This document discusses different types of radiation therapy machines from low to high energy units. It provides details about kilovoltage units including Grenz ray therapy, contact therapy, and orthovoltage therapy. It then discusses megavoltage units including linear accelerators (LINACs). The summary describes LINACs using electromagnetic waves to accelerate electrons, which then produce x-rays either directly or by striking a target. It discusses major LINAC components including the electron gun, waveguide, and treatment head for beam shaping and monitoring.

1. LINAC
 Dr. Neeraj K. Rathee
 Pg Jr II

2. Xray therapy
Kilovoltage units
Grenz ray therapy
Contact therapy
Superficial therapy
Orthovoltage/deep
therapy
Super voltage
therapy
Megavoltage units
Van de graff
generator
Linear accelerators

3. KILOVOLTAGE UNITS
 GRENZ-RAY THERAPY:
 treatment with very soft low-energy x-rays <20 kV. No longer used
due to very low penetrance.

4. CONTACT THERAPY: A contact or endocavitary therapy machine
operates at potentials of 40-50 kV.
SSD of 2.0 cm or less.
Due to very short SSD and low voltage, the skin surface is maximally
irradiated but the underlying tissues are spared .
Useful for tumors <1 to 2 mm deep. The beam is almost completely
absorbed with 2 cm of soft tissue.
Have been used in the treatment of superficial rectal cancers.

5.  SUPERFICIAL THERAPY: x-rays produced at potentials 50-150
kV.
 SSD typically ranges between 15-20 cm.
 This superficial beam is useful for irradiating tumors confined to
about 5-mm depth (~90% depth dose).
 Beyond this depth, the dose drop off is too severe to deliver adequate
depth dose without overdosing of the skin surface.

6.  ORTHOVOLTAGE/ DEEP THERAPY: treatment
with x-rays produced at potentials 150 to 500 kV.
 Most orthovoltage equipment is operated at 200 to 300
kV and 10 to 20 mA.
 The SSD is usually set at 50 cm and the maximum dose
occurs close to the skin surface, with 90% dose at about
2 cm.

7.  Severe limitations to the use of orthovoltage beam in treating lesions
deeper than 2 to 3 cm.
 The greatest limitation is the skin dose
 Now with modern technologies, the skin-sparing properties of higher
energy radiation became the major reason for the modern trend to
megavoltage beams.

8.  SUPERVOLTAGE THERAPY: X-ray therapy in the range of 500 to 1,000 kV.
 Ex: Resonant Transformer Units
 In this apparatus, a combination of the transformer secondary and the capacitance in
parallel exhibits the phenomenon of resonance.
 At the resonant frequency, the oscillating potential attains very high amplitude.
 Thus, the peak voltage across the x-ray tube becomes very large when the
transformer is tuned to resonate at the input frequency.
 Hence, the electrons attain high energies before striking the target.
 a transmission-type target is then used to obtain the x-ray beam on the other side of
the target.

9.  MEGAVOLTAGE THERAPY: X-ray beams of energy 1 MV or
greater.
 The term strictly applies to the x-ray beams, however the γ-ray beams
produced by radionuclides are also commonly included in this
category if their energy is >/=1 MeV.
 Examples of clinical megavoltage machines are :
 Van de Graaff generator
 Linear accelerator
 Betatron and Microtron
 Teletherapy γ-ray units such as Cobalt-60.

10. LINEAR ACCELERATORS

11. Uses EM waves to accelerate charged particles like
electrons to high energies through an accelerator.
Electron trajectories are linear in the accelerator tube, hence
the name ‘LINEAR ACCELERATOR’
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.

12. MAJOR COMPONENTS
 Power Supply
 Modulator
 Magnetron
 Electron Gun
 Wave Guide system
 Accelerator Tube
 Bending Magnet
 Treatment Head
 Treatment table (Couch)

13.  A power supply provides DC power to the modulator, which includes
the pulse-forming network .
 High- voltage pulses from the modulator section are flat-topped DC
pulses of a few microseconds in duration.
 These pulses are delivered to the magnetron and simultaneously to the
electron gun.
 Pulsed microwaves produced in the magnetron are injected into the
accelerator tube or structure via a waveguide system.
 At that proper instant, the electrons, produced by the electron gun, are
also pulse injected into the accelerator structure.

14.  The accelerator structure consists of a copper tube with its interior
divided by copper disks or diaphragms of varying aperture and spacing
.
 The electrons are injected into the accelerator structure with an initial
energy of about 50 keV.
 The electrons interact with the electromagnetic field of the
microwaves.
 The electrons gain energy from the sinusoidal electric field by the
acceleration process.
 As the high-energy electrons emerge from the exit window of the
accelerator structure, they are in the form of a pencil beam of about 3
mm in diameter..

15.  In the low-energy linacs (up to 6 MV) with relatively short accelerator
tube, the electrons are allowed to proceed straight on and strike a target
for x-ray production.
 In the higher-energy linacs, however, the accelerator structure is too
long and, therefore, is placed horizontally or at an angle with respect to
the horizontal.
 The electrons are then bent through a suitable angle (usually about 90
or 270 degrees) between the accelerator structure and the target.
 The bending of the electron beam is accomplished by the beam
transport system consisting of bending magnets, focusing coils, and
other components.

16. MODULATOR
 The Modulator cabinet contains three major components:
 Fan control (cooling the power-distribution system).
 Auxiliary power distribution system (contains the emergency off
button that shuts off the power to the treatment unit )
 Primary power-distribution system .

17. ELECTRON GUN

18. Produces electrons and injects them into the accelerator
structure .
Tungsten Mesh/coil produces electrons by thermionic
emission when voltage is applied in terms of “Filament
current”.
It is pulsed so that the electrons are injected into the waveguide
at the same time as it is energized by the microwaves.
The number of electrons ejected depends upon the
temperature of the filament.

19. WAVE GUIDE
 Accelerator wave guide/accelerator structure, mounted in the gantry:
Horizontally (High-energy machines)
Vertically (Low-energy machines ).
 It is the tube in which the acceleration of the electrons’ takes place

20.  Accelerating electrons tends to diverge, partly by the
mutual coulomb repulsion and mainly by the radial
component of electric field in waveguide structure.
 Electrons are focused back to their path by the use of co-
axial magnetic focusing field generated.
 It is done by the coils which are coaxial with accelerating
waveguide.(Also called as steering coils)

21.  Accelerator tubes accelerate electrons either by traveling or stationary
EM waves of frequency in the microwave region (~3000 Mhz)
 The difference between traveling wave and stationary wave
accelerators is the design of the accelerator structure.
 The traveling wave structure has a terminating load to absorb the
residual power at the end of the structure, thus preventing a
backward reflected wave.
 Standing wave guide structure helps in reducing the accelerating
length due to option of side coupling cavities.

22.  The standing wave structure provides maximum reflection of the
waves at both ends of the structure.
 Hence, a combination of forward and reverse traveling waves gives
rise to stationary waves.
 The standing wave design tends to be more efficient.
 However, it is more expensive and requires installation of an isolator
to prevent reflections from reaching the power source.

23. LINAC

24. MAGNETRON
 It is a device that produces microwaves. It functions as a high-
power oscillator, generating microwave pulses.
 The frequency within each pulse is about 3,000 MHz.
 It has a cylindrical construction, having a central cathode and an
outer anode with resonant cavities made of a solid piece of copper.
 The cathode is heated by an inner filament and the electrons are
generated by thermionic emission.
 A static magnetic field is applied perpendicular to the plane of the
cross section of the cavities and a pulsed DC electric field is applied
between the cathode and the anode.

26.  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.
 The generated microwave pulses are led to the accelerator structure
via the waveguide.
 Typically, magnetrons operate at a 2-MW peak power output to
power low-energy linacs (6 MV or less).

28. ELECTRON BEAM
 Electron beam, as it exits the window of the accelerator tube, is a
narrow pencil about 3 mm in diameter.
 This beam is made to strike a scattering foil to spread the beam
and get a uniform electron fluence across the treatment field.
 The scattering foil is of a thin high-Z metallic foil (lead).
 The thickness of the foil is such that most of the electrons are
scattered instead of suffering bremsstrahlung.
 However, a small fraction still appears as x-ray contamination of the
electron beam.

30. X-RAY BEAM
 Bremsstrahlung x-rays are produced when the electrons are incident
on a target of a high-Z material (tungsten).
 The target is water cooled, and it is thick enough to absorb the incident
electrons.
 As a result of bremsstrahlung-type of interactions ,the electron energy
is converted into a spectrum of x-ray energies
 The average photon energy of the beam is approximately one-third of
the maximum energy.

32. TREATMENT HEAD
 Treatment head comprises of components that are designed to shape
and monitor the treatment beam.
 It contains –
 x-ray target,
 scattering foil,
 flattening filter,
 ion chamber,
 fixed and movable collimator,
 and light localizer system.
 The head provides sufficient shielding against leakage radiation in
accordance with radiation protection guidelines.

33. FLATTENING FILTER
 Since linear accelerators produce electrons in the megavoltage range,
 The x-ray intensity is peaked in the forward direction.
 To make the beam intensity uniform across the field, a flattening
filter is inserted in the beam.
 This filter is usually made of lead, although tungsten, uranium, steel,
aluminum, or a combination has also been used.

34. BEAM COLLIMATION AND MONITORING
 The treatment beam is first collimated by a fixed primary collimator
located immediately beyond the x-ray target.
 The beam is then incident on the dose monitoring chambers.
 The monitoring system consists of ion chambers
 The function of the ion chamber is to
 monitor dose rate,
 integrated dose,
 and field symmetry.

35.  After passing through the ion chambers, the beam is further collimated
by a continuously movable x-ray collimator.
 This collimator consists of two pairs of lead or tungsten blocks (jaws)
which provide a rectangular opening from 0 × 0 to the maximum of
40 × 40 cm2
 They are projected at a standard distance such as 100 cm from the x-
ray source (focal spot on the target).
 In addition to the x-ray jaws, modern LINACS are equipped with
multileaf collimators (MLC) to provide irregularly shaped field
blocking and intensity modulation for intensity-modulated radiation
therapy (IMRT).

36. MULTILEAF COLLIMATORS
 MLC’s are a bank of large number of collimating blocks or
leaves.
 Each can be moved automatically independently to generate a field
of any shape.
 40-80 pairs of leaves,
 each having a width of 1 cm or less as projected at the isocenter.
 Primary transmission through the leaves < 2%. (jaws -1%, and
cerroband –3.5%)

38.  Double focus or single focus leaves.
 Single Focused Leaves : Leaf ends follow beam divergence
 Reduces width of penumbra, allows better conformality.
 Double Focused Leaves : Both Leaf Ends and Leaf Sides match
Beam Divergence
 Double-focused MLCs are difficult to manufacture
 They provide constant beam transmission through a leaf edge,
regardless of its position in the field.

39.  In order to allow fast interleaf
movement, while reducing radiation
transmission, a tongue and groove
design is often used.
 This design in turn leads to some
underdosing in the region of the
tongue
(17 – 25%).

40. MULTILEAF COLLIMATORS
Merits Demerits
Beam shaping is simple and less time
consuming
radiation leakage between the leaves
Software operated, so no need to enter
the room
Larger physical penumbra than
cerroband or jaws.
Overall treatment time is shortened Field boundary is not continous but
jagged due to the multiple leaves
making the field
Constant control and continous
adjustment of field shape is possible
Matching of field is a bit dificult.
Correction of field shape can be done
simply.

41. GANTRY
 Most LINACS are so constructed that the source of radiation can rotate
about a horizontal axis.
 The point of intersection of the collimator axis and the axis of rotation
of the gantry is known as the isocenter. (gantry, collimator and couch
axes)
 Patients are treated mostly in isocenter position in a LINAC.

42. EPID
 EPIDS’ allow to view the portal images instantaneously :
 1) real time images can be displayed on computer screen before
initiating a treatment.
 2) Portal images can also be stored on computer for later viewing or
archiving .
 EPIDs use flat panel arrays of solid state detectors based on
amorphous silicon (a-Si) technology .
 Daily imaging for treatment localization and verification become
feasible

44.  Acquisition, analysis, storage and distribution of portal images can
be performed with relative ease.
 A scintillator converts the radiation beam into visible photons.
 The light is detected by an array of photodiodes implanted on an
amorphous silicon panel.

45. RADIATION SAFETY INTERLOCKS
 Various Interlocks are present in LINAC to avoid any mishappening or
wrong treatment to the patient.
 The interlocking system helps in solving the problem particularly and
easily.
 Safety Interlocks include:
 1) Last Man Out Switch(LMO)
 2) Door interlock
 3) Beam ON/OFF Key etc.,
 Emergency switches are provided at all the systems of an LINAC unit
to completely turn Off the entire unit with only single switch during
emergency situations.

46. TREATMENT TABLE (COUCH)
 Treatment table : It is a mechanically movable motor driven
couch .
 Patient is positioned over the treatment table according to the
desired co-ordinates of planning.
 Patient is immobilized on it using the Immobilization devices.
 Treatment table can be moved Horizontal, Vertical as well as in
Rotational directions.

48.  Hand Pendent:
 It contains all the control switches which can be used to
access the movement of Gantry, Couch, Collimator jaws,
SSD etc.,

50. FIELD LIGHT AND LASERS
 Field Light :
Field localizing device, Used to display the position of the radiation
field on the patient skin.
 The light field size is in congruence with the radiation field size.
 Lasers:
 Lasers guide in determining Isocenter position.
 Also help in positioning of the patient.
 2 Side lasers, 1saggital and 1 Ceiling lasers are mounted on walls of
LINAC unit.

51. COOLING SYSTEM
 Heat dissipation is an important step for maintenance of the machine.
 The x-rays produced are almost the 1 percent of the electron
energy which is striking on the target.
 Hence 99% of the energy is converted to heat.
 This heat is needed to be cooled and that is achieved by the ‘Cooling
system’.
 Cooling system consists of ‘water chiller’ for cooling the water.
 Machine has water inlets and outlets to various parts of the LINAC for
cooling.

52. BETATRON
 Cyclic particle accelerator, first used for radiotherapy in the early
1950s.
 They preceded the LINACS but rarely used now.
 Principle: electron in a changing magnetic field experiences
acceleration in a circular orbit.
 The accelerating tube is a hollow doughnut and is placed between
the poles of an alternating current magnet.
 A pulse of electrons is introduced into this by an injector at the
instant that the AC cycle begins.

54.  As the magnetic field rises, the electrons accelerate continuously
and spin with increasing velocity around the tube.
 By the end of the quarter the electrons have achieved maximum
energy.
 At this instant the electrons are made to exit of the orbit by an
additional attractive force.
 The electrons then are made to produce a broad beam of electrons
or x-rays as needed.

55. CYCLOTRON
 Charged particle accelerator, mainly used for nuclear physics
research.
 source of high-energy protons for proton beam therapy in
radiotherapy.
 More recently, adopted for generating neutron beams.
 Another important use is as a particle accelerator for the production
of certain radionuclides.

56.  Consists of a short metallic cylinder divided into two sections,
usually referred to as Ds.
 The Ds are placed between the poles of a DC producing a constant
magnetic field.
 An alternating potential is applied between the two Ds.
 Positively charged particles such as protons or deuterons are
injected into the chamber at the center of the two Ds.

58.  Under the action of the magnetic field, the particles travel in a circular
orbit.
 The frequency of AC current is adjusted such that as the particle is
accelerated by the electric field of the right polarity.
 With each pass between the Ds, the particle gains energy and the
radius of its orbit increases.

59. Thank you