Lecture (4)

Treatment machines for external beam
radiotherapy

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:
1. Radioactive source
2. Source housing, including beam collimator and source
movement mechanism.
3. Gantry and stand.
4. Patient support assembly.
5. Machine control console.
1-Definition of Teletherapy machines

2-Teletherapy sources
The most widely used teletherapy source uses
60
Co radionuclides contained inside a cylindrical
stainless steel capsule and sealed by welding.
A double welded seal is used to prevent any leakage
of the radioactive material.
The 60
Co adionuclides in a teletherapy source decay
with a half-life of 5.26 years into 60
Ni with the
emission of electrons ( ß particles) with a maximum
energy of 320 keV and two g rays with energies of
1.17 MeV and 1.33 MeV.

Table : Physical properties of radionuclides used in external
beam radiotherapy
2-Teletherapy sources

The emitted g rays constitute the therapy beam; the
electrons are absorbed in the cobalt source or the source
capsule, where they produce relatively low energy and
essentially negligible bremsstrahlung X rays and
characteristic X rays.
Teletherapy sources are usually replaced within one half-
life after they are installed; however, financial
considerations often result in longer source usage.
2-Teletherapysources

3- Teletherapy source housing
The housing for the teletherapy source is called the source
head, and consists of
• A steel shell with lead for shielding purposes .
• Mechanism for bringing the source in front of the
collimator opening to produce the clinical g ray beam.
Currently two methods are in use for moving the
teletherapy source from the beam off into the beam on
position and back:
(i) a source on a sliding drawer
(ii) a source on a rotating cylinder.
Both methods incorporate a safety feature in which the
beam is terminated automatically in the event of a power
failure or emergency.

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

4- Dose delivery with teletherapy machines
The prescribed target dose is delivered with the help
of two treatment timers: Primary and Secondary.
• The primary timer actually controls the treatment
time.
• The secondary timer serves as a backup timer in
case of the primary timer’s failure.

5-The important features of 60Co teletherapy
machines can be summarized
as follows:
● Relatively high energy g ray emission;
● Relatively long half-life;
● Relatively high specific activity;
● Relatively simple means of production.

6- A comparison between cobalt and linear accelerator

3- Simulators and Computed Tomography Simulators

Simulators and CT simulators are important
components of equipment used in radiotherapy.
They cover several crucial steps in the
radiotherapeutic process that are not related to the
actual dose delivery but are nonetheless very
important, as they deal with the determination of
target location, treatment planning and spatial
accuracy in dose delivery.

1. Radiotherapy simulator
A radiotherapy simulator consists of a diagnostic X ray
tube mounted on a rotating gantry, simulating
geometries identical to those found on megavoltage
therapy machines that are either isocentric teletherapy
60Co units or isocentric linacs.
The simulator enjoys the same degrees of freedom as a
megavoltage machine, but rather than providing a
megavoltage beam for treatment it provides a diagnostic
quality X ray beam suitable for imaging, either in the
radiographic mode (image recorded on radiographic film)
or in the fluoroscopic mode (image recorded on a TV
monitor using an image intensifier).

In megavoltage machines, radiation fields are defined with
collimators (upper and lower jaws), while in simulators the
rectangular and square fields are defined with delineator
wires to enable visualization of the target as well as of
healthy tissues adjacent to the target.
A modern simulator covers the following processes:
● Tumour and adjacent normal tissue localization;
● Treatment simulation;
● Treatment plan verification;
● Monitoring of treatment.
1. Radiotherapy simulator

CT simulators are CT scanners equipped with special features that make
them useful for certain stages in the radiotherapeutic process.
The special features typically are:
A flat table top surface to provide a patient position during
simulation that will be identical to the position during treatment on a
megavoltage machine.
 A laser marking system to transfer the coordinates of the tumour
isocentre, derived from the contouring of the CT data set, to the
surface of the patient. Two types of laser marking systems are used:
a gantry mounted laser and a system consisting of a wall mounted
moveable sagittal laser and two stationary lateral lasers.
A virtual simulator consisting of software packages that allow the
user to define and calculate a treatment isocentre and then simulate
a treatment using {Digitally Reconstructed Radiographs } (DRRs).
2. Computed tomography simulator

Question 1: Write the suitable expression for the following definition
A. The
60
Co adionuclides in a teletherapy source decay with a half-life
of ……………………. into ………….. with the emission of electrons ( ß
particles) with a maximum energy of 320 keV and two g rays with
energies of ……………. and …......... MeV.
B. The housing for the teletherapy source is called the …………………, and
consists of
1- …………………………………………………………………………………………………………
2- …………………………………………………………………………………………………………
C. International regulations require that the average leakage of a
teletherapy machine head be less than ……………. mR/h at ……………m
from the source.
D. 60Co machines with an SAD of ……….cm while linacs with an SAD of
………cm.

E. Two methods are in use for moving the teletherapy
source from the beam off into the beam on position and
back:
(i)………………………………………………………………………………………
(ii)………………………………………………………………………………………
F. Simulators and CT simulators are important components
of equipment used in radiotherapy to
…………………………………………………………………………………………
…………………………………….………………………………….
G. A virtual simulator consisting of software packages that
allow the user to define and calculate a treatment
isocentre and then simulate a treatment using…………………
………………………………………………………………………………………..

Q2: Answer The Following
A. The main components of a teletherapy machine are :
1. …………………………………..
2. …………………………………..
3. …………………………………..
4. …………………………………...
5. ……………………………………
B. modern simulator covers the following processes:
1. ………………………………………………………………………………………………………
2. ………………………………………………………………………………………………………
3. ………………………………………………………………………………………………………
4. ………………………………………………………………………………………………………
Q3:Compare between LINAC and Cobalt 60

C. The important features of 60Co teletherapy machines
can be summarized as follows:
1. …………………………………………………………………………………….
2. …………………………………………………………………………………….
3. …………………………………………………………………………………….
4. …………………………………………………………………………………….

Lecture (4)

More Related Content

What's hot (20)

Similar to Lecture (4) (20)

Recently uploaded (20)

Lecture (4)