Chapter 4 Clinical Radiation Generators

Chapter 4 Clinical Radiation Generators
The physics of Radiation Therapy, pp Chapter 4 Clinical Radiation Generators

Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron
Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

X-ray machines for radiotherapy
The main components of a radiotherapy x-ray machine are: • X-ray tube • Ceiling or floor mount for the x-ray tube • Target cooling system • Control console • X-ray power generator

The components of a radiotherapy x-ray machine: • X-ray tube • Applicators

The main components of a typical therapy x-ray tube are:
• Water or oil cooled target (anode) • Heated filament (cathode)

X-ray machines for radiotherapy
With x-ray tubes the patient dose is delivered using a timer and the treatment time must incorporate a shutter correction time. 􀀁 In comparison with diagnostic radiology x-ray tubes, a therapy x-ray tube operates: • At about 10% of instantaneous current. • At about 10 times average energy input. • With significantly larger focal spot and a fixed rather than rotating anode.

Kilovoltage Units Up to above 1950
X-rays generated at voltages up to 300 kVps Still some use in the present era, esp. treatment of superficial skin lesions Kilovoltage Therapy Grenz-Ray Therapy Contact Therapy Superficial Therapy Orthovoltage Therapy or Deep Therapy Supervoltage Therapy

Kilovoltage Units Grenz-Ray Therapy Contact Therapy
Energy : < 20 kV Very low depth of penetration No longer used in R/T Contact Therapy Energy: 40 – 50 kV Short SSD (< 2 cm) Produces a very rapidly decreasing depth dose Max irradiated tissue : skin surface Application: Tumor not deeper than 1 – 2 mm

Kilovoltage Units Superficial Therapy Energy: 50 – 150 kV
HVLs: 1.0- – 8.0-mm Al Applicator or cone attached to the diaphragm SSD: 15 – 20 cm Tube current: 5 – 8 mA Application: tumors confined to about 5-mm depth

Kilovoltage Units Orthovoltage Therapy or Deep Therapy
Energy: 200 – 300 kV Tube current: 10 – 20 mA HVLs: 1 – 4 mm Cu Cones or movable diaphragm (continuous adjustable field size) SSD: 50 cm Application: tumor located < 2 –3 cm in depth Limitation of the treatment: skin dose Depth dose distribution Increase absorbed dose in bone Increase scattering

Kilovoltage Units Supervoltage Therapy Energy: 500 – 1000 kV
Technical problem Insulating the high-voltage transformer Conventional transformer systems were not suitable for producing potential > 300 kVp The problem solved by invention of resonant transformer

Kilovoltage Units Resonant transformer units
Used to generate x-rays from 300 to 2000 kV At resonant frequency Oscillating potential attains very high amplitude Peak voltage across the x-ray tube becomes very large

Megavoltage Therapy X-ray beams of energy > 1 MV
Accelerators or γray produced by radionuclides Examples of clinical megavoltage machines Van de Graaff generator Linear accelerator Betatron Microtron Teletherapy γray units (e.g. cobalt-60)

Clinical x-ray beams In the diagnostic energy range ( kVp) most photons are produced at 90 from the direction of electrons striking the target (x-ray tube). In the megavoltage energy range ( MV) most photons are produced in the direction of the electron beam striking the target (linac).

Van de Graaff Generator
Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Van de Graaff Generator
Electrostatic accelerator Energy of x-rays: 2 MV (typical), up to 10 MV Limiation: size high-voltage insulation No longer produced commercially Technically better machine (e.g. Co-60 units & linear accelerators)

Linear Accelerator Kilovoltage Units Van de Graaff Generator Betatron

LINACS Medical linacs are cyclic accelerators that accelerate
electrons to kinetic energies from 4 to 25 MeV using microwave radiofrequency fields: • 103 MHz : L band • 2856 MHz: S band • 104 MHz: X band 􀀁 In a linac the electrons are accelerated following straight trajectories in special evacuated structures called accelerating waveguides. As far as the accelerating electric field is concerned there are two main classes of accelerator: electrostatic and cyclic. • In electrostatic accelerators the particles are accelerated by applying an electrostatic electric field through a voltage difference, constant in time, whose value fixes the value of the final kinetic energy of the particle. • In cyclic accelerators the electric fields used for particle acceleration are variable and associated with a variable magnetic field. This results in some closed paths along which the kinetic energy gained by the particle differs from zero.

Linac generations During the past 40 years medical linacs have gone
through five distinct generations, each one increasingly more sophisticated: (1) Low energy x rays (4-6 MV) (2) Medium energy x rays (10-15 MV) and electrons (3) High energy x rays (18-25 MV) and electrons (4) Computer controlled dual energy linac with electrons (5) Computer controlled dual energy linac with electrons combined with intensity modulation

Linear Accelerator Use high frequency electromagnetic waves to acelerate charged particles (e.g. electrons) to high energies through a linear tube High-energy electron beam – treating superficial tumors X-rays – treating deep-seated tumors

Linear Accelerator Types of EM wave 1. Traveling EM wave
Required a terminating (“dummy”) load to absorb the residual power at the end of the structure Prevent backward reflection wave 2. Standing EM wave Combination of forward and reverse traveling waves More efficiency Axial beam transport cavities and the side cavities can be independently optimized More expensive Requires installation of a circulator (or insulator) between the power source the structure prevent reflections from reaching the power source

Linear Accelerator Fig 4.5. A block diagram of typical medical linear accelerator

Accelerating waveguide
In the standing wave accelerating structure each end of the accelerating waveguide is terminated with a conducting disk to reflect the microwave power producing a standing wave in the waveguide. Every second cavity carries no electric field and thus produces no energy gain for the electron (coupling cavities In the travelling wave accelerating structure the microwaves enter on the gun side and propagate toward the high energy end of the waveguide. Only one in four cavities is at any given moment suitable for acceleration

Microwave power transmission
􀀁 The microwave power produced by the RF generator is carried to the accelerating waveguide through rectangular uniform waveguides usually pressurized with a dielectric gas (freon or sulphur hexafluoride SF6). 􀀁 Between the RF generator and the accelerating waveguide is a circulator (isolator) which transmits the RF power from the RF generator to the accelerating waveguide but does not transmit microwaves in the opposite direction.

The Magnetron A device that produces microwaves
Functions as a high-power oscillator Generating microwave pulses of several microseconds with repetition rate of several hundred pulses per second Frequency of microwave within each pulse is about 3000 MHz Peak power output: 2 MW (for low-energy linacs, 6MV or less) 5 MW (for higher-energy linacs, mostly use klystrons)

The Magnetron The cathode is heated by an inner filament
Electrons are generated by thermionic emission Pulse E-field between cathode & anode Electron accelerated toward the anode Static B-field perpendicular to the plane of cavities Electron move in complex spirals toward the resonant cavities Radiating energy in form of microwave

The Klystron Not a generator of microwaves Microwave amplifier
Needs to be driven by a low-power microwave oscillator

The Klystron Electrons produced by the cathode
Passed in the drift tube (field-free space) Electrons are accelerated by –ve pulse into buncher cavity Lower level microwave set up an alternating E field across the buncher cavity Electrons arrive catcher cavity Generate a retarding E-field Electrons suffer deceleration KE of electrons converted into high-power microwaves Velocity of e- is altered by the action of E-field (velocity modulation) Some e- are speed up Other are slowed down

Schematic diagram of a modern fifth generation linac

Electron beam transport
Three systems for electron beam bending have been developed: • 90o bending • 270o bending • 112.5o (slalom) bending

The Linac X-Ray Beam Production of x-rays
Electrons are incident on a target of a high-Z material (e.g. tungsten) Target – need water cooled & thick enough to absorb most of the incident electrons Bremsstrahlung interactions Electrons energy is converted into a spectrum of x-rays energies Max energy of x-rays = energy of incident energy of electrons Average photon energy = 1/3 of max energy of x-rays Designation of energy of electron beam and x-rays Electron beam - MeV (million electron volts, monoenergetic) X-ray beam – MV (megavolts, voltage across an x-ray tube, hetergeneous in energy)

Linac treatment head 􀀁 Components of a modern linac treatment head:
• Several retractable x-ray targets (one for each x-ray beam energy). • Flattening filters (one for each x-ray beam energy). • Scattering foils for production of clinical electron beams. • Primary collimator. • Adjustable secondary collimator with independent jaw motion. • Dual transmission ionization chamber. • Field defining light and range finder. • Retractable wedges. • Multileaf collimator (MLC).

Physical Wedge Beamline
2 34

Virtual Wedge Beamline
Dose Rate Control MU/min Jaw Speed Constant mm/sec 3 35

Multi Leaf Collimator (MLC)

Siemens 43 cm 29.2 cm 39.2 cm 55.0 cm 57.6 cm 32 cm Varian Elekta
Isocenter Siemens Varian Source 1.0 cm Resolution Jaw MLC Accessory Holder 43 cm 29.2 cm Elekta 100 cm Y X 1 X 2 39.2 cm 55.0 cm 57.6 cm 32 cm X

Lead or tungsten Opening from 0 x 0 to 40 x 40 cm at SSD 100 cm

Production of clinical x-ray beams
Typical electron pulses arriving on the x-ray target of a linac. Typical values: Pulse height: 50 mA Pulse duration: 2 μs Repetition rate: 100 pps Period: 104 μs

Collimation System In modern linacs the x-ray beam collimation is achieved with three collimation devices: • Primary collimator. • Secondary adjustable beam defining collimator (independent jaws). • Multileaf collimator (MLC). 􀀁 The electron beam collimation is achieved with: • Secondary collimator. • Electron applicator (cone). • Multileaf collimator (under development).

Production of clinical electron beam
􀀁 To activate the electron mode the x-ray target and flattening filter are removed from the electron pencil beam. 􀀁 Two techniques for producing clinical electron beams from the pencil electron beam: • Pencil beam scattering with a scattering foil (thin foil of lead). • Pencil beam scanning with two computer controlled magnets

Narrow pencil about 3 mm in diameter
Uniform electron fluence across the treatment field e.g. lead Electron scatter readily in air Beam collimator must be achieved close to the skin surface

Dose monitoring system
􀀁 Transmission ionization chambers, permanently embedded in the linac clinical x-ray and electron beams, are the most common dose monitors in linacs. 􀀁 Transmission ionization chambers consist of two separately sealed ionization chambers with completely independent biasing power supplies and readout electrometers for increased patient safety.

􀀁 Most linac transmission ionization chambers are permanently sealed, so that their response is not affected by ambient air temperature and pressure. 􀀁 The customary position for the transmission ionization chamber is between the flattening filter (for x-ray beams) or scattering foil (for electron beams) and the secondary collimator.

􀀁 The primary transmission ionization chamber measures the monitor units (MUs). 􀀁 Typically, the sensitivity of the primary chamber electrometer is adjusted in such a way that: • 1 MU corresponds to a dose of 1 cGy • delivered in a water phantom at the depth of dose maximum • on the central beam axis • for a 10x10 cm2 field • at a source-surface distance (SSD) of 100 cm.

􀀁 Once the operator preset number of MUs has been reached, the primary ionization chamber circuitry: • Shuts the linac down. • Terminates the dose delivery to the patient. 􀀁 Before a new irradiation can be initiated: • MU display must be reset to zero. • Irradiation is not possible until a new selection of MUs and beam mode has been made.

Betatron Kilovoltage Units Van de Graaff Generator Linear Accelerator

Betatron Betatron is a cyclic accelerator in which the electrons are
made to circulate in a toroidal vacuum chamber (doughnut) that is placed into a gap between two magnet poles. 􀀁 Conceptually, the betatron may be considered an analog of a transformer: • Primary current is the alternating current exciting the magnet. • Secondary current is the electron current circulating in the doughnut. toroid is a doughnut-shaped object,

Betatron Electron in a changing magnetic field experiences acceleration in a circular orbit Energy of x-rays: 6 – 40 MV Disadvantage: low dose rate Small field size

Microtron Kilovoltage Units Van de Graaff Generator Linear Accelerator
Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Microtron is an electron accelerator that combines the
features of a linac and a cyclotron. 􀀁 The electron gains energy from a resonant wave guide cavity and describes circular orbits of increasing radius in a uniform magnetic field. 􀀁 After each passage through the wave guide the electrons gain an energy increment resulting in a larger radius for the next pass through the wave guide cavity.

Microtron Electron accelerator which combines the principles of both linear accelerator and the cyclotron Advantage: Easy energy selection, small beam energy spread and small size

Cyclotron Kilovoltage Units Van de Graaff Generator Linear Accelerator
Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Cyclotron Charged particle accelerator
Mainly used for nuclear physics research As a source of high-energy protons for proton beam therapy Have been adopted for generating neutron beams recently

Cyclotron Structures Short metallic cylinder divided into two section (Ds) Highly evacuated Placed between the poles of a direct current magnet Alternating potential is applied between two Ds

Cyclotron In a cyclotron the particles are accelerated along a spiral trajectory guided inside two evacuated half-cylindrical electrodes (dees) by a uniform magnetic field produced between the pole pieces of a large magnet (1 T).

Cyclotron Positive charged particles (e.g. protons or deuterons) are injected at the center of the two Ds Under B-field, the particles travel in a circular orbit Accelerated by E-field while passing from one D to the other Received an increment of energy Radius of its orbit increases

Machine Using Radionuclides
Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

The important characteristics of radionuclides useful for
external beam radiotherapy are: • High gamma ray energy (of the order of 1 MeV). • High specific activity (of the order of 100 Ci/g). • Relatively long half life (of the order of several years). • Large specific air kerma rate constant. 􀀁 Of over 3000 radionuclides known only 3 meet the required characteristics and essentially only cobalt-60 is currently used for external beam radiotherapy.

Machines Using Radionuclides
Radionuclides have been used as source of γrays for teletherapy Radium-226, Cesium-137, Cobalt-60 60Co has proved to be most suitable for external beam R/T Higher possible specific activity Greater radiation output Higher average photon energy Radionuclide Half-Life (Years) γRay Energy MeV I- Value ( Rm2_) Ci – h Specific Activity Achieved in Practice (Ci/g) Radium-226 (filtered by 0.5 mm Pt) 1622 0.83 (avg.) 0.825 ~ 0.98 Cesium-137 30.0 0.66 0.326 ~ 80 Cobalt-60 5.26 1.17, 1.33 1/30 ~ 300

Teletherapy sources Teletherapy radionuclides: cobalt-60 and cesium-137 • Both decay through beta minus decay • Half-life of cobalt-60 is 5.26 y; of cesium-137 is 30 y • The beta particles (electrons) are absorbed in the source capsule.

Teletherapy machines Treatment machines used for external beam radiotherapy with gamma ray sources are called teletherapy machines. They are most often mounted isocentrically with SAD of 80 cm or 100 cm. 􀀁 The main components of a teletherapy machine are: • Radioactive source • Source housing, including beam collimator and source movement mechanism. • Gantry and stand. • Patient support assembly. • Machine control console

Teletherapy machines Cobalt-60 teletherapy machine, Theratron-780, AECL (now MDS Nordion), Ottawa, Canada

Cobalt-60 Unit Source Treatment beam Heterogeneity of the beam
From 59Co(n, γ) nuclear reactor Stable 59Co → radioactive 60Co In form of solid cylinder, discs, or pallets Treatment beam 60Co →60Ni + 0β(0.32 MeV) + γ(1.17 & 1.33 MeV) Heterogeneity of the beam Secondary interactions βabsorbed by capsule → bremsstrahlung x-rays (0.1MeV) scattering from the surrounding capsule, the source housing and the collimation system (eletron contamination)

Teletherapy sources To facilitate interchange of sources from one teletherapy machine to another and from one radionuclide production facility to another, standard source capsules have been developed. 􀀁 Teletherapy sources are cylinders with height of 2.5 cm and diameter of 1, 1.5, or 2 cm. • The smaller is the source diameter, the smaller is the physical beam penumbra and the more expensive is the source. • Often a diameter of 1.5 cm is chosen as a compromise between the cost and penumbra.

Teletherapy sources 􀀁 Typical source activity: of the order of Ci ( TBq). 􀀁 Typical dose rates at 80 cm from source: of the order of cGy/min 􀀁 Teletherapy source is usually replaced within one half-life after it is installed. Financial considerations often result in longer source usage.

Teletherapy source housing
􀀁 The source head consists of: • Steel shell with lead for shielding purposes • Mechanism for bringing the source in front of the collimator opening to produce the clinical gamma ray beam. 􀀁 Currently, two methods are used for moving the teletherapy source from the BEAM-OFF into the BEAM-ON position and back: • Source on a sliding drawer • Source on a rotating cylinder

􀀁 Both methods (source-on-drawer and source-on-cylinder) incorporate a safety feature in which the beam is terminated automatically in the event of power failure or emergency. 􀀁 When the source is in the BEAM-OFF position, a light source appears in the BEAM-ON position above the collimator opening, allowing an optical visualization of the radiation field, as defined by the machine collimator.

􀀁 Some radiation (leakage radiation) will escape from the teletherapy machine even when the source is in the BEAM-OFF position. 􀀁 Head leakage typically amounts to less than 1 mR/h (0.01 mSv/h) at 1 m from the source. 􀀁 International regulations require that the average leakage of a teletherapy machine head be less than 2 mR/h (0.02 mSv/h).

Collimator and penumbra
Collimators of teletherapy machines provide square and rectangular radiation fields typically ranging from 5x5 to 35x35 cm2 at 80 cm from the source.

Penumbra The region, at the edge of a radiation beam, over which the dose rate changes rapidly as function of distance from the beam axis 1. Transmission penumbra 2. Geometric penumbra

Transmission penumbra
Source Transmission penumbra The region irradiated by photons which are transmitted through the edge of the collimator block The inner surface of the blocks is made parallel to the central axis of the beam The extent of this penumbra will be more pronounced for larger collimator opening Minimizing the effect The inner surface of the blocks remains always parallel to the edge of the beam Collimator SDD SSD

Radiation source: not a point source
Geometric penumbra Radiation source: not a point source e.g. 60 Co teletherapy → cylinder of diameter ranging from 1.0 to 2.0 cm From considering similar triangles ABC and DEC DE = CE = CD = MN = OF + FN – OM AB CA CB OM OM AB = s (source diameter) OF = SSD DE = Pd ( penumbra) Pd = s (SSD + d – SDD) SDD Parameters determine the width of penumbra

Geometric penumbra (con’t)
Solutions Extendable penumbra trimmer Heavy metal bars to attenuate the beam in the penumbra region Secondary blocks Placed closed to the patient for redifining the field Should not be placed < 15 – 20 cm, excessive electron contaminants Definition of physical penumbra in dosimetry Lateral distance between two specified isodose curves at a specified depth At a depth in the patient, dose variation at the field border Geometric, transmission penumbras + scattered radiation produced in the patient

Heavy Particle Beams Kilovoltage Units Van de Graaff Generator
Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams

Heavy Particle Beams Advantage Including Still experimental
Dose localization Therapeutic gain (greater effect on tumor than on normal tissue) Including neutrons, protons, deuterons, αparticles, negative pions, and heavy ions Still experimental Few institutions because of the enormous cost

Neutrons Sources of high energy neutron beams D-T generators
D-T generator, cyclotrons, or linear accelerators D-T generators 2H + 3H → 4He + 1n MeV Monoenergetic (14 MeV) Isotropic (same yield in all directions) Major problem Lack of sufficient dose rate at the treatment distance 15 cGy/min at 1 m Advantage Its size is small enough to allow isocentric mounting on gantry

Cyclotron 1 4 5 0 Stripping reaction 2H + 9Be → 10Be + 1n
Mostly in forward direction Spectrum of energies (40% - 50% of deuteron energy) Fig Neutron spectra produced by deuterons on beryllium target

Comparative beam characteristics n’s vs. Co-60

Protons and Heavy Ions Energy of therapeutic proton beams
150 – 250 MeV Sources: produced by cyclotron or linear accelerator Major advantage Characteristic distribution of dose with depth Bragg peak

Comparative beam characteristics
Heavy charged particle beams vs. n’s

Main Interactions of Protons
Electronic (a) ionization excitation Nuclear (b-d) Multiple Coulomb scattering (b), small q Elastic nuclear collision (c), large q Nonelastic nuclear interaction (d) p’ p nucleus g, n e (b) (c) (d) (a) q This slide needs to be remade!

Comparative beam characteristics
Electron beams & protons

Why Protons are advantageous
Relatively low entrance dose (plateau) Maximum dose at depth (Bragg peak) Rapid distal dose fall-off Energy modulation (Spread-out Bragg peak) RBE close to unity Depth in Tissue Relative Dose 10 MeV X-rays Modulated Proton Beam Unmodulated Proton Beam

1 mm 4 mm

Range energy relationship for protons
Use to calculate the range for other particles with the same initial velocity R1/R2 = (M1/M2) · (Z2/Z1)2 R1, R2 — particle range M1,M2 — Masses Z2, Z1 — the charges of the two particle e.g. Protons (150 MeV), Deuterons (300 MeV), Helium ions (600 MeV) have same range of about 16 cm water Range  A/Z2

Negative Pions Pi meson (pion, π)
Protons and neutrons are held together by a mutual exchange of pi mesons Mass : 237x of electron Charge : π+, π-, π0 Decay: π+ → μ+ + ν (mean life: 2.54 x 10-18) π- → μ- + ν (mean life: 2.54 x 10-18) π0 → hν1 + hν2(mean life: 2.54 x 10-18) μ— mesons; ν — neutrinos

Negative Pions Pi meson (pion, π) (con’t) Sources:
nuclear reactor Cyclotron or linear accelerator with protons (400 – 800 MeV) and beryllium as target material Energy range of pion interest in R/T — 100 MeV Range in water about — 24 cm Problems Low dose rates Beam contamination High cost

Chapter 4 Clinical Radiation Generators

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