7 NRSL 192 Ir 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)
8 NRSL 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.
9 NRSL 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) : Gy h -1 High dose rate (HDR) 12.0 Gy h -1
10 NRSL 3. High dose rate 192 Ir Source calibration at PDSL Primary standards for HDR 192 Ir sources are not available. PTB/Germany comprises the evaluation of the entire calibration function of the ionization chamber between 30 keV and 60 Co radiation, and a subsequent interpolation for the 192 Ir emission lines weighted with their emission probability. The overall relative uncertainty is 2.5% (k=2).
11 NRSL 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 137 Cs and 60 Co ISO narrow-spectrum x-ray qualities used for chamber calibration Free air chamber at INER Graphite cavity chamber at INER
12 NRSL The set-up for the air kerma calibration for HDR 192 Ir 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 NRSL 4.HDR 192 Ir Source calibration at SSDLs and hospitals 4.1 Free in-air measurement The reference air kerma rate, K R, may be determined from measurements made free in-air using the equation: K R =N K ·(M u /t)·k air ·k scatt ·k n ·(d/d ref ) 2 N K the air kerma calibration factor of the ionization chamber M u 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 k air the correction for attenuation of the primary photons by the air between the source and the chamber k scatt the correction for scattered radiation from the walls, floor, measurement set-up, air, etc. k n the non-uniformity correction factor, accounting for the non-uniform electron fluence within the air cavity d the measurement distance d ref the reference distance of 1 m.
14 NRSL 4.1 Free in-air measurement (cont) Ionization chambers to be used For HDR sources, ionization chambers with volumes greater than 0.5 cm 3 can be used (e.g. Farmer 0.6 cm 3 chamber). For 192 Ir calibrations, it is recommended to use chambers that have a variation of the air kerma calibration factor of less than 5% between 60 Co and 60 keV Air kerma calibration of ionization chamber (N K ) The air kerma weighted average energy of an 192 Ir brachytherapy source is 397 keV, the principle proposed by Goetsch is to calibrate the chamber at 250 kV x-ray quality and at 137 Cs, or at 60 Co if a 137 Cs 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 NRSL 137 Cs calibration point The ionization chamber wall (inner wall and cap) must be thick enough (0.36 g/cm 2 ) to block all electrons emanating from the source or capsule, and to provide CPE in the 137 Cs beam. Air kerma calibration factors, N K, for both the 137 Cs and x-ray beam must be determined with the build up cap (equivalent wall) in place for both beams. The factor A w for the attenuation of the cap and scattering effects of the chamber wall must be taken into account Air kerma calibration of ionization chamber (N K ) (cont) where x = (t/ 9.3×10 22 ) for a wall thickness of t electrons/cm 2
16 NRSL Air kerma calibration of ionization chamber (N K ) (cont) 60 Co calibration point In the event that there is no 137 Cs beam energy at the SSDL, a 60 Co beam may be used as the high energy point using the appropriate build up cap and wall thickness for 60 Co, 0.5g/cm 2. The weighted interpolation factors are given by the following equations where and are the air kerma weighted average energies of 192 Ir and 60 Co gamma rays, respectively, and represents the effective energy (131 keV) of the 250 kV x-ray beam. This results in the following equation for N K,Ir with the weighted air kerma values and
17 NRSL Table IV includes A w factors for different ionization chambers. If the chamber in use is not listed in the table, then A w can be set to 1.000
18 NRSL 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).
19 NRSL 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
20 NRSL f = M ' d (d ' ) 2 = (M d -M s )(d+c) (d) 2 dK/dt = (N K ) f / dt The charge readings after application of the corrections discussed above are denoted here as M d referring to the nominal distances d, A constant reading M s due to scattering is included in each reading M d = M ' d +M s 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 M s, c, and f. Making measurements at more than three distances overdetermines that result, and averaging of several solutions to minimize the error.
21 NRSL 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 NRSL Table V shows the results of a few experimental determinations of the scatter correction using the shadow shield method. In 192 Ir dosimetry it has been shown that the scatter correction factors obtained with the two methods are in a good agreement.
23 NRSL 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, k n 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.
24 NRSL 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. Table VI the values for some commonly used inner wall materials. The relationship between the two theories is given by K n =1/A pn (d)
27 NRSL For spherical ionization chambers, = 0, and the non- uniformity correction factors given by isotropic theory can be directly applied. The A pn (d ) factors for spherical chambers are reproduced in Table X.
28 NRSL 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 k air correction factors at different distances between the source and the ionization chamber.
29 NRSL 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).
30 NRSL 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, K R, may be determined from measurements made free in-air using the equation: K R =N K ·(M u /t)·k air ·k scatt ·k n ·(d/d ref ) 2
31 NRSL Using well-type chambers, the reference air kerma rate can be calculated generally using thr following expression K R =N K k p I max k ion where N K the air kerma rate calibration factor(Gy h -1 m 2 A -1 ) taken from the calibration certificate k p the correction factor for the difference between the actual air pressure and temperature and the reference chamber calibration conditions I max the maximum measured ionization current value with the well type chamber (including the electrometer calibration factor). k ion 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
32 NRSL 4.3 Calculation of well type ionization chamber calibration factor at INER N K =K R /( k p I max k ion ) Mallinckrodt Medical B.V. D35A0832 HDR 192 Ir K R 47.9 mGy m 2 h -1 ( :00 PTB, 2.5%, k=2) mGy m 2 h -1 ( :00), 2% ×10 -3 d -1 Remote afterloader and well type chamber at INER
33 NRSL Nucletron afterloader PTW HDR well-type chamber I max 830 mm
34 NRSL 3% (k=2)
35 NRSL 4.4 Calibration using well type ionization chamber Quality control of well type chamber measurement The complicated energy spectrum of HDR 192 Ir 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 241 Am (average energy 60 keV) and 137 Cs (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 chambers calibration factor for HDR 192 Ir sources has not changed. Further, if chambers is used for 192 Ir source calibrations, the re-calibration interval should be shortened to 2 years.
36 NRSL 4.5 HDR 192 Ir brachytherapy source traceability Chain in Taiwan Standard Chamber Standard Source Users Source Well type Chamber (Primary Lab.) Well type Chamber (user) Primary Laboratory User KRKR.
39 NRSL 192 Ir / ( 137 Cs 60 Co ) X / 30 ~250 kV 60 Co / Well-type chamber / 192 Ir /
40 NRSL 92 93
41 NRSL HDR well-type chamber cavity chamber
42 NRSL 1. TG-21 protocol (Med Phys, 1983) Part I: How to obtain the calibration factors N x (exposure) and N gas (dose to cavity gas) in a Co-60 beam. if only N x is provided by the standard laboratory, then the user needs to convert N x to N gas. Part II: How to use N gas in a users 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:
43 NRSL primary standard (3 cm 3 10 cm 3 30 cm 3 spherical chambers) and calibration system of 60 Co air kerma
44 NRSL Comparison of standard of air kerma for 60 Co R NMI, BIPM 60 Co air kerma rate measurement uncertainty (0.48 %, k=2) at INER
45 NRSL 60 Co air kerma calibration factor (N K ) uncertainty evaluation INER calibration certificate N K (Gy C -1 ) A ion ((4-Q 1 /Q 2 )/3) N X = N K / (Gy C -1 ) TG-21 N X (R C -1 ) Air kerma calibration factor (N K ) uncertainty (0.6 %, k=2)
46 NRSL TG-21 protocol (Part I - 60 Co beam) N gas Cavity –gas calibration factor (Gy C -1 ) N X Exposure calibration factor (R C -1 ) (INER calibration certificate) k 2.58 × (C kg -1 R -1 ) W/e (J C -1 ) A ion Ion-collection efficiency in the users chamber at 60 Co exposure (Calibration certificate) A wall Wall correction factor in the users chamber at 60 Co exposure (Table II or III)(Table II or III) Faction of ionization due to electrons from chamber wall (Fig. 1)(Fig. 1) Stopping –power ratio(wall/air, cap/air) (Table I)(Table I) Energy-absorption coefficient ratio(air/wall, air/cap) (Table I)(Table I)
47 NRSL TG-21 protocol (Part II - users beam) M Ion chamber reading in users beam (correction for polarity, electrometer, temperature and pressure) P ion Ion-recombination correction factor applicable to the calibration o f users beam (Fig. 4)(Fig. 4) P repl A factor that corrects for replacement of water phantom by an ionization chamber (Fig. 5)(Fig. 5) P wall Wall correction factor at users beam ( from Fig. 7, stopping – power ratio from Fig. 2 or Table IV, energy-absorption coefficient ratio from Table IX)Fig. 7Fig. 2 Table IVTable IX
48 NRSL 2. TG-51 protocol (Med Phys 26, , 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.
49 NRSL 100 cm 5 g/cm 2 Primary standard (pancake chamber) and calibration system of absorbed dose to water for 60 Co
50 NRSL Comparison of standard of absorbed dose to water for 60 Co R NMI, BIPM Measurement uncertainty (0.54 %, k=2) of absorbed dose rate to water for 60 Co at INER
51 NRSL Uncertainty evaluation of calibration factor ( ) of absorbed dose to water for 60 Co Absorbed dose to water calibration factor ( ) uncertainty (0.6 %, k=2) INER calibration certificate TG-51 (Gy C -1 )
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, k Q,change in modality, energy, gradient
53 NRSL TG-51 protocol k Q k Q Quality conversion factor(k Q =1 for 60 Co) P ion P ion = Ion-recombination correction factor P TP P TP = Temperature-pressure correction factor P elec = electrometer correction factor P pol P pol =polarity correction factor of ion chamber M raw = uncorrection ion chamber reading
54 NRSL 3.Comparison results for TG-21 and TG-51 dosimetry protocols 13 ( 7, 3, 2, 1) accepted by Radiation Measurements SCI Journal
55 NRSL The characteristics of farmer-type chambers used in this comparison The air gaps between waterproof sleeve and chamber wall<2 mm(AAPM TG-51) PMMA material and thickness<1 mm NE-2561(NE cm 3 ) NE-2571 (0.6 cm 3 ) PTW-30001(0.6 cm 3 ) Cylindrical chamber waterproof sleeves made in INER
56 NRSL ± N D,w /N K The distribution of /N K calibration coefficients ratios for the cylindrical ionization chambers
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 N X -N gas and TG-51 N D,w formalisms (Huq and Andreo, 2001) TG-21TG-51 Step 1 User chamber calibration factor Combined uncertainty N gas 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 k Q 1.0 Combined uncertainty in a Includes 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 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 NRSL Quality assurance for the switch of photon reference dosimetry between TG-21 and TG-51 protocols