radiotherapy equipment
TRANSCRIPT
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ZMT 335
122/3/2017Dr. Nik Noor Ashikin Bt Nik Ab Razak
RADIOTHERAPY & NUC MEDICINE
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TOPIC 5 & 6Radiotherapy Equipment
222/3/2017Dr. Nik Noor Ashikin Bt Nik Ab Razak
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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
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5.0 External Beam Equipment5.1 Low-energy Machines
5.1.1 Superficial Equipment5.1.2 Orthovoltage Units
5.2 Telecurie Units5.2.1 Cs-1315.2.2 Cobalt – 60 Unit
5.3 Linear accelerator (LINAC)422/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
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5.0 External Beam Equipment
522/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
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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
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5.0 External Beam Equipment
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5.1 Low-energy Machines5.1.1 Superficial Equipment5.1.2 Orthovoltage Units
722/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
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• 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
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• 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
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“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
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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
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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
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5.1.1 Superficial Equipment
1322/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
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Part 5, lecture 2: Equipment - superficial, telecurie
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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
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Part 5, lecture 2: Equipment - superficial, telecurie
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X-ray tube
ApplicatorFilter
5.1.1 Superficial Equipment
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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
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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
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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)
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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
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5.1.2 Orthovoltage Units
2022/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
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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
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5.1.2 Orthovoltage Units5.1.2 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
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5.2 Telecurie Units5.2.1 Cs-1315.2.2 Cobalt – 60 Unit
2322/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
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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
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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
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5.2.2 Cobalt – 60 Unit5.2.2.1 Properties5.2.2.2 Application5.2.2.3 Production 5.2.2.4 Source5.2.2.5 Activity5.2.2.6 Half-Life5.2.2.7 Shielding5.2.2.8 Penumbra5.2.2.9 Dose Maximum5.2.2.10 Equipment5.2.2.10 Cobalt – 60 Equipment5.2.2.11 Annual dose to staff5.2.2.12 Gamma Knife 2622/3/2017 Dr. Nik Noor Ashikin Bt Nik Ab Razak
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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
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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 ال 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
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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 ال Photon
Maximum dose at depth of 5 mm
Source to be changed every 4 to 5 yearsLeakage 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
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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
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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
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5.2.2 Cobalt – 60 Unit5.2.2.2 Application
AP
PL
ICA
TIO
N
To treat cancers of the head and neck area, breast, spine, and
extremities
Areas just below the skin surface
Ideal in treating lymph nodes.
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5.2.2 Cobalt – 60 Unit5.2.2.3 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
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5.2.2 Cobalt – 60 Unit5.2.2.3 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.
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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.
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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
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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
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Part VII.14.3 : Radiation Sources in Teletherapy Slide 39
Cobalt source – how does it look?5.2.2.4 Cobalt – 60 Source
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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
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•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
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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.
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• 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)
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Part 5, lecture 2: Equipment - superficial, telecurie 48
5.2.2 Cobalt – 60 Unit5.2.2.8 Penumbra
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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
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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.
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• Source head and transfer mechanism
5.2.2.10 Cobalt – 60 Equipment
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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:
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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:
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5.2.2.10 Cobalt – 60 Equipment
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Picture of a Co source change
5.2.2.10 Cobalt – 60 Equipment
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•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
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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
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Patient positioning collimator
5.2.2 Cobalt – 60 Unit5.2.2.12 GAMMA KNIFE
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Part 5, lecture 2: Equipment - superficial, telecurie
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5.2.2.12 GAMMA KNIFE