HDR Brachytherapy Dosimetry at the NPL

Transcription

1 1 Introduct HDR Brachytherapy Dosimetry at the NPL Brachytherapy is a special procedure in radiotherapy that utilises the irradiat of a target volume (e.g. malignant cells) with radioactive sources placed at short distances from the target. Brachytherapy is the Greek word for short distance treatment. The opposite is teletherapy where an external radiat source is used, e.g. an X-ray tube or a linear accelerator. The radioactive sources are either implanted in the target tissue directly (interstitial brachytherapy) or are placed at distances of the order of a few millimetres from the target tissue, in body cavities such as the uterus, lung, mouth, etc. (intracavitary brachytherapy), or externally on structures such as the eye or the skin (ophthalmic applicators or surface moulds). Shortly after the discovery of radium by Marie and Pierre Curie in 1898, the naturally occurring raduclide 226 Ra and also the daughter nuclide radon ( 222 Rn) were used for cancer treatment (Mould et al. 1994). When artificially produced raduclides became available from nuclear reactors and particle accelerators from the 1950s onwards, many new raduclides entered the brachytherapy field (Thomadsen et al. 2005). Nowadays raduclides used in brachytherapy include photon emitters (e.g. 103 Pd, 125 I, 170 Tm, 169 Yb, 192 Ir, 198 Au, 137 Cs, 60 Co) with mean photon energies ranging from MeV to 1.25 MeV and beta emitters (e.g. 90 Sr, 32 P, 90 Y, 106 Ru) with maximum beta energies ranging from 0.54 MeV to 3.55 MeV. Brachytherapy dose rates cover a very wide range leading to treatment times varying from minutes to months and have been divided into low, medium and high dose rates by the Internatal Commiss for Radiat Units and Measurements (ICRU 1985). Low dose rate (LDR) 0.4 Gy h Gy h -1 Medium dose rate (MDR) 2 Gy h Gy h -1 High dose rate (HDR) >12 Gy h 1 (usually 100 Gy h Gy h -1 ) In the following chapters we will only consider high dose rate (HDR) brachytherapy dosimetry for 192 Ir, at present the most commonly used raduclide worldwide for HDR brachytherapy. 192 Ir is produced in a nuclear reactor via neutron capture by stable 191 Ir and has a half-life of days ± days (DDEP 2004). 192 Ir decays by beta emiss and electron capture to excited states of 192 Pt and 192 Os (Baltas et al. 2007). The daughters decay to the ground states by gamma emiss. The beta particles emitted from the 192 Ir source are either absorbed within the source encapsulat or in the tissue very close to the source, so it is effectively a gamma source. 192 Ir has a very complicated gamma ray spectrum. The average energy of the gamma rays emitted by the Nucletron microselectron-v1 classic 192 Ir-source (obsolete after 31 December 2013), which is placed in a sealed stainless steel HDR Brachytherapy Dosimetry at the NPL Page 1 of 11

2 capsule, is approximately 397 kev (Goetsch et al. 1991). At NPL this type of source has been used for the HDR brachytherapy calibrat service until From 2014 onwards, an HDR 192 Ir Flexisource will be used for the calibrats. There are other brachytherapy afterloaders available, marketed for instance by Varian or Bebig. HDR treatments are delivered by remote controlled afterloaders, which are stepper motor driven systems that transport the radioactive source from a shielded safe into the applicator placed inside the patient. At the end of the pre-programmed dwell time, the source is moved back to the safe. 2 Source characterisat in terms of reference kerma rate In 1985, the ICRU (report no. 38) defined a quantity for the characterisat of photon sources used for brachytherapy, the reference kerma rate (RAKR), which is the kerma rate to, in, at a reference distance of 1 m, corrected for attenuat and scattering (SI units: Gy s -1 ). The kerma rate is given by E K d d tr (1) dt dm where de tr is the sum of the initial kinetic energies of all charged particles liberated by uncharged particles in a small volume of dry with mass d m. The kerma rate can be measured at any suitable distance from the radiat source. The reference kerma rate is given by 2 d R K K (2) d ref where d is the distance from the centre of the source to the measurement point and d 1 m is the reference distance. ref The direct from the source centre to the reference point shall be at right angles to the long axis of the source (ICRU 1985, IAEA 2002). 3 Measurement of RAKR at the hospital Hospital physicists need to characterise their own brachytherapy sources in terms of RAKR. The numerical value of this quantity is usually required as input parameter for the treatment planning system. A calibrated dosemeter comprising an isat chamber and an electrometer is needed to determine RAKR. The isat current produced in the isat chamber being irradiated is measured and then converted to reference kerma rate. Assuming the isat chamber and the electrometer are already calibrated, RAKR can be calculated using the following equat: HDR Brachytherapy Dosimetry at the NPL Page 2 of 11

3 K I k (3) R N KR where K R is the reference kerma rate of the hospital source (Gy s -1 ), I is the corrected isat current (A) measured by the hospital physicist (= isat current corrected to standard atmospheric condits electrometer calibrat coefficient), k is the recombinat correct factor and N K R is the calibrat coefficient of the isat chamber (Gy C -1 ). In the next chapter you will learn how isat chambers for HDR 192 Ir are calibrated against the NPL primary standard and how the calibrat coefficient of the secondary standard,, in terms of Gy C -1 is determined. N K R 4 NPL s kerma standard for HDR 192 Ir brachytherapy sources 4.1 Measurement equat and correct factors The NPL has established a spherical graphite-walled isat chamber as a primary standard for 192 Ir which realises the physical quantity of interest (here: RAKR in terms of Gy s -1 ) from first principles. The cavity chamber was originally used as a high-energy transfer standard for protect level calibrats. The graphite wall of the chamber is 4 mm thick, i.e. there is sufficient build-up material in the chamber wall to provide charged particle equilibrium for secondary electrons produced by 192 Ir -rays (Goetsch et al. 1991). The following measurement equat, based on Bragg-Gray and large cavity theory, applies to the NPL primary standard and shows how the RAKR of an HDR 192 Ir source is determined from the measured isat current. For more informat on cavity theory see the lecture notes on Quantities, Units, and Ionising Radiat Fundamentals. The measurement equat for the 192 Ir primary standard is given by: graph I W S d K 1 en k k i k k k k (4) R fl h att sc dec Tp V e 1 g i d graph ref where K R is the reference kerma rate (Gy s -1 ) at a reference time, I is the isat current (A = C s -1 ) measured at distance d, is the density of dry (kg m -3 ), V is the internal volume of the chamber cavity (m 3 ) which was measured in the Centre for Basic, Thermal and Length Metrology at NPL using a co-ordinate measuring machine. V was found to be E-04 m 3 with an uncertainty of 0.12% (k = 2). W is the average energy (J) required to produce an p in dry per unit charge (C), where W e (33.97 ± 0.05) J C HDR Brachytherapy Dosimetry at the NPL Page 3 of 11

4 g = is the fract of secondary electron energy lost to bremsstrahlung in. The next three terms follow from cavity theory and need to be applied because the isat chamber is made of graphite and not. The values were evaluated by calculat using Monte Carlo techniques. en = is the ratio of the mean photon-energy-fluence-weighted graph photon mass energy-absorpt coefficient of to that of graphite and graph S kfl = is the product of the ratio of the mean electron-fluenceweighted electron mass stopping power of graphite to that of and the fluence perturbat correct factor. i k is the product of further correct factors to be applied to the primary i standard. Some of these were determined by Monte Carlo simulats, i.e. the wall correct factor = which needs to be applied because the collecting volume is surrounded by a graphite wall which attenuates and scatters the primary photon beam, the central electrode correct factor = and the product of the radial and axial non-uniformity correct factor = The following three correct factors were determined by measurement, i.e. the stem scatter correct factor = , the polarity correct factor = and the recombinat correct factor = The latter is dose rate dependent and given for a typical isat current of 20 pa. k h = is the humidity correct factor. k attsc (uncertainty: 0.05%, k = 2) is the combined attenuat and scatter correct factor which corrects the measured isat current at d = m for attenuat and scatter between the source and the point of measurement (see definit of RAKR in sect 2). k depends on temperature and pressure and the attenuat and scatter correct factor is usually found to be in the range ± The next factor, d d 2 ref, follows from the inverse square law and corrects the measured current at distance d = m (see figure 1) to the current that would have been measured at the reference distance d = 1 m. k dec is the decay correct. 192 Ir has a relatively short half-life, 1/2 = days, and the measured isat current at the actual measurement time, t now, needs to be corrected to a reference time, t ref. When the secondary standard isat chamber is calibrated with the calibrated source, the measured isat current is corrected to the same reference time, t ref, before the primary standard to secondary standard ratio is calculated. HDR Brachytherapy Dosimetry at the NPL Page 4 of 11 ref attsc

5 ktp is the temperature and pressure correct factor which converts the isat current measured at temperature T and pressure p to the standard temperature T STD = K (= 20 C) and the standard pressure p STD = mbar. By applying all correct factors of equat 4 to the measured primary standard isat current, I, it is possible to derive the RAKR of the 192 Ir source from first principles. A detailed descript of the primary standard cavity chamber can be found in NPL report DQL-RD 004 (Sander and Nutbrown 2006). The total uncertainty of the source calibrat is estimated to be 0.7% (k = 2). 4.2 Source calibrat set-up at NPL Figure 1 shows the set-up used at NPL for the RAKR measurement mm Lead collimator Cavity chamber (primary standard) Alignment telescope Source in catheter Brachytherapy unit Figure 1. Set-up for 192 Ir HDR source calibrat at NPL (not to scale) The cavity chamber is set up at a source to chamber distance of m and the measured isat current is normalised to the reference distance of 1 m by applying the inverse square law. The lead collimator was developed to reduce the amount of scattered radiat from the floor and the walls of the exposure room reaching the collecting volume of the cavity chamber and to avoid irradiating the chamber stem and connectors. Both the source-tochamber distance and aperture size were chosen to give a uniform field over the whole of the isat chamber, which is circular in cross-sect. 4.3 Traceability to the NPL primary standard The HDR brachytherapy calibrat service at NPL is for dosemeters (isat chambers and electrometers) intended to be used as secondary HDR Brachytherapy Dosimetry at the NPL Page 5 of 11

6 standards or instruments required for measurement of the greatest accuracy. The isat chambers used in the hospital are calibrated directly against the NPL primary standard for HDR 192 Ir. The calibrat is a two-step process: First, the RAKR of the Nucletron microselectron Classic 192 Ir HDR source at NPL is determined using the NPL primary standard cavity chamber. The source is characterised in terms of Gy/s at 1 m. Once the RAKR is known, the 192 Ir source can be used to calibrate an isat chamber. The customer s isat chamber is connected to a calibrated electrometer and the calibrated source is moved to the dwell posit corresponding to the maximum chamber response ( sweet-spot ). The calibrat coefficient of the isat chamber, N K R, is the ratio of the primary standard measurement (RAKR of 192 Ir source) to the secondary standard measurement (isat current measured with the source placed at the sweet-spot ). The isat current is also corrected for recombinat effects and N is given by: K R N K R RAKR isat current (A) k 1 Gy s -1 calibratio n coeffici ent Gy C. (5) The calibrat coefficient, N K R, is reported on the calibrat certificate for the isat chamber and enables the hospital physicist to determine the source strength of the hospital source (see equat 3). 5 Secondary standard isat chambers for 192 Ir brachytherapy sources Well-type isat chambers are suitable secondary standards for HDR 192 Ir brachytherapy sources. Thimble chamber/jig combinats can also be used. However, there is no standardised calibrat procedure for thimble chamber/jig combinats at NPL as the end users have a variety of jigs. Thimble chambers need to be clamped in a suitable calibrat jig with a source to chamber distance of typically 10 cm. There is significant potential for non-reproducible source positing as catheters supported in the calibrat jig may not always be parallel and there are many possible adjustments. In comparison, the well chamber method is less sensitive to source positing errors owing to the 4 measurement geometry inherent in the design. Figure 2 shows a few well-type isat chambers suitable for measuring HDR sources. The source can be moved through a suitable transfer tube or a plastic catheter connected to the treatment head of the afterloader and the insert of the well chamber. Figure 3 shows a thimble chamber clamped on a source calibrat jig. In the bottom right hand corner two transfer tubes can be seen which link the two HDR Brachytherapy Dosimetry at the NPL Page 6 of 11

7 catheters left and right of the thimble chamber to the brachytherapy afterloader. Figure 2. Well chambers for HDR brachytherapy sources Figure 3. Thimble chamber / Nucletron Jig combinat In well-type isat chambers, a 370 GBq 192 Ir source will produce isat currents in the order of, say, 40 na 80 na because the collecting volume is quite large ( 200 cm 3 ) and the source is close to the collecting volume. In thimble chambers (collecting volume 0.7 cm 3 ) mounted on a calibrat jig, the same source will generate isat currents of only a few pa. The calibrat procedure for both instruments is similar. 5.1 Calibrat of a well-type isat chamber for 192 Ir at NPL The well chamber is posited at least 1 m from any wall and 1 m above floor level on a low scatter surface. Before commencing measurements, sufficient time is allowed for the chamber to reach thermal equilibrium with the surrounding. Well chambers are usually left to settle overnight. The well chamber is then connected to a calibrated electrometer. Measurements are taken after a warm-up period of at least 30 minutes during which the electrometer, isat chamber and cables are allowed to settle. The point of maximum response ( sweet-spot ) of the well chamber is found by stepping the 192 Ir source through the chamber and by plotting the corrected isat current versus the dwell posit of the source (see figure 4). The sweet-spot is the reference point and the hospital physicist needs to place the hospital source at the same posit when using the chamber for measuring the RAKR of 192 Ir sources. The 192 Ir source is then sent to the dwell posit corresponding to the maximum chamber response and at least five measurements of the isat current are taken. The electrometer calibrat coefficient is applied and all isat currents are corrected to the same reference time, t ref, as mented in sect 4.1 by applying a decay correct. HDR Brachytherapy Dosimetry at the NPL Page 7 of 11

8 normalised isat current / % of maximu 100% 90% Standard Imaging 1000 Plus PTW / Nucletron SDS 80% 70% 60% centre of source to bottom of well chamber distance / mm Figure 4. Typical response curves of a Standard Imaging 1000 Plus and PTW 33004/Nucletron SDS well chamber for HDR 192 Ir with respect to the source posit. An recombinat correct factor, k reciprocal of the collect efficiency, A, is determined by using the twovoltage technique (Attix 1984). k is given by: 4 I, is also applied. k, which is the k 3 3 I (6) 150 where I 300 is the instrument response in amps with the polarising potential set to 300 V and I 150 is the instrument response in amps with the polarising potential set to 150 V. During calibrat at NPL, the collecting electrode and guard electrode are positive with respect to the outer electrode (chamber housing). This potential gradient will ensure that the negative current is collected by the electrometer. The isat current is normalised to standard atmospheric condits: T STD = 20 C, p STD = mbar. The well chamber calibrat coefficient, applying equat 5. N K R, can now be obtained by The total uncertainty of the well chamber calibrat coefficient is 0.8% (k = 2). HDR Brachytherapy Dosimetry at the NPL Page 8 of 11

9 5.2 Calibrat of a thimble chamber/jig combinat for 192 Ir at NPL For thimble chamber/jig combinats, the calibrat procedure is similar to the well chamber calibrat procedure. When using the Nucletron Source Calibrat Jig, the thimble chamber is set up between two catheters, such that the distance from the centre of the 192 Ir source to the centre of the thimble is 10 cm for both catheters. During calibrat at NPL, for Farmer-type thimble chambers the polarising potential is usually set to 250 V. The collecting electrode and guard electrode are positive with respect to the graphite thimble. This potential gradient will ensure that the negative charge is collected by the electrometer. The point of maximum response ( sweet-spot ) of the thimble chamber is found by stepping the 192 Ir source through both catheters parallel to the long axis of the chamber (see figure 3) and by plotting the corrected isat current versus the dwell posit of the source. The response curves look similar to a well chamber response curve (see figure 4). The corrected isat current is calculated for both channels and the mean current is used for the calculat of the calibrat coefficient, N, again in terms of (Gy C -1 at 1 m). The recombinat correct for thimble chambers set up in the Nucletron calibrat jig and used for measuring HDR 192 Ir sources is negligible and therefore k is taken as unity. The total uncertainty of the thimble chamber calibrat coefficient determined at NPL is 1.1% (k = 2). This is greater than the uncertainty quoted for well chamber calibrat coefficients, which is mainly due to the posital uncertainty in the set up of the thimble chamber in the calibrat jig. The IPEM code of practice for HDR brachytherapy (Bidmead et al. 2010) therefore recommends the use of well-type isat chambers as secondary standards for HDR 192 Ir. K R 6 From RAKR to absorbed dose to water The calibrated isat chambers and electrometers are returned to the brachytherapy centres and the hospital physicists can now use the calibrated instruments to determine the RAKR of their own HDR 192 Ir sources. This will be discussed in more detail in the presentat: HDR Brachytherapy Dosimetry in the Clinic. The isat current is measured with the HDR 192 Ir source located at the point of maximum response of the secondary standard. The RAKR of the hospital source can be determined from equat 3 in terms of Gy s -1 at 1 m, traceable to the NPL primary standard for HDR 192 Ir. There are a number of different afterloaders and source types currently in use in hospitals. If the 192 Ir source type in the hospital is different from that used for the calibrat of the well chamber at NPL, a source geometry correct factor, k sg, is required to correct the calibrat coefficient for any change of the well chamber response due to the geometric differences between the sources (Bidmead et al. 2010). These correct factors will be issued as part of the calibrat certificate. HDR Brachytherapy Dosimetry at the NPL Page 9 of 11

10 Although brachytherapy gamma ray sources are currently calibrated for RAKR, patient dosimetry is based on absorbed dose to water. The American Associat of Physicists in Medicine (AAPM) published the Task Group 43 protocol (Nath et al. 1995) and an update (TG-43U1) by Rivard et al. in This protocol allows the calculat of absorbed dose to water at a point (r, ) near the source from kerma strength 1 or RAKR. Many treatment planning systems are based on the TG-43 formalism. The reference point r 0 1cm, 0 / 2 for dose calculats is chosen to lie on the transverse axis of the source at a distance of 1 cm from its centre. The dose rate to water at a distance of 1 cm is r,θ S Λ D (7) 0 0 where D r, 0 0 is the dose rate to water at the reference point in terms of cgy h -1, S K is the kerma strength in terms of cgy cm 2 h -1 and is the dose rate constant which is defined as the dose rate to water at a distance of 1 cm on the transverse axis of a unit kerma strength source (1 U = 1 cgy cm 2 h -1 ) in a water phantom. The dose rate constant includes the effects of source geometry, the spatial distribut of radioactivity within the source, encapsulat and self-filtrat within the source and scattering in water surrounding the source. The numerical value of depends on the source type. Equat 7 is only a special case and the general equat for the dose rate r, r, can be written as D at point K D G r,θ r,θ S Λ gr Fr,θ K G r 0, (8) 0 S is the kerma strength, is the dose rate constant, r, g is the radial dose funct and r, where K geometry funct, r anisotropy funct as defined in TG-43 (Nath et al. 1995). Tabulated values of G r,, g r and r, G is the F is the F for various brachytherapy sources can be found at the following website of Carleton University, Canada: (Taylor and Rogers 2008). 1 Air kerma strength is the quantity used in North America for the specificat of brachytherapy gamma ray sources. Air kerma strength has units of cgy cm 2 h -1. HDR Brachytherapy Dosimetry at the NPL Page 10 of 11

11 7 References Attix F H 1984 Determinat of A and P in the new AAPM radiotherapy dosimetry protocol, Med. Phys. 11, Baltas D, Sakelliou L and Zamboglou N 2007 The Physics of Modern Brachytherapy for Oncology, CRC Press, Taylor & Francis Group, New York, ISBN Bidmead A M et al The IPEM code of practice for determinat of the reference kerma rate for HDR 192 Ir brachytherapy sources based on the NPL kerma standard, Phys. Med. Biol. 45, DDEP 2004 Decay Data Evaluat Project: LNE-CEA/LNHB (France), PTB (Germany), INEEL (USA), KRI (Russia), LBNL (USA), NPL (United Kingdom), CIEMAT (Spain). ( accessed on 6 December 2007) Goetsch S J, Attix F H, Pearson D W, Thomadsen B R 1991 Calibrat of 192 Ir high-dose-rate afterloading systems, Med. Phys. 18, IAEA 2002 Calibrat of photon and beta ray sources used in brachytherapy, IAEA-TECDOC-1274, IAEA, Vienna, ISSN ICRU 1985 Dose and Volume Specificat for Reporting Intracavitary Therapy in Gynecology, ICRU report 38, Internatal Commiss on Radiat Units and Measurements, Bethesda, MD Mould R F, Battermann J J, Martinez A A, Speiser B L (eds.) 1994 Brachytherapy from Radium to Optimizat, Nucletron Internatal B.V., The Netherlands, ISBN Nath R et al Dosimetry of interstitial brachytherapy sources: Recommendats of the AAPM Radiat Therapy Committee Task Group No. 43, Med. Phys. 22, Rivard M J et al Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculats, Med. Phys. 31, Sander T and Nutbrown R F 2006 The NPL kerma primary standard TH100C for high dose rate 192 Ir brachytherapy sources, NPL report DQL- RD 004, Teddington, ISSN Taylor R E P and Rogers D W O 2008 An EGSnrc Monte Carlo-calculated database of TG-43 parameters, Med. Phys. 35, Thomadsen B, Rivard M and Butler W (eds.) 2005 Brachytherapy Physics, Second Edit, Medical Physics Publishing, AAPM Monograph no. 31, ISBN HDR Brachytherapy Dosimetry at the NPL Page 11 of 11