assumed to be constant over the measurement distances.

It is essential in this method that the changes in distance be precise and accurate, in order to derive the correction c that yields the “true” center-to-center source to chamber distances, d'.

d' = d + c

where

d ： the apparent center-to-center source chamber distance

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距離平方反比方法量測HDR 192Ir射源之參考數據 ( Steven J. Goetsch)

d (cm) c(cm) d'(cm) M d(nC) M s(nC) M s% M' d(nC) f (nC cm2) 10.00 -0.296 9.704 9.5242 0.0146 0.15 9.5096 895.5 15.00 -0.296 14.704 4.1532 0.0146 0.35 4.1386 894.8 20.00 -0.296 19.704 2.3228 0.0146 0.63 2.3082 896.2 25.00 -0.296 24.704 1.4816 0.0146 0.99 1.4670 895.3 30.00 -0.296 29.704 1.0293 0.0146 1.42 1.0147 895.3 35.00 -0.296 34.704 0.7580 0.0146 1.93 0.7434 895.3 39.64 -0.296 39.344 0.5921 0.0146 2.47 0.5775 893.9

f = M'd (d')2 = (M d -M s)(d+c) (d’)2 dK/dt = (NK) f / dt

The charge readings after application of the corrections discussed above are denoted here as Md referring to the nominal distances d, A constant reading Ms due to scattering is included in each reading：

Md = M'd +Ms

Where M'd is the reading from primary photon only. f = M'd (d')2 = (M d -M s)(d+c) 2= constant Any group three such equations, for three distance, can be solved for three unknowns Ms, c, and f. Making measurements at more than three distances overdetermines that result, and averaging of several solutions to minimize the error.

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The shadow shield method has mainly been used to determine the scatter correction factor at a distance of 1 m., a cone of a high Z material is placed between the source and the chamber in order to prevent the primary photons from reaching the chamber. The ratio of the measured charge with and without the shield in place can be used to calculate the scatter correction factor.

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Table V shows the results of a few experimental determinations of the scatter correction using the shadow shield method. In 192Ir dosimetry it has been shown that the scatter correction factors obtained with the two methods are in a good agreement.

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4.1.3 Correction factors for free in-air measurements

•The non-uniformity correction factor

In the measurements of brachytherapy sources free in-air, the non-collimated geometry, with high divergence of the incident photons, Due to the non-uniform photon fluence over the wall, the generation of electrons from the wall varies significantly from place to place in the wall. The net result of this is a non-uniform electron fluence in the air cavity of the chamber. The non-uniformity correction factor, kn depends on the shape and dimensions of the ionization chamber (spherical, cylindrical, internal radius and length); measurement distance and the source geometry (‘point source’, line source, etc.); material in the inner wall of the chamber; energy of the photons emitted from the source.

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is the non-uniformity correction factor obtained from the isotropic theory and is the non-uniformity correction factor according to the anisotropic theory. A'pn(d) takes into account the anisotropic electron fluence within the air cavity and the degree of anisotropy is given by the energy and material dependent factor ω.

Table VI the ω values for some commonly used inner wall materials.

The relationship between the two theories is given by

)()()( dAdAdA pn

KR

pnPn′+= ω

Kn=1/Apn(d)

)(/1 dAKR

pn

)(/1 dApn′

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For spherical ionization chambers, ω = 0, and the non-uniformity correction factors given by isotropic theory can be directly applied. The Apn(d ) factors for spherical chambers are reproduced in Table X.

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4.1.3 Correction factors for free in-air measurements

•Correction for the attenuation of primary photon in air

For determination of the reference air kerma rate from the measured air kerma at the distance d, it is necessary to correct for the attenuation of the primary photons between the source and the ionization chamber. Table XI gives the kair correction factors at different distances between the source and the ionization chamber.

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4.1.3 Correction factors for free in-air measurements

•Correction for transit effects, leakage current and recombination loss While the source moves into the measurement position, and then away after the measurement, the detector measures a signal, referred to as the transit signal. This transit signal strongly depends on the source-to-detector distance, and is significant at the distances used in calibration. Several techniques can be used to eliminate the transit component of the signal: Using an externally-triggered electrometer to collect charge during an interval after the source has stopped moving Subtracting two readings taken for differing intervals to eliminate the transit charge common to both. Using a current reading after the source has stopped moving (if the signal is large enough).

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The importance of electrical leakage currents in the individual dosimetry system should be evaluated since the signal levels measured during calibration are typically 50 to 100 times less than usually encountered in teletherapy measurements. This can be significant for most thimble or Farmer type ionization chambers. Generally if the leakage is greater than 0.1% of the signal, it should be taken into account.

A correction is also needed for the recombination losses and for the ambient temperature and Pressure.

Final ⇒ The reference air kerma rate, KR, may be determined from measurements made free in-air using the equation:

KR=NK·(Mu/t)·kair·kscatt·kn·(d/dref)2

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Using well-type chambers, the reference air kerma rate can be calculated generally using thr following expression

KR=NK⋅ kp ⋅Imax ⋅kion

where

NK：the air kerma rate calibration factor(Gy h-1 m2 A-1) taken from the calibration certificate kp ：the correction factor for the difference between the actual air pressure and temperature and the reference chamber calibration conditions Imax：the maximum measured ionization current value with the well type chamber (including the electrometer calibration factor). kion：the reciprocal of the ion collection efficiency factor Aion calculated as follows Q1 and Q2 are the charge readings at nominal (300 V) and half (150 V) potential

4.2 Calibration using well type ionization chamber

)31(

34

2

1

QQAion ⋅−=

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4.3 Calculation of well type ionization chamber calibration factor at INER

NK=KR /( kp ⋅Imax ⋅kion ) Mallinckrodt Medical B.V. D35A0832 HDR 192Ir

KR：47.9 mGy m2 h-1 (2004.2.6 00:00追溯至德國PTB, 不確定度

2.5%, k=2)⇒原廠測試報告48.85 mGy m2 h-1 (2004.2.6 00:00), 差異≈2%

λ：9.38829×10-3 d-1

Remote afterloader and well type chamber at INER

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Sensitivity curve for PTW well-type chamber

0.50000.55000.60000.65000.70000.75000.80000.85000.90000.95001.00001.0500

0 10 20 30 40 50 60 70 80 90 100 110

Distance from chamber buttom (mm)

Rel

ativ

e io

niza

tion

curre

n

0.999

1.000

1.001

1.002

1.003

1.004

1.005

1.006

1.007

1.008

1.009

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Distance from room wall (mm)

Rel

ativ

e re

adin

gNucletron afterloader 對PTW HDR well-type chamber Imax 設定位置為830 mm

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8.08.18.28.38.48.58.68.78.88.99.09.19.29.39.49.59.69.79.89.9

10.0

Wel

l-typ

e ch

ambe

r 校正因子

(105 G

y m

2 h-1

A-1

)

PTW TN33004Nucletron 077094Nucletron 077091

核能研究所 A B C D量測不確定度3% (k=2)

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4.4 Calibration using well type ionization chamber

Quality control of well type chamber measurement

The complicated energy spectrum of HDR 192Ir includes about 40 energies falling approximately between 50 keV and 700 keV and with an average energy of 397 keV. In practice, it is possible to use 241Am (average energy 60 keV) and 137Cs (average energy 661 keV) check sources for this purpose. The stability of the output of a well type chamber should be check at least 4 times per year. If the periodic constancy checks remain the same to within ±1 %, it can be assumed that the calibration factor for response of the chamber does not change significantly with time at these energies, it may be concluded that the chamber’s calibration factor for HDR 192Ir sources has not changed. Further, if chambers is used for 192Ir source calibrations, the re-calibration interval should be shortened to 2 years.

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4.5 HDR 192Ir brachytherapy source traceability Chain （in Taiwan）

Standard Chamber

Standard Source

User’s Source

Well type Chamber (Primary Lab.)

Well type Chamber

(user)

Primary Laboratory

User

KR

.

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三、校正服務

或

國家游離輻射標準實驗室網站新功能 ⇒儀器校正進度查詢

查詢 : 校正進度

羅東博愛醫院核子醫學科

儀器序號 :

查詢送件(游校)日期自

西元年月日(格式 yyyymmdd, 例如: 20050117)

起之進度

提交 重新設定

http://nrsl.iner.gov.tw/ 點選中文/校正服務/儀器校正進度查詢 或 http://nrsl.iner.gov.tw/CalRec-search.asp

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送校件 送校方式 校正時程 備註 遠隔治療劑量校正追溯

Cavity chamber

客戶自送或 委託廠商送達

按現行作業流程 已開放校正服務

192Ir近接治療劑量校正追溯

Well-type chamber (優先度1)

客戶自送委託廠商送校

(1)兩週前預約，3工作天完成

(2)無預約，接收日起7工作天完成

(3)校正完成後5工作天內寄發校正報告

(1)94.7.1開放校正服務

(2)如有急需可先傳真報告

(3)提供現場游校

192 Ir近接治療射源

(1)廠商送校 (2)現場游校

(1)兩週前預約，當日完成 (2)校正完成後3工作天寄發

校正報告

(1)94.7.1開放校正服務

(2)如有急需可先傳真報告

送校方式與校正時程

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游離腔及192 Ir射源校正費用 •加馬射線空氣克馬：基本費新台幣八千元/部

(能量範圍137Cs、60Co，每增加一能量點加新台幣二千元)

•X射線空氣克馬：基本費新臺幣八千元/部（能量範圍30

~250 kV，每增加一能量點加新臺幣二千元）

•60Co水吸收劑量：新臺幣八千元/部

•Well-type chamber：預估新臺幣一萬五千元/部

•192Ir射源：預估新臺幣一萬元/個

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92及93年每月校正數量統計

校正季規劃 ⇒提昇實驗室人力運用與客戶送校儀器

之校正效率

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校正季規劃實施方式： 1.每年7-9月開放3個月提供HDR well-type chamber校正。 2.每年7-12月提供cavity chamber校正。 3.研討會歸納客戶意見。 4.以e-mail及書面方式告知。 6.於實驗室對外網站公告。 7.非校正季時，假如仍有客戶送件， •請櫃台告知新措施 •收件後校正期程可能延長

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1. TG-21 protocol (Med Phys, 1983)

Part I: How to obtain the calibration factors Nx (exposure) and Ngas(dose to cavity gas) in a Co-60 beam.

if only Nx is provided by the standard laboratory, then the user needs to convert Nx to Ngas.

Part II: How to use Ngas in a user’s beam, which can be any modalities (photon or electron beams) and energies, and the chamber can be placed in a plastic medium.

Dose to cavity gas → dose to medium → dose to water

TG-21 protocol has two major components:

四、遠隔治療劑量標準

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primary standard (3 cm3 、 10 cm3 及 30 cm3

spherical chambers) and calibration system of 60Co air kerma

stemPThatcepionki KKKKKK ×××××=π

ki

wall

air

air

wall

air Lge

Wv

QK πρρ×

×

×

−××

×= ρ

μ1

1 en

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Comparison of standard of air kerma for 60Co R N

MI,

BIPM

0.97

0.98

0.99

1.00

1.01

1.02

1.03

ENEA ARPANSA NIST NRC INER AIST

60Co air kerma rate measurement uncertainty (0.48 %, k=2) at INER

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分析項目(Source component) Type A (%) Type B(%)

空氣克馬率量測(air kerma rate) 0.14 0.20 電流量測(current) 0.13 0.045 溫度修正(temperature) 0.025 氣壓修正(pressure) 0.004 距離(distance) 0.05 平方和開根號(quadratic sum) 0.20 0.21 組合標準不確定度(combined uncertainty) 0.29 擴充不確定度(expanded uncertainty, k=2) 0.58

60Co air kerma calibration factor (NK) uncertainty evaluation

INER calibration certificate ⇒NK(Gy C-1)及Aion((4-Q1/Q2)/3) ⇒NX = NK/8.792×10-3(Gy C-1) ⇒TG-21 NX(R C-1)

Air kerma calibration factor (NK) uncertainty (0.6 %, k=2)

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TG-21 protocol (Part I - 60Co beam)

Ngas：Cavity –gas calibration factor (Gy C-1)

NX：Exposure calibration factor (R C-1) ⇒(INER calibration certificate)

k：2.58 × 10-4 (C kg-1 R-1)

W/e：33.97 (J C-1)

Aion：Ion-collection efficiency in the user’s chamber at 60Co exposure ⇒(Calibration certificate)

Awall： Wall correction factor in the user’s chamber at 60Co exposure (Table II or III)

α：Faction of ionization due to electrons from chamber wall (Fig. 1)

：Stopping –power ratio(wall/air, cap/air) (Table I)

：Energy-absorption coefficient ratio(air/wall, air/cap) (Table I)

)/( ρL

ρµ /en

])/()/)(1()/()/([)/(

aircaplen

capair

airwallen

wallair

wallionxgas LL

AAeWkNNρµραρµρα −+

=

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wallreplionwaterairgaswater PPP)

ρL(MND =

TG-21 protocol (Part II - user’s beam)

M：Ion chamber reading in user’s beam (correction for polarity , electrometer, temperature and pressure)

Pion：Ion-recombination correction factor applicable to the calibration o f user’s beam (Fig. 4)

Prepl：A factor that corrects for replacement of water phantom by an ionization chamber (Fig. 5)

Pwall：Wall correction factor at user’s beam ( α from Fig. 7, stopping –power ratio from Fig. 2 or Table IV, energy-absorption coefficient ratio from Table IX)

waterair

waterair

waterwallen

wallair

wall LLLP

)/()/)(1()/()/(

ρραρµρα −+

=

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2. TG-51 protocol (Med Phys 26, 1847-70, 1999 )

The TG-51 protocol is based on “absorbed dose to water” calibration (also in a Co-60 beam)

The chamber calibration factor is denoted

The calibrated chamber can be used in any beam modality (photon or electron beams) and any energy, in water.

The formalism is simpler than the TG-21, but it is applicable in water only.

Cow,DN 60

49 NRSL

100 cm

5 g/cm2

Primary standard (pancake chamber) and calibration system of absorbed dose to water for 60Co

hrnpfpss

cwcwcenwencavacwater

kkkkkkseWmQD

×××××

+××××××=−−−

,,, )1(])//()/[()/()/( εψρµρµ

50 NRSL

Comparison of standard of absorbed dose to water for 60Co R N

MI,

BIPM

0.97

0.98

0.99

1.00

1.01

1.02

1.03Fricke dosimetryGraphite calorimetryWater calorimetryIonometric dosimetry

PTB ARPANSA NIST NRC OMH INER

Measurement uncertainty (0.54 %, k=2) of absorbed dose rate to water for 60Co at INER

51 NRSL

分析項目(Source component) Type A (%) Type B(%)

空氣克馬率量測(air kerma rate) 0.066 0.255 電流量測(current) 0.13 0.045 溫度修正(temperature) 0.025 氣壓修正(pressure) 0.004 距離(distance) 0.05 平方和開根號(quadratic sum) 0.16 0.26 組合標準不確定度(combined uncertainty) 0.30 擴充不確定度(expanded uncertainty, k=2) 0.60

Uncertainty evaluation of calibration factor ( ) of absorbed dose to water for 60Co

CowDN

60

,

Absorbed dose to water calibration factor ( ) uncertainty (0.6 %, k=2)

CowDN

60

,

INER calibration certificate

⇒TG-51 (Gy C-1) CowDN

60

,

52 NRSL

TG-51 protocol

Requires absorbed dose to water calibration factors, Conceptually easier to understand and simpler to implement

Requires a quality conversion factor, kQ ,change in modality, energy, gradient

N CowD,

60

CowD,Q

Qw

60NMkD =

rawpolelecTPion MPPPPM =

53 NRSL

Cow,DQ

Qw NkMD 60

=

TG-51 protocol

rawpolelecTPion MPPPPM =

kQ＝Quality conversion factor(kQ=1 for 60Co)

Pion = Ion-recombination correction factor

PTP = Temperature-pressure correction factor

Pelec = electrometer correction factor

Ppol =polarity correction factor of ion chamber

Mraw = uncorrection ion chamber reading

54 NRSL

3.Comparison results for TG-21 and TG-51 dosimetry protocols

參與單位：核能研究所國家游離輻射標準實驗室 醫學物理學會 國內13家醫院放射腫瘤科 (北：7, 中：3, 南：2, 東：1) 時間：2002.7.1-2003.8.30 成果：2005.1.20 accepted by Radiation Measurements

SCI Journal

55 NRSL

The characteristics of farmer-type chambers used in this comparison

Chamber type Cavity volume (cm3)

Cavity length (mm)

Cavity inside diameter (mm)

Wall material Wall thickness (mg cm-2)

Buildup cap material

Buildup cap thickness (g cm-2)

Waterproof

Wellhöfer FC65-P 0.67 23 6.2 Delrin 56 POM 0.560 Yes

NE 2571 0.69 24 6.3 Graphite 65 Delrin 0.551 No

PTW 30001 0.60 23 6.1 Acrylic/Graphite 60 PMMA 0.541 No

PTW 30010 0.60 23 6.1 Acrylic/Graphite 57 PMMA 0.541 No

PTW 30006 0.60 23 6.1 Acrylic /Graphite 49 PMMA 0.541 Yes

⇒ The air gaps between waterproof sleeve and chamber wall<2 mm(AAPM TG-51)

⇒ PMMA material and thickness<1 mm

⇒NE-2561(NE-2611，0.3 cm3) NE-2571 (0.6 cm3) PTW-30001(0.6 cm3)

Cylindrical chamber waterproof sleeves made in INER

56 NRSL

1.080

1.090

1.100

1.110

1.120PTW 30006

PTW 30001

Wellhöfer FC65-P

NE 2571

PTW 30010

B C D E F G H I J K L MA

± 0.006

ND

,w/N

K

The distribution of /NK calibration coefficients ratios for the cylindrical ionization chambers

CowDN

60

,

57 NRSL

Estimated relative standard uncertainty (%) in the determination of absorbed dose to water at the reference depth in high energy photon beams using the TG-21 NX-Ngas and TG-51 ND,w formalisms (Huq and Andreo, 2001)

TG-21 TG-51

Step 1：User chamber calibration factor Combined uncertainty

Ngas

0.8

0.6

Step 2：User beam mesurements Combined uncertainty in dosimeter reading a

0.9

Step 3a：Quantities and perturbation factors for the user beam Combined uncertainty in stopping power ratios, perturbation factors and their assignment to beam quality

0.9

Step 3b：Beam quality correction kQ 1.0

Combined uncertainty in 1.5 1.4 CowD60

CowDN

60

,

aIncludes long-term stability of the dosimeter, establishment of reference conditions, measurement of beam quality , and dosimeter reading relative to timer or beam monitor

58 NRSL

6 MV beam quality 10 MV beam quality

TG-21 TG-51 TG-21 TG-51 Accelerator type

Participant code

TPR20,10 wallreplaw PPL ,)/( ρ %dd(10)X kQ TG-51/TG-21

TPR20,10 wallreplaw PPL ,)/( ρ %dd(10)X kQ TG-51/TG-21

A 0.674 1.119 67.2 0.990 1.002 0.736 1.112 74.0 0.979 0.997 B 0.681 1.113 68.1 0.989 1.014 0.732 1.115 73.7 0.979 1.002 C 0.670 1.121 66.6 0.991 1.005 0.733 1.114 73.5 0.980 1.002 D 0.678 1.113 67.4 0.993 1.010 0.729 1.110 73.1 0.985 1.005

Elekta Precise

E 0.701 1.117 68.5 0.988 1.008 0.734 1.113 73.9 0.979 1.002

F 0.665 1.119 65.9 0.992 1.013 0.738 1.107 72.8 0.981 1.012 G 0.678 1.121 67.0 0.990 1.000 0.750 1.108 74.0 0.979 1.001 H 0.677 1.115 67.1 0.990 1.005 0.734 1.107 73.8 0.980 1.002 I 0.670 1.114 67.5 0.990 1.014 0.742 1.107 73.9 0.979 1.012

Siemens Primus

J 0.675 1.114 67.1 0.993 1.011 0.746 1.111 74.9 0.982 1.003

K 0.663 1.116 66.4 0.991 1.004 0.737 1.108 73.2 0.980 1.000 L 0.675 1.113 67.3 0.990 1.013 0.735 1.108 73.5 0.980 1.007 Varian

2100EX M 0.668 1.113 65.8 0.992 1.010 0.737 1.109 72.8 0.981 1.003

Comparison results of absorbed dose to water for 6 MV and 10 MV photon beams determined by following the recommendations of the TG-21 and TG-51 protocols

gas

CowD

wallreplaw

Q

TGQw

TGQw

NN

PPLk

DD

60

,

,21

51

)/()()(

×=−

−

ρ

59 NRSL

Ionization chambers calibrated by INER

Determination of CowDN

60

, and NK calibration coefficients

Within the range of 1.100 ± 0.006

Re-calibration

No Yes

Chamber calibration verification

AAPM TG-21

Comparison of clinical photon reference dosimetry for different protocols

Comparison of absorbed dose to water calibrations

Difference ≤ 1.5%

No Pass

Yes

Calculation of CowDN

60

, /NK ratio

AAPM TG-51

Review and redo

Quality assurance for the switch of photon reference dosimetry between TG-21 and TG-51 protocols

60 NRSL