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ZMT 335
122/3/2017
Dr. Nik Noor Ashikin Bt Nik Ab Razak
RADIOTHERAPY & NUC MEDICINE
TOPIC 5 & 6
Radiotherapy Equipment
222/3/2017
Dr. Nik Noor Ashikin Bt Nik Ab Razak
OBJECTIVE
322/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
To understand
the design and
functionality of
the equipment
To review
physics and
technology of
external beam
radiotherapy
equipment
5.0 External Beam Equipment
5.1 Low-energy Machines
5.1.1 Superficial Equipment
5.1.2 Orthovoltage Units
5.2 Telecurie Units
5.2.1 Cs-131
5.2.2 Cobalt – 60 Unit
5.3 Linear accelerator (LINAC)
422/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
5.0 External Beam Equipment
522/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
Therapeutic
x-ray
equipment
• 10 kVp - 150 kVp (superficial);
• 150 kVp - 400 kVp (orthovoltage/ deep)
Radioactive
sources ( γ ray
equipment)
• Cobalt 60 & Cesium 137
MV
accelerators for
X and electron
therapy
• Linear accelerator
6
5.0 External Beam Equipment
5.1 Low-energy Machines
5.1.1 Superficial Equipment
5.1.2 Orthovoltage Units
722/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
• 10-15 kVp
• Treatment of inflammatory disorders
(Langerhans’ cells), Bowen’s disease,
patchystage mycosis fungoides,
herpes simplex
1.Grenz rays
• Superficial skin lesions
• Endocavitary treatments for curative
intent (rectal)
2.Contact therapy
5.1 Low-energy Machines
•Low-energy machines: Uses x-rays generated at voltages up to 500kVp
• 50-150 kVp
• Skin cancer and tumors no
deeper than 0.5 cm
3.Superficial equipment
• 150-500 kVp
• Skin, mouth, and cervical
carcinoma
• Experience limitation in the
treatment of lesions deeper
than 2 to 3 cm.
4.Orthovoltage machines
5.1 Low-energy Machines
“conventional”
X Ray tube
with electrons
accelerated by
an electric field
filtration
important
Stationary anode
(in contrast to
diagnostic tubes
which have a
rotating anode to
allow for a smaller
focal spot)
Part 5, lecture 2: Equipment - superficial,
telecurie
10
5.1 Superficial / Orthovoltage equipment5.1 Low-energy Machines
5.1 Superficial / Orthovoltage equipment
Can not reach
deep-seated
tumors with an
adequate dosage
of radiation
Do not spare
skin and
normal tissues.
LIMITATIONS
OF
LOW ENERGY
MACHINES
5.1 Low-energy Machines
Part 5, lecture 2: Equipment - superficial, telecurie 12
5.1 Superficial / Orthovoltage equipment
50 to 150kVp
small skin lesions
maximum applicator size
typically < 7cm
typical FSD < 30cm
beam quality measured in HVL
aluminium (0.5 to 8mm)
Superficial
150 to 500kVp
applicators or diaphragm
skin lesions, bone metastases
FSD 30 to 60cm
beam quality in HVL copper (0.2
to 5mm)
Orthovoltage
5.1 Low-energy Machines
5.1.1 Superficial Equipment
1322/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
Part 5, lecture 2: Equipment - superficial,
telecurie
14
Superficial X Ray tube (Philips RT 100)
• Manufacturers picture...
X Ray tube
Cooling
water Target
Applicator/
collimator
5.1.1 Superficial Equipment5.1.1 Superficial Equipment
Part 5, lecture 2: Equipment - superficial,
telecurie
15
X-ray tube
ApplicatorFilter
5.1.1 Superficial Equipment
X-ray
produced at
50-150 kV
Varying thickness
of filtration
(usually 1-6 mm
Al) are added to
harden the beam to
a desire degree
Superficial
treatment are
usually given
with the help of
applicators or
cones attachable
to the diaphragm
of the machine
SSD range 15
to 20 cm
5.1.1 Superficial Equipment
17
Superficial x-ray equipment (cont)
• Dose is highly dependent on source-skin distance, filtration and
applicator area.
5.1.1 Superficial Equipment
15 cm FSD cones
25cm FSD cones
5.1.1 Superficial Equipment5.1.1 Superficial Equipment
Usually operated
at 5-8 mA
Beyond this depth, the
dose drop-off is too
severe to deliver
adequate depth dose
without considerable
overdosing of the skin
surface
Useful for
irradiating
tumor confined
to about 5 mm
depth (~90%
depth dose)
5.1.1 Superficial Equipment
• Short focus to skin distance (FSD)
and hence high output and large
influence of inverse square law
• Calibration difficult due to strong
dose gradient i.e. dose fall off and
electron contamination
Issues with
Superficial
radiotherapy
5.1.2 Orthovoltage Units
2022/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
Uses conventional
X-ray tube
Energy range 150-
500 kV X-rays
Mostly used around
250 - 300 kVp
Applicators are used
in superficial therapy
Treatment depths of
around 20 mm
Penetration sufficient for
palliative treatment of bone
lesions relatively close to the
surface (ribs, spinal cord)
5.1.2 Orthovoltage Units5.1.2 Orthovoltage Units
5.1.2 Orthovoltage Units
5.1.2 Orthovoltage Units
• Higher dose to bone - photoelectric
absorption
• Maximum dose on the surface
hence higher skin dose
• Treatment to a depth of only a few
centimeters possible
• Low energy, hence high scattered
radiation and larger penumbra
Disadvantages
Of Deep X-ray
5.2 Telecurie Units
5.2.1 Cs-131
5.2.2 Cobalt – 60 Unit
2322/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
Part 5, lecture 2: Equipment - superficial, telecurie 24
5.2 Telecurie Units
Features
of a
Teletherapy
Source
high energy
gamma ray
emission high
specific air
kerma rate
constant
simple
means of
production
Low cost
high
specific
activity
long half-
life
Part 5, lecture 2: Equipment - superficial, telecurie 25
5.2.1 137-Cs
137-Cs
Photon energy 0.66MeV
Relatively large source to relatively low specific activity
Medium FSD (around 60cm)
No isocentric mounting - similar to orthovoltage equipment in set-up
Not sold anymore and should not be in use
5.2.2 Cobalt – 60 Unit
5.2.2.1 Properties
5.2.2.2 Application
5.2.2.3 Production
5.2.2.4 Source
5.2.2.5 Activity
5.2.2.6 Half-Life
5.2.2.7 Shielding
5.2.2.8 Penumbra
5.2.2.9 Dose Maximum
5.2.2.10 Equipment
5.2.2.10 Cobalt – 60 Equipment
5.2.2.11 Annual dose to staff
5.2.2.12 Gamma Knife 2622/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
Part VII.14.3 : Radiation Sources in Teletherapy Slide 28
Natural Cobalt (59Co)
COBALT - Kobald, from the German for goblin or evil spirit. Discovered in 1735. Brittle hard metal similar
to iron and nickel. Found in minerals and meteorites. Salts and glass oxides are deep blue in colour.
5.2.2 Cobalt – 60 Unit
5.2.2 Cobalt – 60 Unit
ORTHOVOLTAGE UNIT
150-500 KV x-rays
Maximum dose on the skin
Treatment to a depth of few centimeters
Higher absorption by bone
non uniform dose distribution
Higher side scatter hence larger penumbra
Telecobalt Unit
1.25 MeV ‫ﻻ‬ Photon
Maximum dose at depth of 5 mm
Relatively uniform dose absorption
Higher penetration deep seated tumours
Relatively uniform distribution
More of forward scatter, lesser penumbra
Mostly isocentric unit
5.2.2 Cobalt – 60 Unit
5.2.2 Cobalt – 60 Unit
Linear Accelerator
4 to 21 MV photon beams
Maximum dose at higher depth with energy
No radioactive source
Radiation only when the source is switched is ON
Uniform dose absorption
1mm source – nearly point source
Small Penumbra
Electron beam of various energies possible
Telecobalt Unit
1.25 MeV ‫ﻻ‬ Photon
Maximum dose at depth of 5 mm
Source to be changed every 4 to 5 years
Leakage radiation present even while the beam is
off
Relatively uniform distribution
1-2 cm source diameter
Larger penumbra
Gamma Photon only
5.2.2 Cobalt – 60 Unit
31
Introduced in the
1950’s, being replaced
by linacs.
The first practical
radiation therapy
treatment unit to
provide a significant
dose below the skin
surface and
simultaneously spare
the skin the harsh
effects of earlier
methods.
Still used in developing
countries: simpler
design, cost, little tech
support.
5.2.2 Cobalt – 60 Unit
32
Photon energy
around 1.25MeV
Specific activity
large enough for
FSD of 80cm or
even 100cm
Therefore,
isocentric set-up
possible
Constantly emit
radiation
60Co source must
be shielded in a
protective housing
(source head).
source head is a
steel shell filled
with lead (may be
up to 2 ft in
diameter
PROPERTIES
5.2.2.1 Properties
33
5.2.2 Cobalt – 60 Unit
5.2.2.2 Application
APPLICATION
To treat cancers of the
head and neck area,
breast, spine, and
extremities
Areas just below the skin
surface
Ideal in treating lymph nodes.
34
5.2.2 Cobalt – 60 Unit
5.2.2.3 Production of Cobalt – 60
1
• Cobalt: produced in nuclear reactors by the irradiation of neutrons
of the common stable form of 59Co.
2
• The 59Co nucleus absorbs a neutron in the reactor and becomes
60Co.
3
• Radioactive 60Co produces a useful therapy beam when it
undergoes beta decay
35
5.2.2 Cobalt – 60 Unit
5.2.2.3 Production of Cobalt – 60
4
• The nucleus emits a beta particle and then two photons, 1.17 MeV
and 1.33 MeV for an effective energy of 1.25 MeV
5
• 60Co  60Ni+ + B- + neutrino (v) + gamma rays
6
• Radioactive 60Co emits radiation in the form of high energy gamma
rays in an effort to return to its more stable state.
36
5.2.2 Cobalt – 60 Unit5.2.2.4 Cobalt – 60 Source
Of the close to 300 natural nuclides and over 3000 artificially produced
radionuclides, only four meet the teletherapy source requirements (Co-60, Cs-137,
Eu-152, and Ra-226) and only cobalt-60 is actually used in practice.
37
5.2.2 Cobalt – 60 Unit5.2.2.4 Cobalt – 60 Source
• 1 to 3 centimetersDiameter of a 60Co source
• Encased in multiple layers of welded metal to prevent
contamination of the environment and to absorb β-
particles produced by the decay process.
Source Form: Pellets of radioactive
60Co
• Smaller source with less penumbra for the same beam
intensity
• Less hazard of contamination should a source ever
become exposed to the environment.
Source Form: 60Co fused into a solid
cylinder
Part VII.14.3 : Radiation Sources in Teletherapy Slide 38
How does a teletherapy Cobalt source look?
3500 pellets; 275 Ci/g; 7700 Ci
5.2.2.4 Cobalt – 60 Source
Part VII.14.3 : Radiation Sources in Teletherapy Slide 39
Cobalt source – how does it look?
5.2.2.4 Cobalt – 60 Source
40
5.2.2 Cobalt – 60 Unit5.2.2.5 Cobalt – 60 Activity
SI unit: Curies (Ci)
3.7 x 1010 Becquerel
(Bq)
1 Bq = 1 disintegration
per second
also defined in rhm units
(roentgens per hour at 1
meter)
Most sources have an
activity of 750-9000 Ci,
typically 3000-9000 Ci
used in radiation therapy
41
•Half-life: the time necessary for a radioactive material to decay to half or
50% of its original intensity.
• Requires a correction factor for this decay of about 1% per month in all
treatment calculations.
• Source must be replaced at about five year intervals.
•The half-life of 60Co is 5.26 years.
5.2.2 Cobalt – 60 Unit5.2.2.6 Cobalt – 60 Half-Life
42
5.2.2 Cobalt – 60 Unit5.2.2.7 Cobalt – 60 Shielding
Cerrobend (Lipowitz metal):
Lower melting point than Pb, cheaper
50% Bismuth
26.7% Lead
13.3% tin
10% Cadmium (a toxic metal can get
into bloodstream
Density ratio of Cerrobend to Lead:
1.2 cm Cerrobend to 1 cm lead.
5 HVL is needed to reduce
intensity
A thickness of 7.2 cm of
Cerrobend needed, 6 cm lead.
46
• Penumbra: the area at the edge of the radiation beam at which the dose rate changes rapidly as
a function of distance from the beam axis.5.2.2 Cobalt – 60 Unit5.2.2.8 Penumbra
1
• Describes the edge of the field having full radiation intensity for the
beam compared with the area at which the intensity falls to 0.
2
• The larger the source size, the larger the penumbra
3
• Larger field sizes are necessary to cover the same amount of tissue
adequately compared to the linac.
4
• Geometric penumbra typically wide because source diameter is large
(>2cm)
Part 5, lecture 2: Equipment - superficial, telecurie 48
5.2.2 Cobalt – 60 Unit5.2.2.8 Penumbra
49
5.2.2 Cobalt – 60 Unit5.2.2.8 Penumbra
The transmission
penumbra can be
reduced by using
satellite
collimators,
penumbra trimmers
or trimmer bars
Trimmers are
metal bars that
attenuate the
edge of the
beam providing
a sharper field
edge.
It Should be
placed no closer
than 15 cm
from the
patients skin to
reduced
electron
contamination
(increased skin
dose) by metal
devices.
Provides
enough distance
for the
secondary
electrons
produced by the
trimmer bars to
lose sufficient
energy
REDUCING PENUMBRA
52
5.2.2 Cobalt – 60 Unit5.2.2.9 Dose Maximum
Dose maximum
(Dmax): when a
greater percentage
of dose occurs
below the skin
surface
Dmax is the depth
of maximum
buildup, in which
100% of the dose
is deposited
For 60Co, Dmax
occurs at 0.5 cm
below the skin
surface.
Part 5, lecture 2: Equipment - superficial,
telecurie
53
• Source head and transfer mechanism
5.2.2.10 Cobalt – 60 Equipment
Part 5, lecture 2: Equipment - superficial,
telecurie
54
5.2.2.10 Cobalt – 60 Equipment
• shield against the
primary cobalt-60
beam
Primary barriers
• shield against
leakage radiation and
radiation scattered
from the patient
Secondary barriers
Typical cobalt-60 teletherapy installation:
55
11. Beam On/Beam Off
5.2.2 Cobalt – 60 Unit5.2.2.10 Cobalt – 60 Equipment
• Turning the beam on requires physically exposing the source either by
moving it into position or by removing shields around the source.
Turning the beam
• the compressor generates air pressure by pushing the source horizontally
into position over the collimator opening.
Air pressure:
• the motor rotates a wheel 180 degrees by placing the source over the
collimator opening.
Rotating wheel:
Part 5, lecture 2: Equipment - superficial,
telecurie
56
5.2.2.10 Cobalt – 60 Equipment
Part 5, lecture 2: Equipment - superficial,
telecurie
57
Picture of a Co source change
5.2.2.10 Cobalt – 60 Equipment
Part 5, lecture 2: Equipment - superficial,
telecurie
58
•Assume:
• 200 days, 8hours per day working time per year
• 10% of this time in treatment room
• 3 Gy h-1 typical dose averaged over all locations of the staff member in the
treatment room
•Dose = 200 x 8 x 0.1 x 3 Gy
•  0.5mGy/year (half of dose limit for general public)
5.2.2 Cobalt – 60 Unit5.2.2.11 Annual dose to staff
5.2.2 Cobalt – 60 Unit5.2.2.12 GAMMA KNIFE
Therefore it is also known as the stereotactic surgery
Patient wears a specialized helmet that is surgically fixed to their skull (brain tumor
remains stationary at target point of the gamma rays)
It is placed in a circular array in a heavily shielded assembly
Aims gamma radiation through a target point in the patient's brain.
Contains 201 cobalt-60 sources of approximately 30 curies each
60
Patient positioning collimator
5.2.2 Cobalt – 60 Unit5.2.2.12 GAMMA KNIFE
Part 5, lecture 2: Equipment - superficial,
telecurie
62
5.2.2.12 GAMMA KNIFE

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Radiotherapy Equipment

  • 1. ZMT 335 122/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak RADIOTHERAPY & NUC MEDICINE
  • 2. TOPIC 5 & 6 Radiotherapy Equipment 222/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
  • 3. OBJECTIVE 322/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak To understand the design and functionality of the equipment To review physics and technology of external beam radiotherapy equipment
  • 4. 5.0 External Beam Equipment 5.1 Low-energy Machines 5.1.1 Superficial Equipment 5.1.2 Orthovoltage Units 5.2 Telecurie Units 5.2.1 Cs-131 5.2.2 Cobalt – 60 Unit 5.3 Linear accelerator (LINAC) 422/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
  • 5. 5.0 External Beam Equipment 522/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
  • 6. Therapeutic x-ray equipment • 10 kVp - 150 kVp (superficial); • 150 kVp - 400 kVp (orthovoltage/ deep) Radioactive sources ( γ ray equipment) • Cobalt 60 & Cesium 137 MV accelerators for X and electron therapy • Linear accelerator 6 5.0 External Beam Equipment
  • 7. 5.1 Low-energy Machines 5.1.1 Superficial Equipment 5.1.2 Orthovoltage Units 722/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
  • 8. • 10-15 kVp • Treatment of inflammatory disorders (Langerhans’ cells), Bowen’s disease, patchystage mycosis fungoides, herpes simplex 1.Grenz rays • Superficial skin lesions • Endocavitary treatments for curative intent (rectal) 2.Contact therapy 5.1 Low-energy Machines •Low-energy machines: Uses x-rays generated at voltages up to 500kVp
  • 9. • 50-150 kVp • Skin cancer and tumors no deeper than 0.5 cm 3.Superficial equipment • 150-500 kVp • Skin, mouth, and cervical carcinoma • Experience limitation in the treatment of lesions deeper than 2 to 3 cm. 4.Orthovoltage machines 5.1 Low-energy Machines
  • 10. “conventional” X Ray tube with electrons accelerated by an electric field filtration important Stationary anode (in contrast to diagnostic tubes which have a rotating anode to allow for a smaller focal spot) Part 5, lecture 2: Equipment - superficial, telecurie 10 5.1 Superficial / Orthovoltage equipment5.1 Low-energy Machines
  • 11. 5.1 Superficial / Orthovoltage equipment Can not reach deep-seated tumors with an adequate dosage of radiation Do not spare skin and normal tissues. LIMITATIONS OF LOW ENERGY MACHINES 5.1 Low-energy Machines
  • 12. Part 5, lecture 2: Equipment - superficial, telecurie 12 5.1 Superficial / Orthovoltage equipment 50 to 150kVp small skin lesions maximum applicator size typically < 7cm typical FSD < 30cm beam quality measured in HVL aluminium (0.5 to 8mm) Superficial 150 to 500kVp applicators or diaphragm skin lesions, bone metastases FSD 30 to 60cm beam quality in HVL copper (0.2 to 5mm) Orthovoltage 5.1 Low-energy Machines
  • 13. 5.1.1 Superficial Equipment 1322/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
  • 14. Part 5, lecture 2: Equipment - superficial, telecurie 14 Superficial X Ray tube (Philips RT 100) • Manufacturers picture... X Ray tube Cooling water Target Applicator/ collimator 5.1.1 Superficial Equipment5.1.1 Superficial Equipment
  • 15. Part 5, lecture 2: Equipment - superficial, telecurie 15 X-ray tube ApplicatorFilter 5.1.1 Superficial Equipment
  • 16. X-ray produced at 50-150 kV Varying thickness of filtration (usually 1-6 mm Al) are added to harden the beam to a desire degree Superficial treatment are usually given with the help of applicators or cones attachable to the diaphragm of the machine SSD range 15 to 20 cm 5.1.1 Superficial Equipment
  • 17. 17 Superficial x-ray equipment (cont) • Dose is highly dependent on source-skin distance, filtration and applicator area. 5.1.1 Superficial Equipment 15 cm FSD cones 25cm FSD cones
  • 18. 5.1.1 Superficial Equipment5.1.1 Superficial Equipment Usually operated at 5-8 mA Beyond this depth, the dose drop-off is too severe to deliver adequate depth dose without considerable overdosing of the skin surface Useful for irradiating tumor confined to about 5 mm depth (~90% depth dose)
  • 19. 5.1.1 Superficial Equipment • Short focus to skin distance (FSD) and hence high output and large influence of inverse square law • Calibration difficult due to strong dose gradient i.e. dose fall off and electron contamination Issues with Superficial radiotherapy
  • 20. 5.1.2 Orthovoltage Units 2022/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
  • 21. Uses conventional X-ray tube Energy range 150- 500 kV X-rays Mostly used around 250 - 300 kVp Applicators are used in superficial therapy Treatment depths of around 20 mm Penetration sufficient for palliative treatment of bone lesions relatively close to the surface (ribs, spinal cord) 5.1.2 Orthovoltage Units5.1.2 Orthovoltage Units
  • 22. 5.1.2 Orthovoltage Units 5.1.2 Orthovoltage Units • Higher dose to bone - photoelectric absorption • Maximum dose on the surface hence higher skin dose • Treatment to a depth of only a few centimeters possible • Low energy, hence high scattered radiation and larger penumbra Disadvantages Of Deep X-ray
  • 23. 5.2 Telecurie Units 5.2.1 Cs-131 5.2.2 Cobalt – 60 Unit 2322/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
  • 24. Part 5, lecture 2: Equipment - superficial, telecurie 24 5.2 Telecurie Units Features of a Teletherapy Source high energy gamma ray emission high specific air kerma rate constant simple means of production Low cost high specific activity long half- life
  • 25. Part 5, lecture 2: Equipment - superficial, telecurie 25 5.2.1 137-Cs 137-Cs Photon energy 0.66MeV Relatively large source to relatively low specific activity Medium FSD (around 60cm) No isocentric mounting - similar to orthovoltage equipment in set-up Not sold anymore and should not be in use
  • 26. 5.2.2 Cobalt – 60 Unit 5.2.2.1 Properties 5.2.2.2 Application 5.2.2.3 Production 5.2.2.4 Source 5.2.2.5 Activity 5.2.2.6 Half-Life 5.2.2.7 Shielding 5.2.2.8 Penumbra 5.2.2.9 Dose Maximum 5.2.2.10 Equipment 5.2.2.10 Cobalt – 60 Equipment 5.2.2.11 Annual dose to staff 5.2.2.12 Gamma Knife 2622/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
  • 27.
  • 28. Part VII.14.3 : Radiation Sources in Teletherapy Slide 28 Natural Cobalt (59Co) COBALT - Kobald, from the German for goblin or evil spirit. Discovered in 1735. Brittle hard metal similar to iron and nickel. Found in minerals and meteorites. Salts and glass oxides are deep blue in colour. 5.2.2 Cobalt – 60 Unit
  • 29. 5.2.2 Cobalt – 60 Unit ORTHOVOLTAGE UNIT 150-500 KV x-rays Maximum dose on the skin Treatment to a depth of few centimeters Higher absorption by bone non uniform dose distribution Higher side scatter hence larger penumbra Telecobalt Unit 1.25 MeV ‫ﻻ‬ Photon Maximum dose at depth of 5 mm Relatively uniform dose absorption Higher penetration deep seated tumours Relatively uniform distribution More of forward scatter, lesser penumbra Mostly isocentric unit 5.2.2 Cobalt – 60 Unit
  • 30. 5.2.2 Cobalt – 60 Unit Linear Accelerator 4 to 21 MV photon beams Maximum dose at higher depth with energy No radioactive source Radiation only when the source is switched is ON Uniform dose absorption 1mm source – nearly point source Small Penumbra Electron beam of various energies possible Telecobalt Unit 1.25 MeV ‫ﻻ‬ Photon Maximum dose at depth of 5 mm Source to be changed every 4 to 5 years Leakage radiation present even while the beam is off Relatively uniform distribution 1-2 cm source diameter Larger penumbra Gamma Photon only 5.2.2 Cobalt – 60 Unit
  • 31. 31 Introduced in the 1950’s, being replaced by linacs. The first practical radiation therapy treatment unit to provide a significant dose below the skin surface and simultaneously spare the skin the harsh effects of earlier methods. Still used in developing countries: simpler design, cost, little tech support. 5.2.2 Cobalt – 60 Unit
  • 32. 32 Photon energy around 1.25MeV Specific activity large enough for FSD of 80cm or even 100cm Therefore, isocentric set-up possible Constantly emit radiation 60Co source must be shielded in a protective housing (source head). source head is a steel shell filled with lead (may be up to 2 ft in diameter PROPERTIES 5.2.2.1 Properties
  • 33. 33 5.2.2 Cobalt – 60 Unit 5.2.2.2 Application APPLICATION To treat cancers of the head and neck area, breast, spine, and extremities Areas just below the skin surface Ideal in treating lymph nodes.
  • 34. 34 5.2.2 Cobalt – 60 Unit 5.2.2.3 Production of Cobalt – 60 1 • Cobalt: produced in nuclear reactors by the irradiation of neutrons of the common stable form of 59Co. 2 • The 59Co nucleus absorbs a neutron in the reactor and becomes 60Co. 3 • Radioactive 60Co produces a useful therapy beam when it undergoes beta decay
  • 35. 35 5.2.2 Cobalt – 60 Unit 5.2.2.3 Production of Cobalt – 60 4 • The nucleus emits a beta particle and then two photons, 1.17 MeV and 1.33 MeV for an effective energy of 1.25 MeV 5 • 60Co  60Ni+ + B- + neutrino (v) + gamma rays 6 • Radioactive 60Co emits radiation in the form of high energy gamma rays in an effort to return to its more stable state.
  • 36. 36 5.2.2 Cobalt – 60 Unit5.2.2.4 Cobalt – 60 Source Of the close to 300 natural nuclides and over 3000 artificially produced radionuclides, only four meet the teletherapy source requirements (Co-60, Cs-137, Eu-152, and Ra-226) and only cobalt-60 is actually used in practice.
  • 37. 37 5.2.2 Cobalt – 60 Unit5.2.2.4 Cobalt – 60 Source • 1 to 3 centimetersDiameter of a 60Co source • Encased in multiple layers of welded metal to prevent contamination of the environment and to absorb β- particles produced by the decay process. Source Form: Pellets of radioactive 60Co • Smaller source with less penumbra for the same beam intensity • Less hazard of contamination should a source ever become exposed to the environment. Source Form: 60Co fused into a solid cylinder
  • 38. Part VII.14.3 : Radiation Sources in Teletherapy Slide 38 How does a teletherapy Cobalt source look? 3500 pellets; 275 Ci/g; 7700 Ci 5.2.2.4 Cobalt – 60 Source
  • 39. Part VII.14.3 : Radiation Sources in Teletherapy Slide 39 Cobalt source – how does it look? 5.2.2.4 Cobalt – 60 Source
  • 40. 40 5.2.2 Cobalt – 60 Unit5.2.2.5 Cobalt – 60 Activity SI unit: Curies (Ci) 3.7 x 1010 Becquerel (Bq) 1 Bq = 1 disintegration per second also defined in rhm units (roentgens per hour at 1 meter) Most sources have an activity of 750-9000 Ci, typically 3000-9000 Ci used in radiation therapy
  • 41. 41 •Half-life: the time necessary for a radioactive material to decay to half or 50% of its original intensity. • Requires a correction factor for this decay of about 1% per month in all treatment calculations. • Source must be replaced at about five year intervals. •The half-life of 60Co is 5.26 years. 5.2.2 Cobalt – 60 Unit5.2.2.6 Cobalt – 60 Half-Life
  • 42. 42 5.2.2 Cobalt – 60 Unit5.2.2.7 Cobalt – 60 Shielding Cerrobend (Lipowitz metal): Lower melting point than Pb, cheaper 50% Bismuth 26.7% Lead 13.3% tin 10% Cadmium (a toxic metal can get into bloodstream Density ratio of Cerrobend to Lead: 1.2 cm Cerrobend to 1 cm lead. 5 HVL is needed to reduce intensity A thickness of 7.2 cm of Cerrobend needed, 6 cm lead.
  • 43.
  • 44.
  • 45.
  • 46. 46 • Penumbra: the area at the edge of the radiation beam at which the dose rate changes rapidly as a function of distance from the beam axis.5.2.2 Cobalt – 60 Unit5.2.2.8 Penumbra 1 • Describes the edge of the field having full radiation intensity for the beam compared with the area at which the intensity falls to 0. 2 • The larger the source size, the larger the penumbra 3 • Larger field sizes are necessary to cover the same amount of tissue adequately compared to the linac. 4 • Geometric penumbra typically wide because source diameter is large (>2cm)
  • 47.
  • 48. Part 5, lecture 2: Equipment - superficial, telecurie 48 5.2.2 Cobalt – 60 Unit5.2.2.8 Penumbra
  • 49. 49 5.2.2 Cobalt – 60 Unit5.2.2.8 Penumbra The transmission penumbra can be reduced by using satellite collimators, penumbra trimmers or trimmer bars Trimmers are metal bars that attenuate the edge of the beam providing a sharper field edge. It Should be placed no closer than 15 cm from the patients skin to reduced electron contamination (increased skin dose) by metal devices. Provides enough distance for the secondary electrons produced by the trimmer bars to lose sufficient energy REDUCING PENUMBRA
  • 50.
  • 51.
  • 52. 52 5.2.2 Cobalt – 60 Unit5.2.2.9 Dose Maximum Dose maximum (Dmax): when a greater percentage of dose occurs below the skin surface Dmax is the depth of maximum buildup, in which 100% of the dose is deposited For 60Co, Dmax occurs at 0.5 cm below the skin surface.
  • 53. Part 5, lecture 2: Equipment - superficial, telecurie 53 • Source head and transfer mechanism 5.2.2.10 Cobalt – 60 Equipment
  • 54. Part 5, lecture 2: Equipment - superficial, telecurie 54 5.2.2.10 Cobalt – 60 Equipment • shield against the primary cobalt-60 beam Primary barriers • shield against leakage radiation and radiation scattered from the patient Secondary barriers Typical cobalt-60 teletherapy installation:
  • 55. 55 11. Beam On/Beam Off 5.2.2 Cobalt – 60 Unit5.2.2.10 Cobalt – 60 Equipment • Turning the beam on requires physically exposing the source either by moving it into position or by removing shields around the source. Turning the beam • the compressor generates air pressure by pushing the source horizontally into position over the collimator opening. Air pressure: • the motor rotates a wheel 180 degrees by placing the source over the collimator opening. Rotating wheel:
  • 56. Part 5, lecture 2: Equipment - superficial, telecurie 56 5.2.2.10 Cobalt – 60 Equipment
  • 57. Part 5, lecture 2: Equipment - superficial, telecurie 57 Picture of a Co source change 5.2.2.10 Cobalt – 60 Equipment
  • 58. Part 5, lecture 2: Equipment - superficial, telecurie 58 •Assume: • 200 days, 8hours per day working time per year • 10% of this time in treatment room • 3 Gy h-1 typical dose averaged over all locations of the staff member in the treatment room •Dose = 200 x 8 x 0.1 x 3 Gy •  0.5mGy/year (half of dose limit for general public) 5.2.2 Cobalt – 60 Unit5.2.2.11 Annual dose to staff
  • 59. 5.2.2 Cobalt – 60 Unit5.2.2.12 GAMMA KNIFE Therefore it is also known as the stereotactic surgery Patient wears a specialized helmet that is surgically fixed to their skull (brain tumor remains stationary at target point of the gamma rays) It is placed in a circular array in a heavily shielded assembly Aims gamma radiation through a target point in the patient's brain. Contains 201 cobalt-60 sources of approximately 30 curies each
  • 60. 60 Patient positioning collimator 5.2.2 Cobalt – 60 Unit5.2.2.12 GAMMA KNIFE
  • 61.
  • 62. Part 5, lecture 2: Equipment - superficial, telecurie 62 5.2.2.12 GAMMA KNIFE