Beam Production, characteristics and shaping

Transcription

1 Kantonsspital Luzern Beam Production, characteristics and shaping Dr. Manfred Sassowsky Cantonal Hospital Lucerne (KSL) Institute for Radio-Oncology X-ray production 60 Co units Linear Accelerators Beam characteristics Beam shaping Beam production / / Dr. M. Sassowsky / KSL 1

2 Literature 1. E.B. Podgorsak (Technical Editor): Radiation Oncology Physics: A Handbook for Teachers and Students, IAEA, Vienna, 2005, ISBN , 2. TRS 398: Absorbed Dose Determination in External Beam Radiotherapy, IAEA, Vienna, H. Reich (Hrsg.): Dosimetrie ionisierender Strahlung, B.G. Teubner, Stuttgart, ISBN (out of print) 4. H. Krieger: Strahlenphysik, Dosimetrie und Strahlenschutz (2 volumes), B.G. Teubner, 2001, ISBN X and ISBN Recommendations of the Swiss Society of Radiobiology and Medical Physics ( Beam production / / Dr. M. Sassowsky / KSL 2

3 X ray production: Principle Heated cathode: AC voltage U H Thermoelectric emission of electrons Tube voltage U accelerates electrons towards anode Electrons interact with anode material ~ U (See lecture "Basic radiation physics") H => X rays Kinetic energy of electrons: W = e U e.g.: U = 100 kv => W = 100 kev Cathode (heated filament) e - e - e - e - e - e - e - U Vacuum X rays Anode Beam production / / Dr. M. Sassowsky / KSL 3

4 X ray production: Focussing In particular for diagnostic X ray beams: small spot size needed (higher image resolution) Focusing using guard electrode with voltage U f ; negative with respect to ~ U H Cathode (heated filament) Guard electrode e - e - e - e - X rays e - Anode cathode e - U f U Vacuum Beam production / / Dr. M. Sassowsky / KSL 4

5 X ray production: Efficiency η = k Z U Efficiency: Z = atomic number of anode material U = tube voltage 9 1 k = constant; k 10 V Numerical example: Z = 74 (Wo) U = 100 kv => η 0.01 i.e.: - only 1% of electron kinetic energy is transformed into radiation energy - 99% are dissipated in heat Technical challenge: cooling of anode => rotating anode Beam production / / Dr. M. Sassowsky / KSL 5

6 X ray production: Rotating anode tube Vacuum Dielectric cooling oil Cathode Anode Stator Rotor Focal spot Exit window Beam production / / Dr. M. Sassowsky / KSL 6

7 X ray production: Applications Naming conventions Diagnostic - 2D images - Computed Tomography (CT) images => see lesson "Imaging for radiotherapy" Therapeutic (dermatology) W = 10 kev kev W = 100 kev kev W > 1 MeV Superficial X rays Othovoltage X rays Megavoltage X rays Beam production / / Dr. M. Sassowsky / KSL 7

8 X ray production: Photon energy spectrum As discussed in lecture "Basic radiation physics" - Continous X rays - Characteristic X rays Relative Intensity W max = e U Photon energy Beam production / / Dr. M. Sassowsky / KSL 8

9 X ray production: Filtering Diagnostic applications: - Low energy photons to not traverse patient - Do not contribute to diagnostic image - But: lead to dose deposition => use of Cu and/or Al filters to decrease intensity of low energy photons Relative Intensity Increasing filter thickness = e U Photon energy max Beam production / / Dr. M. Sassowsky / KSL 9 W

10 60 Co units: 60 Co decay 60 Co decays to 60 Ni by beta-decay Half life t 1/2 = 5.26 years Excited state of 60 Ni de-excites to ground state by two subsequent gamma-decays Specific activity a: a = A m = N t A 1/ 2 ln 2 M A N A t 1/2 M = Activity = Avogadro s number = Half life = Molar mass Co 5.26 a 0.31 MeV β 1.17 MeV γ 1.33 MeV γ Ni a Bq/g (pure 60 Co) a Bq/g (technically achievable with 25% 60 Co) Beam production / / Dr. M. Sassowsky / KSL 10

11 60 Co units: 60 Co sources Source shape: pellets or disks 60 Co produced using neutrons from a nuclear reactor: 59 Co + n 60 Co + γ d = 1 cm... 2 cm Self absorption of gamma radiation in source: A eff l l x A x A μ μ da e = dx e = 1 l μ l 0 0 = ( μl e ) The longer the source, the higher the self absorption Typical values: μ cm -1 (disks), l 10 cm => A eff 0.25 A i.e. about ¾ of gamma radiation is absorbed in source Beam production / / Dr. M. Sassowsky / KSL 11

12 60 Co units: Shutter mechanisms Rotating cylinder Sliding drawer source W = Wolfram Ur = Uranium Pb = lead source Operated by electromotor Safety mechanism: mechanical spring for emergency retraction Beam production / / Dr. M. Sassowsky / KSL 12

13 60 Co units: Treatment head Source Light bulb Fibre optic Primary collimator Secondary collimators Display of field size Depleted uranium Penumbra trimmer Lead Wolfram Beam production / / Dr. M. Sassowsky / KSL 13

14 Linear accelerators: Introduction Need for higher beam energies to treat deep-seated tumors Electrostatic acceleration limited to U 1 MV (discharges) => Use of particle accelerators Originally developped for research in elementary particle physics This lecture: only linear accelerators will be treated (time constraints) Basic idea: use moderate acceleration voltages many times to obtain higher total acceleration voltage Typical modern high energy linear accelerators (Linac): - Two photon energies (e.g. 6 MV, 18 MV) - Several electron energies (e.g. 6, 9, 12, 16, 20 MeV) Beam production / / Dr. M. Sassowsky / KSL 14

15 Linear accelerators: Electron gun: thermionic generation of electrons Accelerating waveguide: accelerates electrons using RF waves RF Generator Beam transport: transfer of electrons from accelerating waveguide to scatter foil or target Scatter foil: spreading of electrons for electron beams Target: generates photons from incident electrons Filter: flattening of photon beam Monitor chambers RF generator Power converters Stand Overview Electron gun RF Accelerating waveguide Scatter foil / Target + Filter Monitor chambers Gantry Beam transport Treatment couch Isocenter Beam production / / Dr. M. Sassowsky / KSL 15

16 Linear accelerators : RF Generation RF generator Electron gun Accelerating waveguide Beam transport Scatter foil / Target + Filter Monitor chambers RF Isocenter Power converters Treatment couch Stand Gantry Beam production / / Dr. M. Sassowsky / KSL 16

17 Linear accelerators: RF Generation (1) RF = Radio Frequency Used here as synonym for a high power, high frequency, electromagnetic wave Key element: Klystron High power RF amplifier Typical output: P 10 MW during Δt 5 μs f RF 3 GHz High voltage power supply charging Pulse forming network Klystron Low power RF pulse High power RF pulse Beam production / / Dr. M. Sassowsky / KSL 17

18 Linear accelerators: RF Generation (2) Klystron: Low power RF excites standing waves in 1st cavity Unbunched electron beam enters 1st cavity Bunching of electron beam: velocity modulation Drift region: bunch size decreases Bunched electron beam excites 2nd resonant cavity High power RF extracted from cavity Low power RF Cathode High power RF Resonant RF cavities Drift tube Anode Beam production / / Dr. M. Sassowsky / KSL 18

19 Linear accelerators : Electron gun RF generator Electron gun Accelerating waveguide Beam transport Scatter foil / Target + Filter Monitor chambers RF Isocenter Power converters Treatment couch Stand Gantry Beam production / / Dr. M. Sassowsky / KSL 19

20 Linear accelerators: Electron gun (1) Triode gun: pulsed electron bunches Thermoelectric emission of electrons (heated cathode) Anode voltage U positive with respect to cathode (15 kv kv) Grid voltage U g negative with respect to cathode (typ. 150 V) => Electrons can not pass grid, are kept in region between cathode and grid Cathode (heated filament) Accelerating waveguide Grid electrode Electrons Anode ~ U H U g U Beam production / / Dr. M. Sassowsky / KSL 20

21 Linear accelerators: Electron gun (2) Grid voltage U g set to zero during short time interval => Some electrons can pass grid, are accelerated towards anode Electron bunch Cathode (heated filament) Accelerating waveguide Grid electrode Electrons Anode ~ U H U g U g time U Beam production / / Dr. M. Sassowsky / KSL 21

22 Linear accelerators: Electron gun (3) Periodic pulsing of grid voltage Repeated electron bunches Cathode (heated filament) Accelerating waveguide Grid electrode Electrons Anode ~ U H U g U g time U Beam production / / Dr. M. Sassowsky / KSL 22

23 Linear accelerators: Acceleration RF generator Electron gun Accelerating waveguide Beam transport Scatter foil / Target + Filter Monitor chambers RF Isocenter Power converters Treatment couch Stand Gantry Beam production / / Dr. M. Sassowsky / KSL 23

24 Linear accelerators: Acceleration (1) Principle: use moderate acceleration voltages many times Explained using Wideroe accelerator (only of historical importance, but convenient to explain principle) Drift tubes, varying length L, alternating polarities Driven by RF Generator, frequency f RF, period T RF = 1 / f RF Acceleration of electron bunch in gaps between drift tubes Synchronicity condition: vt L = RF 2 RF generator ~ Gun L Drift tubes Beam production / / Dr. M. Sassowsky / KSL 24

25 Linear accelerators: Acceleration (2) RF generator ~ t = T 0 RF generator ~ t = T T RF RF generator ~ t = T 0 + T RF Beam production / / Dr. M. Sassowsky / KSL 25

26 Linear accelerators: Acceleration (3) Modern linac: RF waves in accelerating guide - Travelling wave - Standing wave RF in E RF out Travelling wave: - Hollow conducting cylinder as wave guide - Filled with discs (irises) Propagation speed (group velocity) of RF wave depends on geometry: - length of cells - Inner vs. outer diameter of irises Matched with electron speed 5 cm cm Beam production / / Dr. M. Sassowsky / KSL 26

27 Linear accelerators: Acceleration (4) Pictorial view: electrons ride close to the crest of the RF wave Bunch size is further decreased: - Faster electrons: o advance o experience lower accelerating field o are thus brought back to bunch - Slower electrons: o lag behind o experience higher accelerating field o are thus brought back to bunch V el V wave Beam production / / Dr. M. Sassowsky / KSL 27

28 Linear accelerators: Acceleration (5) More advanced travelling wave structure Showing RF input and output Beam production / / Dr. M. Sassowsky / KSL 28

29 Linear accelerators: Acceleration (6) RF forward RF reflected E Acceleration using standing waves End of accelerating waveguide is made reflecting for RF Forward RF wave is reflected back Superposition of forward and reflected RF wave yields a standing wave Maxima and minima of standing wave do not travel, but stay at fixed positions Beam production / / Dr. M. Sassowsky / KSL 29

30 Linear accelerators: Acceleration (7) More advanced standing wave structures Advantage of standing wave structure: Can be made shorter than travelling wave structure (while obtaining the same electron energy) "side coupled" cavities Beam production / / Dr. M. Sassowsky / KSL 30

31 Linear accelerators: Acceleration (8) Typical time structure of electron beam: Time Beam production / / Dr. M. Sassowsky / KSL 31

32 Linear accelerators : Beam transport RF generator Electron gun Accelerating waveguide Beam transport Scatter foil / Target + Filter Monitor chambers RF Isocenter Power converters Treatment couch Stand Gantry Beam production / / Dr. M. Sassowsky / KSL 32

33 Linear accelerators: Beam transport (1) Bend electron beam onto target Energy selection Charged particle in magnetic field: - Lorentz-Force: F L = q v B - Radius of curvature r : p r = qb q v p B = Charge = Velocity = Momentum = Magnetic induction Other magnet geometries are possible omitted here Beam production / / Dr. M. Sassowsky / KSL 33

34 Linear accelerators: Beam transport (2) Steering coils for fine-tuning beam position and beam angle Beam production / / Dr. M. Sassowsky / KSL 34

35 Linear accelerators : Scatter foil / Target + Filter RF generator Electron gun Accelerating waveguide Beam transport Scatter foil / Target + Filter Monitor chambers RF Isocenter Power converters Treatment couch Stand Gantry Beam production / / Dr. M. Sassowsky / KSL 35

36 Linear accelerators: Scatter foil (1) Left: Electron beam from accelerating structure is only a few mm in diameter ( Pencil beam ) Right: Treatment requires wide beams with a flat transverse beam profile S: Scatter foil scatters electrons to transform pencil beam into beam useful for treatment Beam production / / Dr. M. Sassowsky / KSL 36

37 Linear accelerators: Scatter foil (2) One scatter foil: limited transverse homogeneity and field size => - Multiple scattering foils - Additional ring shaped foils for low energy beams Beam production / / Dr. M. Sassowsky / KSL 37

38 Linear accelerators: Scatter foil (3) Homogeneous transverse beam profile but: - Energy loss - Energy straggling - Photon contamination from Bremsstrahlung Maximize scattering, minimise adverse effects => materials with high atomic number Z are preferred Beam production / / Dr. M. Sassowsky / KSL 38

39 Linear accelerators: Target / filter (1) Treatment with photons requires wide beams with a flat transverse beam profile Target Photons Filter Target: electrons interact with nuclei and emit photons (Bremsstrahlung); materials with high atomic number (Z) Filter: used to homogenise transverse beam profile; preferably materials with high Z Intensity distribution Collimator Beam production / / Dr. M. Sassowsky / KSL 39

40 Linear accelerators: Target / filter (2) Effects of filter on photon beam: Reduction of beam intenstiy in center of beam Reduction of total beam intensity Compton interaction and Bremsstrahlung => Decrease of photon energy Preferred absorption of low energy photons => Increase of average beam energy Contamination of beam with secondary electrons => Increase of skin dose Modification of depth dose distribution Photon energies above ~10 MeV: nuclear photo effect => Contamination of beam with neutrons Activation of materials in treatment head Beam production / / Dr. M. Sassowsky / KSL 40

41 Linear accelerators: Target / filter (3) Thin target Thick target Average photon energy Higher Lower Yield Lower Higher Electron stopper Yes No Cooling requirements Low High Target e - e - Electron stopper Target Filter Primary collimators Filter Primary collimators Beam production / / Dr. M. Sassowsky / KSL 41

42 Linear accelerators: Target / filter (4) Examples for different filter shapes a) Pb for low energies b) Pb or Wo for energies up to 15 MeV c) Fe with Pb core (25 MV Photons) d) Low Z (Al or steel) for high energies Beam production / / Dr. M. Sassowsky / KSL 42

43 Linear accelerators: Target / filter (5) Effects of electron-beam mis-steering a) Nominal beam b) Beam inclined c) Beam displaced d) Beam divergent Beam production / / Dr. M. Sassowsky / KSL 43

44 Linear accelerators: Monitor chambers RF generator Electron gun Accelerating waveguide Beam transport Scatter foil / Target + Filter Monitor chambers RF Isocenter Power converters Treatment couch Stand Gantry Beam production / / Dr. M. Sassowsky / KSL 44

45 Linear accelerators: Monitor chambers (1) Monitor chambers measure: - MU (monitor units) - Beam symmetry Calibrated so that 1 MU = 1cGy under reference conditions Transmission ionisation chamber - Two electrodes on HV (U) - Beam ionises air molecules - Charge separation in electric field - Charge (Q= I dt) => MU Beam I U Beam production / / Dr. M. Sassowsky / KSL 45

46 Linear accelerators: Monitor chambers (2) Located downstream of scatter foil / filter Two independent systems Each system: two sectors Differential signals => Beam symmetry Beam production / / Dr. M. Sassowsky / KSL 46

47 Beam Characteristics: Absorbed dose to water Absorbed dose is the deposited energy per mass: D = de dm SI unit is the gray (Gy): [ E] [ D] = [ m] = J = kg Gy 1 Gy = 1 Water is used as reference material (Properties similar to tissue, availability, physical properties well defined) J kg Beam production / / Dr. M. Sassowsky / KSL 47

48 Beam Characteristics: Depth dose photons (1) Percentage Depth Dose (PDD) curve of photons in water D ex = exit dose D surface Buildup region Depth in water Beam production / / Dr. M. Sassowsky / KSL 48

49 Beam Characteristics: Depth dose photons (2) Percentage Depth Dose (PDD) curves of photons in water Photon beams ranging from 60 Co to 25 MV cm 2 field Source to Surface Distance (SSD) = 100 cm Beam production / / Dr. M. Sassowsky / KSL 49

50 Beam Characteristics: Depth dose electrons (3) Percentage Depth Dose (PDD) curve of electrons in water R th = therapeutic range R p = practical range R max = maximum range D skin D surface Buildup region Bremsstrahlung tail Bremsstrahlung underground = R 50 Depth in water Beam production / / Dr. M. Sassowsky / KSL 50

51 Beam Characteristics: Depth dose electrons (4) Percentage Depth Dose (PDD) curve of electrons in water D rel 100 Electron energy ranging from 4 MeV to 30 MeV 50 Depth in water (cm) Beam production / / Dr. M. Sassowsky / KSL 51

52 Beam Characteristics: Transverse beam profile Transverse beam profile Relative dose as function of a transverse coordinate 100 Varies with depth D rel (%) D 0 50 Field size x Penumbra region Beam production / / Dr. M. Sassowsky / KSL 52

53 Beam Characteristics: Beam quality specification Superficial and orthovoltage X Rays: Half Value Layer (HVL) Megavoltage X rays: Tissue to Phantom Ratio Q = TPR 20,10 = D 20 / D 10 Higher TPR 20,10 => more penetrating beam Electrons: R 50 SCD = 100 cm d = 20 cm D 20 D 10 Field size = cm 2 d = 10 cm Beam production / / Dr. M. Sassowsky / KSL 53

54 Beam Characteristics: Isodose distributions Isodose distributions: contours of equal relative dose Photons ( 60 Co) Electrons (9 MeV, 20 MeV) Beam production / / Dr. M. Sassowsky / KSL 54

55 Beam shaping: Collimation (photons) Collimation - Primary collimators define maximum field size - Secondary collimators define actual field size in two transverse directions Target Primary collimator Beam s eye view: Filter Y Rotation of collimator assembly Monitor chambers X1 Y2 X2 Secondary collimators Y direction Secondary collimators X direction 0 X Y1 Beam production / / Dr. M. Sassowsky / KSL 55

56 Beam shaping: Collimation (electrons) Collimation - Primary collimators - Secondary collimators - Tertiary collimator (electron applicator) Scattered electrons used to saturate field edges Secondary collimator Without electron applicator Electron Applicator With electron applicator Beam production / / Dr. M. Sassowsky / KSL 56

57 Beam shaping: Wedges (1) Wedge: device or method to achieve a linear tilt of isodose curves in one transverse direction Wedge angle α: angle of isodose curve w.r.t. curve without wedge (at reference depth) α Beam production / / Dr. M. Sassowsky / KSL 57

58 Beam shaping: Wedges (2) Hard wedge: - Inserts in treatment head - Progressive decrease in intensity across the beam - Reduction of beam intensity => wedge transmission factor - Influence on beam quality Dynamic wedge: - Closing motion of one collimator jaw during irradiation - Modulation of dose rate Beam production / / Dr. M. Sassowsky / KSL 58

59 Beam shaping: Blocks Block: device to adjust transverse field shape to target volume Mold for block is cut from styrofoam Block is made by casting Pb alloy (low melting point) in the mold Individually manufactured for each field => time consuming and labour intensive Beam s eye view: Cut A-A : Cut B-B : A A B B D D x x Beam production / / Dr. M. Sassowsky / KSL 59

60 Beam shaping: Multi Leaf Collimators (MLC) MLC: device to adjust transverse field shape to target volume Made from single small leafs Each leaf can be moved to its individual position under computer control Beam s eye view: Leaf shape: Leaf movement A Leaf movement Cut Beam production / / Dr. M. Sassowsky / KSL 60

61 The end... Thank you for your attention! Questions? Beam production / / Dr. M. Sassowsky / KSL 61