2. • The 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
4. EVOLUTION OF X-RAY THERAPY:
GRENZ RAY THERAPY:
The term Grenz ray therapy is used to describe treatment
with beams of very soft (low-energy) x-rays produced at
potentials below 20 kV.
Because of the very low depth of penetration such radiations
are no longer used in radiation therapy.
5. CONTACT THERAPY:
A contact therapy or endocavitary machine operates at potentials of
40 to 50 kV and facilitates irradiation of accessible lesions at very
short source to surface distances (SSDs).
The machine operates typically at a tube current of 2 mA.
Applicators available with such machines can provide an SSD of
2.0 cm or less.
6. When this beam is incident on a patient, the skin surface is
maximally irradiated but the underlying tissues are spared to an
increasing degree with depth.
This quality of radiation is useful for tumors not deeper than 1 to
2 mm. The beam is almost completely absorbed with 2 cm of soft
tissue. Endocavitary x-ray machines have been used in the
treatment of superficial rectal cancers.
7. SUPERFICIAL THERAPY:
The term superficial therapy applies to treatment with x-rays produced at
potentials ranging from 50 to 150 kV. Varying thicknesses of filtration
(usually 1- to 6-mm aluminum) are added to harden the beam to a
desired degree.
The superficial treatments are usually given with the help of applicators
or cones attachable to the diaphragm of the machine. The SSD typically
ranges between 15 and 20 cm. The machine is usually operated at a
tube current of 5 to 8 mA.
8. Superficial beam of the quality shown 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 considerable overdosing of the
skin surface.
9. ORTHOVOLTAGE THERAPY:
The term orthovoltage therapy, or deep therapy, is used to describe
treatment with x-rays produced at potentials ranging from 150 to 500
kV.
Most orthovoltage equipment is operated at 200 to 300 kV and 10 to
20 mA.
The maximum dose occurs close to the skin surface, with 90% of
that value occurring at a depth of about 2 cm.
Thus, in a single-field treatment, adequate dose cannot be delivered
to a tumor beyond this depth.
10. Severe limitations to the use of orthovoltage beam in treating
lesions deeper than 2 to 3 cm.
The greatest limitation is the skin dose, which becomes
prohibitively large when adequate doses are to be delivered to
deep- seated tumors.
Increased absorbed dose in bone and increased scattering that
make orthovoltage beams unsuitable for the treatment of tumors
behind bone.
11. SUPER-VOLTAGE THERAPY:
X-ray therapy in the range of 500 to 1,000 kV has been
designated as high- voltage therapy or supervoltage therapy
Major problem is insulating the high-voltage transformer.
12. MEGAVOLTAGE THERAPY:
X-ray beams of energy 1 MV or greater can be classified as
megavoltage beams.
Although the term strictly applies to x-ray beams, γ-ray beams
produced by radionuclides are also commonly included in this
category if their energy is 1 MeV or greater.
13. Examples of clinical megavoltage machines are
accelerators such as
• Van de Graaff generator,
• linear accelerator,
• betatron and microtron,
• teletherapy γ-ray units such as cobalt-60.
14.
15. CO-60 TELETHERAPY UNIT:
Cobalt 60 teletherapy units appear similar to early generation
clinical linear accelerators.
Most feature a rotating gantry and variable jaw collimation
located in the treatment head.
A light field and optical distance indicator is incorporated into
the treatment head to facilitate patient alignment.
Most Co-60 units use a source-to-axis-distance of 80cm
which compensates for the lower output as compared with a
linear accelerator.
16. When not in use, the source is stored in a heavily shielded
compartment of the treatment head.
Activation and deactivation of the beam is accomplished
by mechanically or pneumatically moving the source from
its shielded storage space to the treatment position
17. The major problem with these units is the
-decaying source, the source needs to be replaced every 5-7 years and is
becoming more and more expensive and is also hard to get.
-reduced output resulting in increased treatment times which in turn will
effectively reduce the patient output.
-Disposal of spent source is another major problem
-the dose-rate is determined by the amount of cobalt source in the machine
and cannot be regulated.
-the edges of the beam are less sharply defined- less precision in dose
delivery.
18. ADVANTAGES OF CO-60 UNIT OVER LINAC:
-lower capital and installation cost
-lower servicing and maintenance
-lesser dependance on reliable electric power
-simplicity of design
-ease of operation
57. GANTRY:
Source of radiation can rotate about a horizontal axis.
As the gantry rotates, the collimator axis (supposedly
coincident with the central axis of the beam) moves in a
vertical plane.
The point of intersection of the collimator axis and the
axis of rotation of the gantry is known as the isocenter
58.
59. MECHANICAL ISOCENTRE:
Point about which the Linac and the couch rotate
RADIATION ISOCENTRE:
Point where the radiation beams intersect when the
gantry, collimeter or couch are rotated
Ideally these two points are one and the same
60.
61. The main advantage of linear accelerators is that the
particles are able to reach very high energies without the
need for extremely high voltages.
The main disadvantage is that, because the particles travel
in a straight line, each accelerating segment is used only
once. This means that the only way of achieving particle
beams with even higher energy is to undertake the expense
of adding segments to the length of the linac.
