7 二、192Ir近接治療劑量標準IAEA-TECDOC-1274：Calibration of photon and beta ray sources used in brachytherapyGuidelines on standardized procedures at Secondary Standards Dosimetry Labotatories (SSDLs) and hospitals (March 2002)
8 1.Characterization of brachytherapy source Gamma ray sourcesThe 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 sourcesThe 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.
9 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
10 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).
11 free air chambers for low and medium energy x-ray qualities To obtain the response functions, the chambers were calibrated against primary air kerma standards:free air chambers for low and medium energy x-ray qualitiesgraphite cavity chambers for gamma rays for 137Cs and 60CoISO narrow-spectrum x-ray qualities used for chamber calibrationFree air chamber at INERGraphite cavity chamber at INER
12 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
13 4.HDR 192Ir Source calibration at SSDLs and hospitals 4.1 Free in-air measurementThe 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)2NK： the air kerma calibration factor of the ionization chamberMu： the measured charge collected during the time t and corrected for ambienttemperature and pressure, recombination losses and transit effects duringsource transfer in the case of afterloading systemskair ： the correction for attenuation of the primary photons by the air betweenthe source and the chamberkscatt： the correction for scattered radiation from the walls, floor, measurementset-up, air, etc.kn ： the non-uniformity correction factor, accounting for the non-uniformelectron fluence within the air cavityd ： the measurement distancedref ： the reference distance of 1 m.
14 4.1 Free in-air measurement (con’t) 4.1.1 Ionization chambers to be usedFor 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.
15 4.1.2 Air kerma calibration of ionization chamber (NK) (con’t) 137Cs calibration pointThe 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.where x = (t/ 9.3×1022) for a wall thickness of t electrons/cm2
16 4.1.2 Air kerma calibration of ionization chamber (NK) (con’t) 60Co calibration pointIn 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 equationsandwhere 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
17 Table IV includes Aw factors for different ionization chambers 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
18 4.1.3 Correction factors for free in-air measurements Measurement distancesA 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).
19 4.1.3 Correction factors for free in-air measurements The scatter correction factorTwo methods have been used to determine the scatter correction: the multiple distance method and the shadow shield methodIn 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 + cwhered ： the apparent center-to-center source chamber distance
20 Where M'd is the reading from primary photon only. 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 +MsWhere M'd is the reading from primary photon only.f = M'd (d')2 = (M d -M s)(d+c) 2= constantAny 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.f = M'd (d')2 = (M d -M s)(d+c)(d’)2 dK/dt = (NK) f / dt
21 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.
22 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.
23 4.1.3 Correction factors for free in-air measurements The non-uniformity correction factorIn 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 theshape and dimensions of the ionization chamber (spherical,cylindrical, internal radius and length);measurement distance and the source geometry (‘point source’,line source, etc.);¾material in the inner wall of the chamber;¾energy of the photons emitted from the source.
24 The relationship between the two theories is given by Kn=1/Apn(d)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 w.Table VI the w values for some commonly used inner wall materials.
27 For spherical ionization chambers, w = 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.
28 4.1.3 Correction factors for free in-air measurements Correction for the attenuation of primary photon in airFor 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.
29 4.1.3 Correction factors for free in-air measurements Correction for transit effects, leakage current andrecombination lossWhile 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 aninterval after the source has stopped moving¾ Subtracting two readings taken for differing intervals to eliminatethe transit charge common to both.Using a current reading after the source has stopped moving (if thesignal is large enough).
30 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
31 4.2 Calibration using well type ionization chamber Using well-type chambers, the reference air kerma rate can be calculated generally using thr following expressionKR=NK kp Imax kionwhereNK：the air kerma rate calibration factor(Gy h-1 m2 A-1) taken from thecalibration certificatekp ：the correction factor for the difference between the actual air pressureand temperature and the reference chamber calibration conditionsImax：the maximum measured ionization current value with the well typechamber (including the electrometer calibration factor).kion：the reciprocal of the ion collection efficiency factor Aion calculated asfollowsQ1 and Q2 are the charge readings at nominal (300 V) and half (150 V)potential
32 NK=KR /( kp Imax kion ) 4.3 Calculation of well type ionization chamber calibration factor at INERNK=KR /( kp Imax kion )Mallinckrodt Medical B.V. D35A0832 HDR 192IrKR：47.9 mGy m2 h-1 ( :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 ( :00), 差異2%： ×10-3 d-1Remote afterloader and well type chamber at INER
33 Nucletron afterloader 對PTW HDR well-type chamber Imax設定位置為830 mm
35 4.4 Calibration using well type ionization chamber Quality control of well type chamber measurementThe 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.
36 4.5 HDR 192Ir brachytherapy source traceability Chain （in Taiwan） Primary LaboratoryWell type Chamber(Primary Lab.)Standard ChamberStandard SourceUserWell type Chamber(user)User’s SourceKR.
42 四、遠隔治療劑量標準 1. TG-21 protocol (Med Phys, 1983) TG-21 protocol has two major components: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
43 primary standard (3 cm3 、10 cm3 及30 cm3 spherical chambers) and calibration system of 60Co air kerma
44 Comparison of standard of air kerma for 60Co RNMI, BIPM60Co air kerma rate measurement uncertainty (0.48 %, k=2) at INER
46 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)：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)
47 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 ( from Fig. 7, stopping –power ratio from Fig. 2 or Table IV, energy-absorption coefficient ratio from Table IX)
48 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 denotedThe 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.
49 Primary standard (pancake chamber) and calibration system of absorbed dose to water for 60Co 100 cm5 g/cm2
50 Comparison of standard of absorbed dose to water for 60Co RNMI, BIPMMeasurement uncertainty (0.54 %, k=2) of absorbed dose rate to water for 60Co at INER
51 Uncertainty evaluation of calibration factor ( ) of absorbed dose to water for 60CoAbsorbed dose to water calibration factor ( ) uncertainty (0.6 %, k=2)INER calibration certificateTG (Gy C-1)
52 TG-51 protocol Requires absorbed dose to water calibration factors, Conceptually easier to understand and simpler to implementRequires a quality conversion factor, kQ ,change in modality, energy, gradient
53 TG-51 protocol kQ＝Quality conversion factor(kQ=1 for 60Co) Pion = Ion-recombination correction factorPTP = Temperature-pressure correction factorPelec = electrometer correction factorPpol =polarity correction factor of ion chamberMraw = uncorrection ion chamber reading
54 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：成果： accepted by Radiation Measurements SCI Journal
55 The characteristics of farmer-type chambers used in this comparison Cylindrical chamber waterproof sleeves made in INERNE-2561(NE-2611，0.3 cm3)NE-2571 (0.6 cm3)PTW-30001(0.6 cm3) The air gaps between waterproof sleeve and chamber wall<2 mm(AAPM TG-51) PMMA material and thickness<1 mm
56 The distribution of /NK calibration coefficients ratios for the cylindrical ionization chambers ND,w/NK± 0.006
57 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-21TG-51Step 1：User chamber calibration factorCombined uncertaintyNgas0.80.6Step 2：User beam mesurementsCombined uncertainty in dosimeter reading a0.9Step 3a：Quantities and perturbation factors for the user beamCombined uncertainty in stopping power ratios, perturbation factors and their assignment to beam qualityStep 3b：Beam quality correction kQ1.0Combined uncertainty in1.51.4aIncludes long-term stability of the dosimeter, establishment of reference conditions, measurement of beam quality , and dosimeter reading relative to timer or beam monitor
58 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
59 Quality assurance for the switch of photon reference dosimetry between TG-21 and TG-51 protocols