Brachytherapy RDTH 3120

Transcription

1 Brachytherapy RDTH 3120 URL : Prof. S.Vynckier UCL, Brussels This PowerPoint file is taken from the lecture : «dosimetry : RDTH3120» at the UCL

2 What is brachytherapy (Curiethérapie) The word brachytherapy is derived from the ancient Greek words βραχυς, which means short or close, and θεραπεια, which has several meanings including medical treatment or therapy. Brachytherapy is a form of radiotherapy in which radioactive sources are placed inside or near the tissue to be irradiated. With this form of treatment, a high dose can locally be deliver to the tumor, with a rapid dose fall-off in the surrounding healthy tissues due to the inversesquare law

3 History From: RF Mould, JJ Batterman, AA Martinez, BL Speiser (eds), Brachytherapy: from Radium to Optimization, Nucletron, 1994 The first successful brachytherapy treatments were performed soon after Marie and Pierre Curie s discovery of radium in This picture shows how radium ( 226 Ra) surface applicators were used for the treatment of skin cancer

4 Brachytherapy today Courtesy of Nucletron

5 Applications Four types of brachytherapy applications are commonly distinguished: Surface applications: source molds or flexible surface applicators are used to position the sources at a fixed distance from the lesion (e.g. skin cancer) to be irradiated. Intracavitary applications: the sources are inserted into a natural body cavity, often using specially designed applicators for precise source positioning. Interstitial applications: the sources are inserted into the tumor itself by means of e.g. needles or catheters: in a temporary implant, the sources are removed when the treatment has been completed; in a permanent implant, the sources are left in place. Intravascular brachytherapy: catheter-based delivery of radiation to prevent restenosis after angioplasty

6 Courtesy of Nucletron Intracavitary examples The most common intracavitary application of brachytherapy is for gynecological tumors (e.g. cervical cancer) Other intracavitary applications include e.g. cancer of the rectum and nasopharynx So-called intraluminal applications include e.g. bronchial or esophageal cancer

7 Interstitial examples Common interstitial applications include prostate cancer, breast cancer (breast conserving therapy), and cancers in the head and neck region. Interstitial treatment is also used for cancers of the brain, pancreas, lung, soft tissues, etc. Generally, temporary implants allow better control of the dose distribution (more accurate source positioning and, in HDR brachytherapy, control of dwell times) However, permanent implantation is a one-time procedure and may therefore be preferable for tumors that are difficult to reach (e.g. in the abdomen) Courtesy of Nucletron

8 Intravascular brachytherapy Schematic overview of the principle of intravascular brachytherapy (IVB). The left-hand column represents the normal chain of events, where the initial success of an angioplasty treatment may be limited by the occurrence of restenosis and repeated intervention may be necessary. The right-hand column shows how intravascular irradiation may be used to prevent restenosis. IVB is performed in coronary as well as peripheral arteries. Courtesy of Nucletron

9 Contact treatments: plaque treatments Choroidal melanoma Rethinoblastoma Choroidal hemangioma... Treatment techniques Enucleation Plaque therapy Proton therapy... b-ray plaques 90 Sr + 90 Y (E ave = MeV b) (E max = 2.28 MeV b) (T 1/2 = years) 106 Ru Rh (E ave = 1.43MeV b) (E max = 3.54 MeV b) (T 1/2 = 368 days) gold plaques loaded with seeds : 125 I (27-35 kev photons) (T 1/2 = 59.4 days) 103 Pd (20-23 kev photons) (T 1/2 = 17.0 days)

10 Fundus photograph of the choroidal melanoma

11 Echography of the choroidal melanoma Normal eye Melanoma

12 125 I ophthalmic plaques Longitudinal view of a 125 I seed Self made gold plaques Bebig plaques with inserts

13 106 Ru ophthalmic plaques Silverwindow window 0.1mm 0.1mmthickness thickness Radioactive part 0.2 mm thickness Radioactive part 0.2 mmthickness Silvershell Silver 0.9 mmthickness shell 0.3 mm thickness

14 Dose planning for 125 I gold plaques

15 Dose planning for b-plaques ( 106 Ru+ 106 Rh)

16 From manual to remote afterloading Courtesy of Nucletron Up until the 1950 s, brachytherapy sources were prepared and put in place manually in most cases, resulting in high doses to the medical personnel involved. The picture shows an example of a surface mold used to treat a skin lesion

17 From manual to remote afterloading A first improvement was the development of manual afterloading techniques. In this approach, unloaded needles, catheters or applicators are carefully implanted. Hereafter, the radioactive seeds, wires or tubes are inserted. Besides a reduction of staff exposure, these techniques also improved the accuracy of dose delivery. Manual afterloading of (relatively weak) LDR brachytherapy sources is still being performed today. In the 1960 s the first remotely controlled afterloaders were developed. The use of these machines completely eliminated staff exposure and made possible the use of much stronger sources

18 Manual afterloading Example of manual afterloading, using 192 Ir wires for the treatment of a tumor in an eyelid. Note the moveable lead shields at both sides of the patient. From: European School of Medical Physics (ESMP), Archamps

19 Remote afterloading Example of a modern afterloader, the microselectron HDR V2 from Nucletron B.V. It contains a small, sealed, 10 Ci 192 Ir stepping source, mounted at the end of a stainless steel drive wire. The afterloader is connected to the implanted applicator, catheter or needle using flexible transfer tubes. The device is able to position the source at a preprogrammed series of source positions with millimeter accuracy. The dose distribution can be optimized by adjusting the dwell time at each source position. Courtesy of Nucletron

20 Remote afterloading Emergency button Hand cranks if everything else fails Safe, holding the active source and a dummy source From: European School of Medical Physics (ESMP), Archamps 2002 Optopair to verify source position Indexer face with 18 source channels Stepper motor with shaft encoder (additional DC motor available for source retraction in case of failure) Indexer Radiation monitor Transfer tube connector Battery pack available in case of power failure

21 Remote afterloading Courtesy of Nucletron The indexer guides the source into one of the 18 source channels. Before the active source is inserted into any of the channels, a dummy source is inserted first to check for obstructions etc. Courtesy of Nucletron

22 Remote afterloading Courtesy of Nucletron As illustrated by this autoradiograph, an important advantage of a stepping source is that the dose distribution can be modified by altering the source positions and the dwell times (i.e., the time spent at each source position). Each of the four dose distributions in this example were produced by a single source in a single catheter

23 Remote afterloading No staff allowed in the treatment room during irradiation, but treatment can be interrupted when patient care is necessary. Courtesy of Nucletron Afterloader Treatment room has shielded walls Treatment control station Treatment planning

24 HDR versus LDR In low dose rate (LDR) brachytherapy, irradiation times are of the order of hours to days (surface, intracavitary and temporary implants) or even days to weeks (permanent implants). In high dose rate (HDR) brachytherapy, doses are typically delivered in a faction of an hour, using much stronger sources. Although HDR brachytherapy requires more expensive equipment, necessities room shielding and increases personnel demands, it has some practical advantages: possibility of treatment on an out-patient basis; possibility to treat more patients in the same amount of time; decreased patient discomfort. Radiobiologically, there are important differences between HDR and LDR brachytherapy

25 HDR versus LDR From: S. Nag, High Dose rate Brachytherapy: a Textbook, Futura, 1994 In low dose rate (LDR) brachytherapy, (almost) all of the potential for repair is being utilized. In contrast, (almost) no repair takes place in HDR treatments. Lower doses are needed with HDR than with LDR to achieve the same cell kill

26 HDR versus LDR From: S. Nag, High Dose rate Brachytherapy: a Textbook, Futura, 1994 One way in which HDR treatment can be made (more) equivalent to LDR irradiation is by means of fractionation

27 HDR versus LDR Besides fractionation, there are several other ways to improve the clinical efficacy of HDR brachytherapy, making use of the dosimetric and treatment advantages that HDR has over LDR: possibility to optimize the dose distribution by adjusting source positions and dwell times, resulting in a more conformal dose distribution; more precise dose delivery due to immobilization; possibility to move (some) sensitive healthy tissues away from the source during treatment. The clinical outcome of HDR has been shown to be comparable to LDR for many indications. Given the practical advantages, this makes HDR preferred modality in many cases

28 Common radionuclides Nuclide Main emission Half-life Energy (kev) Half-value thickness (mm Pb) Air kerma rate constant (µgy m 2 h -1 MBq -1 ) 32 P β d 1710 max (696 avg) 60 Co γ y 1173, (1252 avg) 90 Sr/ 90 Y β y 2280 max* (935 avg)* 103 Pd X-rays d (21 avg) 106 Ru/ 106 Rh β d 3541 max (1413 avg) 125 I X-rays d (28 avg) 137 Cs γ y Ir γ d (354 avg) 198 Au γ d * Maximum and average beta energies are given for the daughter nuclide

29

30 Examples of Iodine sources MED3631 from North American InterSource 125 from IBt Symmetra from Uromed/Bebig Models 6702 and 6711 from Amersham

31 Examples of Palladium sources MED3633 from North American InterSource 103 from IBt Model 200 from Theragenics

32 Beta sources 90 Sr/ 90 Y intravascular source for the treatment of in-stent restenosis in coronary arteries. This source from Novoste Corporation consists of a train of tiny seeds that is advanced to the lesion through a catheter by means of hydraulic pressure. Gold marker seeds at both ends of the source train are used to localize the source under fluoroscopy. Kindly provided by Wim Dries, Catharina Hospital, Eindhoven 106 Ru/ 106 Rh eye plaques from Bebig GmbH. These are temporarily (2-14 days) stitched to the eye to treat eye melanoma. Some plaques have cut-outs for the iris or optic nerve

33 Source strength The first brachytherapy sources contained 226 Ra, and the source strength of such a sources was simply specified by the mass (in mg) of radium contained within the source. A more general quantity is activity, i.e. the number of disintegrations per unit time taking place within the source. The SI unit for activity is Bq (Becquerel, the number of disintegrations per second), but for historical reasons many people still use the Ci (Curie, 1 Ci = Bq). The major problem with both of these quantities is that, for a given radionuclide, the dose rate at a given point outside the source depends not only on the amount of radioactivity inside the source but also on the attenuation, scattering and filtration of the emitted radiation in the source material and capsule (often called self-absorption)

34 Source strength Nowadays, the quantity used to specify the source strength of brachytherapy gamma sources is the air kerma strength defined by the American Association of Physicists in Medicine (AAPM) as the product of air kerma rate in free space and the square of the distance of the calibration point from the source center along the perpendicular bisector. This definition is only valid if the distance between the source and the detector is large enough that they can be treated as a point source and a point detector, respectively. With "in free space" one means that the measurement must be corrected for air attenuation and photon scattering (i.e. the interaction of the radiation with the air between source and detector and with surrounding media such as the walls of the measurement room), so the result equals that of a hypothetical measurement in an infinite vacuum. The unit recommended for air kerma strength is µgy h -1 m 2. perpendicular bisector or transverse axis

35 Source strength Similarly, the International Commission on Radiation Units and measurements (ICRU) defines the reference air kerma rate of a source as the kerma rate to air, in air, at a reference distance of one meter, corrected for air attenuation and scattering. The corresponding unit is µgy h -1 at 1 m. Although defined somewhat differently, this quantity numerically equals the air kerma strength defined by the AAPM. While the air kerma strength and reference air kerma rate are proportional to the activity of the source, they are a much better measure of the strength of a source because selfabsorption (which, due to production tolerances, may not even be equal for different sources of the same type) is taken into account

36 Source strength Once a source has been calibrated in terms of air kerma strength or reference air kerma rate, its strength can also be specified as apparent activity, defined as the activity of a bare point source of the same radionuclide that produces the same air kerma rate at 1 m. This may for example be convenient within the context of radiation protection regulations (e.g. transport), where an estimate of the source activity may be required. The equivalent mass of radium (mg-ra eq) is derived by dividing the air kerma strength or reference air kerma rate by the air kerma rate constant (in µgy h -1 mg -1 m 2 ) of a 226 Ra point source filtered by 0.5 mm Pt. This is mainly of interest for historical reasons, e.g. for comparison with past treatments

37 Source specification (cont d) User calibration : well type ionization chamber with a calibration traceable to the national standard for each type of brachytherapy sources

38 3D dose distribution The 3D dose distribution about a source is determined by the following factors: The inverse square law. The particle fluence about a point source in vacuum falls off with the square of the distance to the source. For a source of finite extend one can calculate it as the integral of the contributions of infinitesimal volume elements over the radioactive volume. The interaction of the emitted particles with the materials within the source itself and around it. The parameters that include the influence of interactions on the dose distribution are the type(s) of particle emitted by the radionuclide, the energy spectrum of the emitted radiation and the composition and density of the materials involved

39 3D dose distribution Some common terminology: the radial depth-dose distribution or radial depth-dose curve describes the variation of the dose rate along the transverse axis or perpendicular bisector as a function of distance from the source center in a given medium; the anisotropy describes the variation of the dose rate as a function of the angle with the source axis. the dose distribution about a source or implant is commonly depicted using isodose curves (lines connecting points of equal dose) perpendicular bisector or transverse axis source axis

40 3D dose distribution Med Phys 22(2), ,

41 TG-43 protocol Nowadays, the dose calculation formalism recommended by Task Group 43 (TG-43) of the AAPM is the generally accepted method to express the dose distribution about (most) brachytherapy sources

42 y TG-43 protocol P(r,) P(r 0, 0 ) r 1 2 The dose distribution in water is described in a polar coordinate system with its origin at the source centre, as the product of a number of parameters: L z D ( r, θ ) = D( r0, θ0) [ G( r, θ ) / G( r0, θ0)] g( r) F( r, θ ) Here, D(r 0,θ 0 ) equals the dose rate in water at the reference point that is located at a distance of r 0 = 1 cm on the transverse bisector of the source, i.e., at θ 0 = π/2. For gamma sources, D(r 0,θ 0 ) = S k Λ, where S k is the air kerma strength and Λ is the dose rate constant, defined as the dose rate in water at the reference point per unit air kerma strength

43 TG-43 protocol y P(r,) P(r 0, 0 ) r The geometry factor is defined as: v G( r ) = v v v 2 [ ρ r r r V A ( ) /( ) ] d v ρ ( r ) dv A 1 2 D(r,)=D(r 0, 0 ) [G(r,)/G(r 0, 0 )] g(r) F(r,) where the activity distribution equals the activity per unit volume at and is an infinitesimal volume element located at the same position. This function reduces to: 1 θ 2 θ1 G ( r) = for a point source; G( r, θ ) = for a line source. 2 r Lrsin( θ ) Here, L is the active length of the source and the angles θ 1 and θ 2 are indicated in the figure. L z

44 TG-43 protocol y P(r,) P(r 0, 0 ) r The radial dose function is defined as: 1 2 L D(r,)=D(r 0, 0 ) [G(r,)/G(r 0, 0 )] g(r) F(r,) z g ( r) = G( r 0, θ ) 0 G( r, θ ) 0 D( r, θ ) D( r 0 0, θ ) 0 As the influence of the inverse square law is accounted for by the geometry factor, it can be said that the radial dose function accounts for the influence of the interaction of the emitted radiation in the medium and source materials on the depth-dose distribution along the transverse axis

45 TG-43 protocol y P(r,) P(r 0, 0 ) r The anisotropy function is defined as: 1 2 L D(r,)=D(r 0, 0 ) [G(r,)/G(r 0, 0 )] g(r) F(r,) z F ( r, θ ) = G( r, θ0) G( r, θ ) D( r, θ ) D( r, θ ) 0 the anisotropy function accounts for the anisotropy of the dose rate distribution relative to the transverse axis, due to self-absorption, oblique filtration of primary photons through the encapsulation materials, and scattering of photons in the medium. Note that, in principle, the geometry factor accounts for the anisotropy resulting from the spatial distribution of the radioactivity in the source

46 Measurements Phantoms: different geometries for the measurements of the dose distribution in solid water WT1 and RW1 Detectors : LiF TLD-100 microcubes from Harshaw 1x1x1 mm 3 Calibration at 6MV with an energy factor of

47 Phantoms geometry Radial function Anisotropy function 10 TLD position Source 2 cm 3cm 5cm Two types of solid water of slightly different composition WT1: r=1.015 RW1: r=

48 Radial function distances (cm) calculations in WT1 measurements in WT1 calculations in WT Pd 125 I measurements in WT1 calculation of Meigooni and al. measurements of Meigooni and al. calculations of Meigooni and al. measurement of Meigooni and al distance from the source (cm

49 Comparison with literature Pd I distance calculations in WT1 measurements in WT1 calculations in WT1 measurements in WT1 calculation of Meigooni and al. measurements of Meigooni and al. calculations of Meigooni and al. measurement of Meigooni and al distance from the source (cm)

50 difference Anisotropy function anisotropy function f(r,q) angle q 125 I 103 Pd calculations at 3cm in WT1 for iodine measurements at 3cm for iodine calculations at 2cm in RW1 for palladium measurement at 2cm for palladium angle q

51 Dose calculation algorithm for β-particles : point kernels from : Vynckier and Wambersie, PMB 1982 J(x) = B ( ρν x)2 c 1 ρν x c exp(1 ρν x c ) ] + ρν xexp(1 ρν x) ρν 1 ρν x xexp( f 2 2 ) } with [ ] 0 for c <ρνx and J(x) 0 for ρνx >f. α is expressed by : α -1 =3c2-(c2-1)e +(3 + f)exp(1 - f) -4 exp(1 - (f/2))

52 From: CA Joslin, A Flynn, EJ Hall (eds), Principles and Practice of Brachytherapy Using Afterloading Systems, Edward Arnold, 2001 Imaging and reconstruction The 3D positions of implanted needles, catheters, applicators or, in case of evaluating permanent implants, the sources themselves, may for example be determined by means of an orthogonal set of X-ray images. Needles and catheters for temporary implants are filled with X- ray markers (e.g. a steel wire with tungsten markers at every cm) before the images are taken

53 Imaging and reconstruction From: European School of Medical Physics (ESMP), Archamps 2002 Orthogonal reconstruction of a gynecological applicator

54 Imaging and reconstruction Courtesy of Nucletron Reconstruction may also be done by means of a treatment simulator

55 Imaging and reconstruction From: CA Joslin, A Flynn, EJ Hall (eds), Principles and Practice of Brachytherapy Using Afterloading Systems, Edward Arnold, when a simulator is used for reconstruction, the use of the variable angle technique allows one to select the imaging angles that provide the best image quality and/or the best visual separation between sources or catheters in a complex implant

56 Imaging and reconstruction From: European School of Medical Physics (ESMP), Archamps 2002 Compared to a bronchial implant with only a few catheters (see right-hand picture, where the catheters contain X-ray markers), a permanent prostate implant (left-hand picture) is much more difficult to reconstruct. The large number of sources makes manual reconstruction very tedious, and sources may be overlapping from any viewing angle. This creates a need for better imaging modalities and computerized reconstruction

57 Imaging and reconstruction From: European School of Medical Physics (ESMP), Archamps 2002 Example of automated reconstruction of an HDR gynecological applicator using computed tomography (CT) images. The images also show the source dwell positions within the 3 source channels, and the calculated isodose curves

58 Imaging and reconstruction uterus cervix bladder rectum From: European School of Medical Physics (ESMP), Archamps 2002 An important advantage of magnetic resonance imaging (MRI) is that soft tissues are much better visible than with CT. This makes it easier to localize not only the applicator, but also the cancerous tissue to be irradiated as well as the radiosensitive tissues (such as the bladder and rectum) to which the dose should be kept to a minimum

59 Imaging and reconstruction Courtesy of Nucletron CT and MRI reconstruction require the use of applicators that contain no metal parts. The right-hand picture shows a recent MRI and CT compatible gynecological applicator made of carbon tubes and plastic parts, while the left-hand picture shows an older applicator containing metal parts

60 Imaging and reconstruction bladder prostate urethra rectum From: European School of Medical Physics (ESMP), Archamps 2002 Ultrasound (US) imaging is also being used, especially for prostate implants. The left-hand figure shows how the sources are implanted via needles that are inserted through a guiding template (blue) into the prostate (yellow). The needle position is constantly monitored using a trans-rectal US (TRUS) probe (pink), in order to avoid damage to the urethra or bladder. In addition, a camera is often inserted into the bladder via a catheter to monitor the bladder wall

61 Treatment planning Courtesy of Nucletron Screenshots of a modern treatment planning program. The left hand picture shows a reconstructed breast implant with isodose curves, the right-hand picture shows a CT image of a prostate implant together with the isodose curves predicted for a HDR brachytherapy boost plus subsequent external beam fractions

62 Système de Paris: exemple A. Définition : Système dosimétrique prévisionnel de curiethérapie basé sur une répartition régulière de sources d Ir 192 B. Règles d implantations : 1. Sources parallèles, rectilignes ; Plan central : plan perpendiculaire aux sources en leur centre; 3. Débit de Kerma uniforme le long de chaque ligne et identique pour toutes les sources ; 4. Lignes équidistantes. Exemples : Disposition en ligne Disposition en carré Disposition en triangle

63 Système de Paris C. Distribution de dose : Symétrie des isodoses circulaire ; Source = axe de symétrie ; La surface des isodoses entourant chaque ligne a la forme d un cigare allongé. Y X Y X Y Z Z Coupe dans le plan central (Y = 0)

64 Système de Paris D. Points de base : La distribution de dose est caractérisée par le débit de dose des points de base. La disposition des points de base assure une distribution homogène. Exemples : DB placés au milieu de 2 sources DB placé au centre du carré DB placés à l intersection des médiatrices des triangles isodose 100 % isodose 85 %

65 Système de Paris E. Paramètres et volumes : Volume irradié : Volume délimité par l isodose 50 % Volume traité : Volume délimité par l isodose 85 % Axe de mesure de la longueur traitée : ligne des DB L t Isodose 85 % e 1 e 2 - Longueur traitée : fonction de la géométrie de l implant, - L t est la moyenne des longueurs élémentaires - Epaisseur traitée : e t est la moyenne des épaisseurs élémentaires. d 1 m 1 - Marge de sécurité et débord latéral : pour des implants - complexes tels qu un implant en triangle m 2

66 2. Materials Implant of 7 needles in triangle

67 2. Materials Implant of 7 needles in triangle Plato V14.2 (Nucletron)