clinical radiation generators

CLINICAL RADIATION GENERATORS

P SATHISH KUMAR RNT MEDICAL COLLEGE UDAIPUR

Introduction

The aim of radiotherapy is maximum and uniform dose to the tumor volume and minimal dose to the normal tissue or organ at risk .

The successful of radiation therapy

treatment was depend on ability of radiation generating equipment.

Requirements of clinical radiation generators

High particle energy for penetration High particle flux for sufficient dose

rate Energy efficient Compact Not too expensive Reliable Simple to operate Safe

Brief History 1895 K.Roentogen discovers X-rays. 1913 W.E.Coolidge develops vacuum X-ray tube. 1931 E.O.Lawrence develops a cyclotron. 1932 1MV Van de Graaff accelerator installed, Boston, MA

(USA). 1939 First medical cyclotron for neutron therapy, Crocker, CA

(USA). 1946 20MeV electron beam therapy with a Betatron, Urbana,

IL (USA). 1952 First Co-60 teletherapy units, Saskatoon (Canada). 1956 First 6MeV linear accelerator, Stanford, CA (USA). 1958 First proton beam therapy (Sweden). 1959 First scanning electron beam therapy, Chicago, IL (USA). 1976 First pion beam therapy, LAMPF, NM (USA). 1990 First hospital based proton therapy, Loma Linda, CA

(USA).

Types of radiation generators

kVp therapy machines Van de Graff accelerator Betatron Microtron Linear accelerator Cyclotron Radioactive source based therapy

units

Kilovoltage Therapy Units

Photons are generated by hitting a solid target with electrons via Bremsstrahlung interactions.

The electron energy is below 300 keV or the acceleration potential is smaller than 300 kV.

The photon energy spectrum is broad

Type of kV therapy

Grenz-ray therapy: < 20 kV Contact therapy: 40 kV ~ 50 kV Superficial therapy: 50 kV ~ 150 kV Orthovoltage (or deep) therapy: 150

kV ~ 500 kv Super voltage therapy: 500 kV ~

1000 kV Megavoltage therapy: 1 MV

Contact Therapy

Accelerating potential = 40 to 50 kV. Tube current = 2 mA SSD = 2 cm Beam hardening by 0.5-to 1.0-mm

aluminum filter. Useful for tumors not deeper than

1mm

Superficial therapy

Accelerating potential = 50 to 150 kV. 1-to 6-mm aluminum for beam

hardening. Half-value layer (HVL) = 1-to 8-mm Al. Applicators or cones for field collimation. SSD = 15 to 20 cm. Tube current = 3 –8 mA. Useful for tumors confined to about 5-

mm dept

Orthovoltage therapy or Deep therapy

Potential = 150 to 500 kV. Beam current = 10 to 20 mA. HVL = 1 to 4 mm Copper (Cu). 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

Depth dose of kVp X-rays

A Grenz rays, HVL=0.0 4mm Al, φ=33cm, SSD=10cm

B Contact therapy, HLV= 1.5mm Al, φ=2cm, SSD=2cm

C Superficial therapy, HV L=3mm

Al, φ=3.6cm, SSD=20cm

D Orthovoltage, HVL=2mm Cu, 10x10cm2, SSD=50cm

E Co-60 γ-rays, 10x10cm2, SSD=80cm

Supervoltage therapy (1)

Energy: 500 –1000 kV

Conventional transformer systems were not suitable for producing potential > 300 kVp

The problem solved by invention of resonant transformer

Supervoltage therapy (2)

Resonant transformer units Used to generate x-rays from 300 to 2000 kV

At resonant frequencyOscillating potential attains very high amplitudePeak voltage across the x-ray tube becomes very large

Megavoltage Therapy

X-ray beams of energy > 1 MeV Accelerators or γ ray produced by

radionuclides Examples of clinical megavoltage

machinesVan de Graaff generator Linear accelerator Betatron Microtron Teletherapy γray units (e.g. cobalt-60)

Van de Graaff generator

The Van de Graaff machine is an electrostatic accelerator designed to accelerate charged particles.

In radiotherapy, the unit accelerates electrons to produce high-energy x-rays, typically at 2 MV.

In this machine, a charge voltage of 20 to 40 kV is applied across a moving belt of insulating material. A corona discharge takes place and electrons are sprayed onto the belt.

Van de Graaff generator

• These electrons are carried to the top where they are removed by a collector connected to a spherical dome.

• As the negative charges collect on the sphere, a high potential is developed between the sphere and the ground.

• This potential is applied across the x-ray tube consisting of a filament, a series of metal rings, and a target.

• The rings are connected to

resistors to provide a uniform drop of potential from the bottom to the top

X-rays are produced when the electrons strike the target. Van de Graaff machines are

capable of reaching energies up to 10 MV, limited only by size and required high-voltage insulation.

Van de Graaff and resonant transformer units for clinical use are no longer produced commercially.

The reason for their demise is the emergence of technically better machines such as cobalt-60 units and linear accelerators.

Betatron

Betarton was the first electron accelerator used for radiotherapy in early 1950s.

An electron in a changing magnetic field experiences accelerator in a circular orbit.

Can produce 6 MeV to 40 MeV electrons. Electron beam current is low for photon

therapy (or low dose rate). The field is small.

LINEAR ACCELERATOR

he linear accelerator (linac) is a device that

uses high-frequency electromagnetic waves to accelerate charged particles such as electrons to high energies through a linear tube.

The high-energy electron beam itself can be used for treating superficial tumors, or it can be made to strike a target to produce x-rays for treating deep-seated tumors

LOW ENERGY PHOTONS (4–8 MV):

straight-through beam; fixed flattening filter; external wedges; symmetric jaws; single transmission ionization chamber; isocentric mounting.

MEDIUM ENERGY PHOTONS (10–15 MV) and electrons: bent beam; movable target and flattening filter; scattering foils; dual transmission ionization chamber; electron cones

LINAC GENERATIONS

HIGH ENERGY PHOTONS (18–25 MV) AND ELECTRONS: dual photon energy and multiple electron energies; achromatic bending magnet; dual scattering foils or scanned electron pencil beam; motorized wedge; asymmetric or independent collimator jaws

High energy photons and electrons: computer controlled operation; dynamic wedge; electronic portal imaging device (EPID); multileaf collimator (MLC.

COMPONENTS OF MODERN LINACS

Linacs are usually mounted isocentrically and the operational systems are distributed over five major and distinct sections of the machine, the

● Gantry;● Gantry stand or support;● Modulator cabinet;●Patient support assembly (i.e. treatment

table);● Control console

The main beam forming components of a modern medical linac are usually grouped into six classes.

(i) Injection system(ii) RF power generation system(iii) Accelerating waveguide(iv) Auxiliary system(v) Beam transport system(vi) Beam collimation and beam

monitoring system.

INJECTION SYSTEM

The injection system is the source of electrons; it is essentially a simple electrostatic accelerator called an electron gun.

Two types of electron gun are in use as sources of electrons in medical linacs:

Diode type; Triode type

Electrons are thermionically emitted from the heated cathode, focused into a pencil beam by a curved focusing electrode and accelerated towards the perforated anode through which they drift to enter the accelerating waveguide.

RADIOFREQUENCY POWER GENERATION SYSTEM

The microwave radiation used in the

accelerating waveguide to accelerate electrons to the desired kinetic energy is produced by the RF power generation system, which consists of two major components

An RF power source; A pulsed modulator.

The RF power source is either a magnetron or a klystron. both are devices that use electron

acceleration and deceleration in a vacuum for the production of high power RF fields.

MAGNETRON

The magnetron is a device that produces microwaves.

MAGNETRON

MAGNETRON

It functions as a high-power oscillator, generating microwave pulses of several microseconds' duration and with a repetition rate of several hundred pulses per second.

The magnetron has a cylindrical construction, having a central cathode and an outer anode with resonant cavities machined out of a solid piece of copper

MAGNETRON

The electrons emitted from the cathode are accelerated toward the anode by the action of the pulsed DC electric field.

Under the simultaneous influence of the magnetic field, the electrons move in complex spirals toward the resonant cavities, radiating energy in the form of microwaves.

MAGNETRON

The generated microwave pulses are led to the accelerator structure via the waveguide.

Typically, magnetrons operate at 2 MW peak power output to power low-energy linacs (6 MV or less).

Although most higher-energy linacs use klystrons, accelerators of energy as high as 25 MeV have been designed to use magnetrons of about 5 MW power.

KLYSTRON

The klystron is not a generator of microwaves but rather a microwave amplifier.

It needs to be driven by a low-power microwave oscillator.

The electrons produced by the cathode are accelerated by a negative pulse of voltage into the first cavity, called the buncher cavity, which is energized by low-power microwaves.

klystron

klystron

The microwaves set up an alternating electric field across the cavity.

The velocity of the electrons is altered by the action of this electric field to a varying degree by a process known as velocity modulation.

Some electrons are speeded up while others are slowed down and some are unaffected.

klystron

This results in bunching of electrons as the velocity-modulated beam passes through a field-free space in the drift tube.

The electrons suffer deceleration, and by the principle of conservation of energy, the kinetic energy of electrons is converted into high-power microwaves.

ACCELERATING WAVEGUIDE

Waveguides are evacuated or gas filled metallic structures of rectangular or circular cross-section used in the transmission of microwaves.

The length of the accelerating waveguide depends on the final electron kinetic energy, and ranges from ~30 cm at 4 MeV to ~150 cm at 25 MeV.

Two types of waveguide are used in linacs:

1. RF power transmission waveguides 2. accelerating waveguides.


The power transmission waveguides transmit the RF power from the power source to the accelerating waveguide in which the electrons are accelerated.

The electrons are accelerated in the accelerating waveguide by means of an energy transfer from the high power RF fields, which are set up in the accelerating waveguide and are produced by the RF power generators.


The simplest kind of accelerating waveguide is obtained from a cylindrical uniform waveguide by adding a series of discs (irises) with circular holes at the centre, placed at equal distances along the tube.

These discs divide the waveguide into a series of cylindrical cavities that form the basic structure of the accelerating waveguide in a linac.

Two types of accelerating waveguide have been developed for the acceleration of electrons:

Travelling wave structure Standing wave structure

In the travelling wave structure the microwaves enter the accelerating waveguide on the gun side and propagate towards the high energy end of the waveguide.

TRAVELLING WAVEGUIDES

These guides have relatively low shunt impedances compared to standing waveguide systems therefore, they need to be physically longer to achieve the same output energy.

Electrons from the gun end enter a velocity of 0.8 c (at 80 kV) where c is the velocity of light.

After the first 30 cm, they are travelling at velocities close to c.

This first part of the guide is called the buncher section.

Thereafter, further energy gain results in a relativistic mass increase, and the iris separation remains constant.

WAVE GUIDE

In this configuration only one in four cavities is at any given moment suitable for electron acceleration, providing an electric field in the direction of propagation.

Standing wave structure

In the standing wave structure each end of the accelerating waveguide is terminated with a conducting disc to reflect the microwave power, resulting in a buildup of standing waves in the waveguide.

In this configuration, at all times, every second cavity carries no electric field and thus produces no energy gain for the electrons.

These cavities therefore serve only as coupling cavities and can be moved out to the side of the waveguide structure, effectively shortening the accelerating waveguide by 50%.

Electron beam transport

In low energy linacs the target is embedded in the accelerating waveguide and no beam transport between the accelerating waveguide and target is required.

Bending magnets are used in linacs operating at energies above 6 MeV, where the accelerating waveguides are too long for straight-through mounting.

The accelerating waveguide is usually mounted parallel to the gantry rotation

axis and the electron beam must be bent to make it strike the X ray target or be able to exit through the beam exit window.

● 90º bending;● 270º bending (achromatic);● 112.5º (slalom) bending

Electrons moving in a magnetic field will be bent in a trajectory dependent on the energy of the individual electron.

The requirement for the generation of satisfactory clinical beams is that all the electrons shall exit from the accelerating tube within a small focal spot and at the correct angle.

the electron beam at the input to the bending chamber will have a very narrow energy spread so that all the electrons will automatically follow the same trajectory.

precise control of beam energy is difficult to achieve and a more stable solution is to design a so-called achromatic bending system where electrons of different energies exit at the same point and in the same direction in spite of their having followed different trajectories

A 270o Magnet Using Hyperbolic Pole Faces

more energetic electrons (larger radii) enter closer spaced regions with higher bending fields, and less energetic electrons encounter lower bending fields between the wider pole spaces.

all electrons that entered on axis at 08 should converge again at the same point at 2708. The deflection is without dispersion with energy (i.e. achromatic).

Linac treatment head

The linac head contains several components that influence the production, shaping, localizing and monitoring of the clinical photon and electron beams.

The important components found in a typical head of a fourth or fifth generation linac include.


Electrons originating in the electron gun are accelerated in the accelerating waveguide to the desired kinetic energy and then brought, in the form of a pencil beam, through the beam transport system into the linac treatment head, where the clinical photon and electron beams are produced.

linac include: Several retractable X ray targets; Flattening filters and electron scattering

foils (also called scattering filters); Primary and adjustable secondary

collimators; Dual transmission ionization chambers;

A field defining light and a range finder; Optional retractable wedges; Optional MLC

The important components found in a typical head of a fourth or fifth generation


Clinical photon beams are produced with a target–flattening filter combination.

Clinical electron beams are produced by retracting the target and flattening filter from the electron pencil beam and Either scattering the pencil beam with a single or dual scattering foil .

The primary collimator defines a maximum circular field, which is then further truncated with an adjustable rectangular collimator consisting of two upper and two lower independent jaws and producing rectangular and square fields with a maximum dimension of 40 × 40 cm2 at the linac isocentre.

DOSE MONITORING SYSTEM

IEC specifies in detail the standards for radiation monitors installed in clinical electron linacs.

It deals with standards for the type of radiation detectors, display of monitor units (MUs),

Most common dose monitors in linacs are transmission ionization chambers permanently imbedded in the linac clinical photon and electron beams to monitor the beam output continuously during patient

treatment.

Most linacs use sealed ionization chambers to make their response independent of ambient temperature and pressure.

For patient safety, the linac dosimetry system usually consists of two separately sealed ionization chambers with completely independent biasing power supplies and readout electrometers.

if the primary chamber fails during patient treatment, the secondary chamber will terminate the irradiation, usually after an additional dose of only a few per cent above the prescribed dose has been delivered.

Cyclotron

Charged particles are accelerated by electric field cyclically.

Charged particles fly in circular orbits in magnetic field.

The radius of the orbits increases as the particle speed increases.

Synchrotron was invented to overcome the energy limit.

Medical applications of cyclotron

Used to generate high energy protons and heavy ions for therapy.

Used to accelerate deuterons to produce neutrons.

Used for the production of radionuclide's . i.e. for PET

Diagram of Cyclotron

Machines using radionuclides

Radionuclides

Half-life[years]

energy [MeV]

Γvalue[Rm2/Ci hr]

[Ci/g]

Cs -137 30 0.66 0.325 50

Co 60 5.26 Avg 1.25 1.33 200

Radioactive source based therapy unit

Co-60 unit 60Co is generated from 59Co via 59Co(n,γ)

60Co reactions. The half-life for 60Co is 5.26 years. 60Co decays to 60Ni with the emission of

βparticles and two photons (1,17 MeV and 1.33 MeV) per disintegration.

The Co source is a cylindrical capsule of 1 cm diameter and 2 cm long, causing relative large penumbra.

β particles are absorbed in the Co-metal and the stainless steel capsule.

Cobalt 60 unit

Cobalt-60 head

1 Cobalt-60source; 2 tungsten cylinder; 3 enriched uranium; 4, lead; 5 laser source; 6collimator; 7 g-rays

Source Housing

The housing for the source is called the sourcehead .

It consists of a steel shell filled with lead for shielding purposes and a device for bringing the source in front of an opening in the head from which the useful beam emerges. Also, a heavy metal alloy sleeve is provided to form an additional primary shield when the source is in the off position.

A number of methods have been developed for moving the source from the off position to the on position.

(a) the source mounted on a rotating wheel inside the sourcehead to carry the source from the off position to the on position;

(b) the source mounted on a heavy metal drawer plus its ability to slide horizontally through a hole running through the sourcehead in the on position the source faces the aperture for the treatment beam and in the off position the source moves to its shielded location and a light source mounted on the same drawer occupies the on position of the source

The 60Co source, usually in the form of a solid cylinder, discs, or pallets, is contained inside a stainless steel capsule and sealed by welding.

This capsule is placed into another steel

capsule, which is again sealed by welding. The double-welded seal is necessary to prevent any leakage of the radioactive material.

All of the above mechanisms incorporate a safety feature in which the source is returned automatically to the off position in case of a power failure.

Beam Collimation and Penumbra

A collimator system is designed to vary the size and shape of the beam to meet the individual treatment requirements.

The simplest form of a continuously adjustable diaphragm consists of two pairs of heavy metal blocks

Each pair can be moved independently to obtain a square- or a rectangle-shaped field. Some collimators are multivane type (i.e., multiple blocks to control the size of the beam).

The term penumbra, in a general sense, means the region, at the edge of a radiation beam, over which the dose rate changes rapidly as a function of distance from the beam axis.

penumbra

the penumbra width increases with an increase in source diameter, SSD, and depth but decreases with an increase in SDD.

The geometric penumbra, however, is independent of field size as long as the movement of the diaphragm is in one plane; that is, SDD stays constant with an increase in field size.

Thank you

clinical radiation generators

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