Introduction to Hospital accelerators

Transcription

1 Introduction to Hospital accelerators Lars Hjorth Præstegaard Aarhus University Hospital Outline Electron linacs for radiotherapy: Overview Beam transport How to change the energy? Treatment head design Medical cyclotrons: Basic principles Applications 1

2 Overview of electron linacs for radiotherapy Main components Acceleration structure Bending magnet Microwave amplifier Electron gun Gantry Treatment head Couch Waveguide 2

3 Gantry and couch degrees of freedom Gantry Collimator Isocenter: Intersection of gantry rotational axis and collimator rotational axis Couch Main manufacturers of medical linacs Elekta Varian 4-22 MeV TW accelerator 4-22 MeV SW accelerator X-ray treatment: Fast electrons + target Intense bremsstrahlung (x-rays) Electron treatment: Fast electrons + scattering foil Spray of fast electrons 3

4 Beam transport Beam transport in electron medical linac Goals: 1. Transport of beam from linac to target/foil 2. Small beam spot at target/foil Linac Target Bending magnet Solenoid x-rays 4

5 Bending magnet: Concepts Chromatic deflection Achromatic deflection Large beam spot Varian / Siemens HE Provides a small isocenter height Achromatic deflection Elekta Varian (low energy) Bending magnet: Varian Clinac HE Coil Magnet poles 5

6 Bending magnet: Varian Clinac HE Energy slit: Slit at position with nonzero dispersion Target How to change beam energy of linac for radiotherapy? 6

7 Frenkvens Frekvens Change of RF input power Change of RF input power Change of the electric field Change of the energy, capture efficiency, and energy spread Only optimum capture efficiency and energy spread for a particular RF input power (beam energy) Problem for multi-energy linac Design compromise Siemens SW linac: 6 MeV Large energy spread Capture efficiency: 36% Small percentage of electrons reach the target Low dose rate + stray rad Accelerations-effektivitet Siemens linac SW : 18 MeV Small energy spread Capture efficiency: 44% Large percentage of electron reach the target Accelerations-effektivitet Change of RF frequency t 2 Buncher t 1 Elekta Synergy TW accelerator RF frequency shift Change of phase velocity Desynchronization of wave and particle Particle energy decrease Only small change of effective electric field in the buncher section Design of optimum capture efficiency and energy spread for a wide range of beam energies. High dose rate for all treatment energies Used for Elekta TW accelerator 7

8 Change of number of active cells Motorized energy switch: Modification of cavity coupling Varian Clinac Buncher +RF input Reduced electric field Same electric field in buncher for all beam energies Design of optimum capture efficiency and energy spread for wide range of beam energies Efficient transfer of electrons from gun to target for all energies High dose rate for all energies Change of input beam current Change of input beam current Change of beam loading Change of the electric field: SW accelerator: TW accelerator: Change of energy Change of capture efficiency and energy spread Change of energy No change of capture efficiency and energy spread Input beam current used for energy feedback loop for Elekta TW accelerator 8

9 Treatment head design Varian Clinac treatment head Flattening filters Target Dual monitor chamber Secondary collimators 120 leaf multi-leaf collimator 9

10 X-ray target Varian Clinac x-ray target: X-ray target: Located inside vacuum Conversion of electron beam to bremsstrahlung (x-ray treatment) Target materials: Target materials affects x-ray yield and spectrum Copper/water for cooling 6 MeV bremsstrahlung spectrum: Primary collimator Primary collimator: Large tungsten block defining the maximum treatment field size Effective shielding of leakage radiation Usually opening with a conical shape Round maximum field size 10

11 X-ray flattening filter 1. Bremsstrahlung is forward-peaked 2. Convenient with flat dose profile Use flattening filter: Cancellation of off-axis intensity variation Reduction in dose rate Off-axis photon energy spectrum variation High energy Varian flattening filters: Low energy Monitor chamber Dual transmission ionization chamber: Determination of treatment beam dose Two chambers: Redundant dose determination TrueBeam dual transmission monitor chamber 11

12 Light field Light field = Extend of treatment field Secondary collimators (jaws) Varian secondary collimators (tungsten): Secondary collimators: 12

13 Multi-leaf collimator (MLC) MLC: 2 rows of thin tungsten blades Detailed shaping of the treatment field Typical leaf width: 5 mm MLC Varian 120 leaf MLC (leaf width: 5 mm) Photon cascade Water Compton scattering Low ionization (dose) at water surface = Dose buildup e - Pair production e - Photoelectric effect e + Bremsstrahlung e - e - Annihilation e + Photoelectric effect e - Photoelectric effect = Ionization 13

14 Relative dose X-ray depth dose curves Dose buildup Depth of max. dose increases with energy Surface dose 25 % 6 MV 15 MV Decreasing dose: Attenuation + inverse square law Depth (mm) Dose buildup provides skin sparing Electron treatment Detailed field shaping: Lead end-frame cut-out Elekta electron applicator 14

15 Relative dose Electron dose versus depth Ionization tracks (20 MeV electrons): Dose High ionization (dose) at surface Water Electron range depends on electron energy Depth Bremsstrahlung tail Electron depth dose curves High dose at surface Depth (mm) 6 MeV 9 MeV 12 MeV 15 MeV 18 MeV Peak for low energy: 1. Increase of electron fluence with depth (scattering ~ 1/E kin2 ) 2. Reduced radiation loss (less smear out of Bragg peak) Bremsstrahlung tail Electrons are used for treatment close to the skin (only 1 treatment field) 15

16 Example: 5 field prostate x-ray treatment Transverse view of patient pelvis Field 2 Field 1 Field 3 Prostate target Field 5 Field 4 Overlap of all fields at cancer target Large dose in cancer target relative to dose in healthy tissue Example: Intensity-modulated RT (IMRT) IMRT: Modulation of each field with MLC Dose distribution fits better to the cancer target 16

17 Imaging systems on-board accelerator Imaging systems Verification of patient position kv source on robotic arm MV radiation head MV detector on robotic arm KV detector on robotic arm Imaging systems: 2D MV imaging 2D KV imaging (good contrast) 2D KV imaging + gantry rotation: CT scanning of pt. in treatment position Medical cyclotrons for proton therapy and isotope production 17

18 The classical cyclotron Magnetic field Circular orbit Circular orbit: Centrifugal force = Magnetic Lorentz force mv r 2 evb z m: Mass of ion. q: Number of charges e: Elementary charge v: Speed of ion r: Radial position of ion B z : Magnetic field strength in the z direction z Angular frequency: rev v r eb m z eb z m 0 Gyro frequency or cyclotron frequency Typically MHz 18

19 Isochronous cyclotron CW high intensity proton beam Simultaneous acceleration of low/high energy protons Revolution period should be energy independent (isochronous cyclotron) Isochronous cyclotron: eb z rev Constant m 0 c B z (r) = (r)b(0), r R rev Field increases versus r + r is limited Problem: Reduced orbit separation More difficult extraction Isochronous cyclotron Methods for increasing the field versus r: 1. Decreasing pole gab versus r 2. Trimming coils 3. Increase hill/valley ratio IBA C235 Isochronism: The magnet poles must be machined and shimmed with very high accuracy (relative field error below 10-4 ) 19

20 Transverse focusing Betatron tune: Number of oscillation periods per revolution Weak focusing: Less than one oscillation per revolution Strong focusing: More than one oscillation per revolution Cylindrical symmetric magnet pole: Radial betatron tune: r2 =1-n Vertical betatron tune: z2 =n n r B z B r z Both radial and vertical focusing r2 =1-n > 0 and z2 =n > 0 0<n<1 (weak focusing) Small field decrease versus r B r F B z Vertical focusing Problem: Isochronous cyclotron: B z (r) = (r)b(0) Vertical focusing: Magnetic field versus r Vertical Lorentz force: F z e v B v r rb Not compatible! Solution: Increasing field versus r + additional vertical focusing Vertical defocusing for isochronous cyclotron =0 for field with no dependence on 0 for field with dependence on Additional vertical focusing: Non-cylindrical magnet pole 20

21 Vertical focusing: Azimuthally varying Field Azimuthally-varying field (AVF): AVF B 0: AVF + spiral pole geometry: Additional vertical focusing Isochronous cyclotron with strong transverse focusing Vertical focusing: IBA medical cyclotron 21

22 Applications of medical cyclotrons Cyclotron for radionuclide production GE PETtrace 700 RF input Targets for radionuclide production RF cavity RF cavity Spiral sectors RF input 22

23 Cyclotron-produced PET radionuclides Acceleration of protons or deuterium by a cyclotron Medical cyclotron at PET center in Aarhus IBA Cyclone 18 Twin: Particles: Protons Beam energi:18 MeV (protons) Number of targets: 8 Shielding:1.8 m concrete 23

24 Positron emission tomography (PET) 511 kev 511 kev Cyclotron-based proton therapy facility Danish Center for Proton Therapy (DCPT): Beam line Energy selection system (ESS) Degrader: Energy adjustment Research room Gantry ( tons) Gantry ( tons) Gantry ( tons) Cyclotron Time line: March 2015: Equipment contract July 2017: Installation of equipment October 2018: First patient 24

25 Cyclotron: Energy change Courtesy by PSI Apertures Apertures Dipole Energy slit Energy slit: Selection of treatment energy Cyclotron Degrader Dipole Degrader + ESS transmission: Degrader: Beam energy Beam size, divergence, and energy spread Dose deposition in the patient Dose distribution of raw beam: Passive scattering: Pencil beam scanning (PBS): Simple Straight forward treatment of moving organs Excess dose to normal tissue Significant neutron dose Patient-specific collimators and compensators No requirement of patientspecific collimators and compensators Interplay effect for moving organs 25

26 Gantry Varian gantry in Munich Proton gantry: Weight: t Diameter: ~10 m Speed: 360/min Treatment center accuracy: < 1 mm Treatment room Varian ProBeam: Gantry nozzle Imaging system Patient couch 26

27 Literature Medical Electron Accelerators, C. J. Karzmark, C. S. Numan, and E. Tanabe, McGraw-Hill, T. Stammbach, Introduction to cyclotrons, CERN Accelerator School: Cyclotrons, linacs and their applications, La Hulpe, Belgium, 28 Apr - 5 May, 1994 ( P. Heikkinen, Cyclotrons, CERN Accelerator School: 5 th general accelerator physics course, Jyväskylä, Finland, 7-18th September, 1992 ( _member_