Radiation Generators Peter J. Biggs Ph.D., Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114 AAPM Review Course – July 19, 2014 Radiation Generators Topics to be Covered: A. Kilovoltage x-ray units B. Cobalt teletherapy units & Gamma Knife C. Linear accelerators, Tomotherapy, Cyberknife & IORT D. Cyclotron (protons) E. Properties of megavoltage beams AAPM Review Course – July 19, 2014 Evolution of Radiation Generators 1895 X-Rays Radium therapy ~1951 Betatrons Cobalt-60 Van de Graaff ~1962 generators Linear accelerators Cyclotrons Viewray Synchrotrons Tomotherapy Cyberknife Gamma knife Present AAPM Review Course – July 19, 2014 Kilovoltage Therapy Units (Non- Isocentric) • Contact therapy ( 50 kV): <100 kV still has – Often short SSD, high dose rate machines with small clinical use applicator sizes – HVL 2mm Al, with sharp dose fall-off for skin irradiation • Superficial therapy: (50-150kV): – SSD’s typically up to 20 cm – HVL up to 8mm Al (150kV) • Orthovoltage therapy (150-500kV): – Typical energies are 250 and 300 kV with SSD’s up to 50cm – HVL up to 4mm Cu All beam qualities have maximum dose at or close to the surface and use fixed applicator treatment. 250 kVp is the “gold standard” for radiobiology. AAPM Review Course – July 19, 2014 Orthovoltage Unit – Mainstay of Early Radiation Therapy Tungsten 1. Stationary, scatter target Radiation (W in Cu block) shield 2. HVL ~ mm Cu CathodeFilament Copper Cooling shield fins 3. Dose rate (current machines): - ~260 R/min; HVL = 1 mm Cu - ~180 R/min; HVL = 2 mm Cu - 50 cm SSD Beryllium Glass Thin X-ray 4. Target angle is ~26°-32° for Envelope Window glass beam large field size (20x20) 5. Tube is oil cooled AAPM Review Course – July 19, 2014 Orthovoltage Applicators –SSD Rx BJR specifies PDDs for both diaphragm limited fields and closed applicators AAPM Review Course – July 19, 2014 Cobalt-60 Teletherapy Units • Source drawer contained in thick, steel encased lead housing to reduce leakage to <0.02 mSv/hr at 1m • Safety systems ensure that the unit fails in the “OFF” position • Depth of maximum dose is 0.5 cm for a 10x10 cm2 field; this drops rapidly with increasing field size due to electron contamination • 60Co is still the standard beam quality for calibrating ionization chambers (ND,W & Nx) • These units now come under homeland security regulations AAPM Review Course – July 19, 2014 Theratron 780 - Cutaway View AAPM Review Course – July 19, 2014 Cobalt 60 Source Construction Lid of inner stainless 1. source comes with a Standard thread Steel capsule min. diameter of 1.5cm Wafers to fill space and can either be in the form of pellets or a Snap ring solid slug. Thus, the Holes penumbra is much Heavy For worse than for linacs. metal wrench 2. clinical sources are Lid of typically 104 Ci, giving outer capsule ~240 rad/min at isocentre (80 cm) Inert gas arc Outer capsule welded seals 3. doubly encapsulated Of inner and Inner stainless source outer capsule Steel capsule AAPM Review Course – July 19, 2014 Cutaway of Gamma Knife 201 sources Activity on loading = 6600 Ci Slide courtesy of Elekta AAPM Review Course – July 19, 2014 Overview of Linac Components - Gun - Waveguide - Power supply - Modulator - Treatment Head AAPM Review Course – July 19, 2014 WHAT KIND OF POWER SOURCE IS NEEDED FOR LINEAR ACCELERATORS? 1. Why not DC? - Problems of electrical breakdown; physical size of electrical equipment 2. Apply technique of repeated pulses, viz. V n v - need oscillating form of power supply 3. Leads to principle of cyclic and linear accelerators AAPM Review Course – July 19, 2014 WHAT KIND OF POWER SOURCE IS NEEDED FOR LINEAR ACCELERATORS? 4. Wavelength has to be short enough to accelerate electrons in a reasonable distance 5. S band microwave technology, developed for radar in WWII, has a frequency ~ 3 GHz, or 10 cm 6. High power is also needed to ensure sufficient energy gain per cycle. AAPM Review Course – July 19, 2014 GENERATION OF HIGH POWER MICROWAVE PULSES This image cannot currently be displayed. 1. At high frequencies, ordinary circuits become impractical (radiation loss, skin effect) 2. Hollow cavities as a form of resonant circuit 3. The quality factor, or Q value, of a resonant circuit or cavity is defined as Energy stored incycle Q Energylost per cycle For a circuit, Q = 102, whereas for cavity, Q = 104 AAPM Review Course – July 19, 2014 Cavity Principle ~ (LC)-1/2 AAPM Review Course – July 19, 2014 GENERATION OF HIGH POWER MICROWAVE PULSES 1. Achieved through devices called magnetrons and klystrons 2. To understand these devices, need to consider properties of cavity resonators 3. Cavity resonators feature in both power sources and accelerating structures AAPM Review Course – July 19, 2014 Cavity Principle - I Current Charge E-field H-field AAPM Review Course – July 19, 2014 Cavity Principle - II Current Charge E-Field B-Field AAPM Review Course – July 19, 2014 Cavity Principle - III Current Charge E-field B-field AAPM Review Course – July 19, 2014 ROLE OF RESONANT CAVITIES IN LINEAR ACCELERATORS 1. Cavity acts as an acceleration module 2. Multiple cavity arrangement can act as RF amplifier -klystron 3. Multiple cavity arrangement can act as a high power oscillator -magnetron AAPM Review Course – July 19, 2014 Magnetron Principle Illustrating the theory of the cylindrical magnetron Bc depends on electron velocity and hence accelerating voltage AAPM Review Course – July 19, 2014 Magnetron Principle - II Increasing the magnetic field eventually leads to a sudden drop in current Theoretical 100 Gauss Cut-off value AAPM Review Course – July 19, 2014 Magnetron - Detail Approximately 2MW peak power for 4 MV AAPM Review Course – July 19, 2014 Magnetron Action -I Charge +,- Em From Ep Charge +,- E From E m p S Anode Cathode Cavity Drift Space AAPM Review Course – July 19, 2014 Magnetron Action - III Space-Charge distribution andelectronpathsinan8- cavity magnetron, when oscillating. AAPM Review Course – July 19, 2014 SlideAAPM courtesy Review Course of Christian – July 19, 2014Wolff ChristianAAPM Wolff: Review www.radartutorial.euCourse – July 19, 2014 Principle of Klystron Low level Amplified microwaves First Cavity Second Cavity high power to be amplified (Buncher) (Catcher) microwaves Hot wire Electron filament Bunches Cathode Drift Tube Electron Electron Stream Beam Collector AAPM Review Course – July 19, 2014 Klystron with Four Sections approx 20 MW peak power AAPM Review Course – July 19, 2014 Acceleration Principle of Waveguide surfing analogy ! AAPM Review Course – July 19, 2014 Principle of Phase Stability Electric field forces particles to “bunch” Principle discovered independently by Veksler (1944) and McMillan (1945) AAPM Review Course – July 19, 2014 Disc-Loaded Waveguide vph > c vph < c AAPM Review Course – July 19, 2014 Travelling Waveguide the amplitude of the e/m wave progressively decreases along the guide Remaining beam power is dumped at the end AAPM Review Course – July 19, 2014 Mass-Energy Relation for Electron As energy increases, so does velocity AAPM Review Course – July 19, 2014 Cut-Away of Travelling Guide gun end narrower wider AAPM Review Course – July 19, 2014 Standing WG principle - I AAPM Review Course – July 19, 2014 Standing WG principle - II E Field Maxima AAPM Review Course – July 19, 2014 Standing WG principle - III Note: the overall e/m amplitude decreases with beam loading E Field Maxima AAPM Review Course – July 19, 2014 Standing WG - Schematic Microwave Power Rectangular Waveguide Electric Field Coupling note that Cavity travelling waveguides are longer than standing waveguides Accelerating due to side Cavity Electron Beam coupled Longitudinal Section Cross Section cavities AAPM Review Course – July 19, 2014 Waveguide for 4 MV Linac vacuum SF6 connection transmission guide gun HV connection ceramic disk waveguide vacuum cooling target gun target cooling AAPM Review Course – July 19, 2014 Schematic of Waveguide AAPM Review Course – July 19, 2014 Electron Beam Current, X-Ray output vs. Electron Energy AAPM Review Course – July 19, 2014 X-Ray Output for Various Energies Beam Loading: a. heavy b. optimal c. light AAPM Review Course – July 19, 2014 Schematic of Energy Switch Fully closed – Partially closed – field reversal field reduction AAPM Review Course – July 19, 2014 Principle of Energy Switch Purdy & Goer, NIMRIP:B 11:1090-1095; 1985 Also used in the Mobetron IOERT machine AAPM Review Course – July 19, 2014 X-Ray Outputs with Energy Switch 6x Mode 18x Mode 6% Slit 6% Slit Relative Beam Intensity Energy(MeV) AAPM Review Course – July 19, 2014 Energy Switch AAPM Review Course – July 19, 2014 In-Line, No Magnet Linac Gantry Standing Wave Accelerator Structure Axis of Gantry Rotation Scale: Feet 024 6810 AAPM Review Course – July 19, 2014 WG Parallel to Rotation Axis Gantry Accelerator Structure Beam Bending Magnet Axis of Gantry Rotation Scale: Feet 024 6810 AAPM Review Course – July 19, 2014 WG at Angle Relative to Rotation Axis Gantry Klystron Beam transport system Accelerator structure Klyst Axis of gantry ron rotation Power supply room Treatment room Scale: feet Old Varian Clinac 35 Note that Elekta machines use a travelling w/g without a pit for the gantry and a magnetron AAPM insteadReview Courseof a klystron – July 19, 2014 90° vs.270° Magnet Schemes H. Enge Rev.Sci.Instr. 34:385-389; 1963 AAPM Review Course – July 19, 2014 Alternative Magnet Schemes AAPM Review Course – July 19, 2014 Beam Envelope of X-Ray Production AAPM Review Course – July 19, 2014 Principle of Flattening Filter note that the flattening filter design is a compromise between flat fields at small and large field sizes – hence the horns at max. field size AAPM Review Course – July 19, 2014 Flattening Filter, Scattering Foil Carousel electron scattering end of foil waveguide Clinac 18 flattening filter AAPM Review Course – July 19, 2014 Top of Gantry (at 270°) Energy switch AAPM Review Course – July 19, 2014 High Energy Linac With No Covers AAPM Review Course – July 19, 2014 Thin vs.