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OTHER MACHINES
Chapter 10 S&S
Chapter 7 W/L Part 2
1
Low Energy Machines
• Limitations:
– Can not reach deep-seated tumors with an
adequate dosage of radiation.
– Do not spare skin and normal tissues.
• Kilovoltage filtration:
– Inherent: glass envelop (similar to x-ray
tube), insulating oil between tube and
housing.
– Added: clinical use, thin sheets between
target and collimator to decrease weak x-rays
that add skin dose (Cu, Al).
2
Central Axis Depth Dose
• Central axis depth dose and physical
penumbra are related to beam quality.
• The central axis depth dose distribution for
a specific beam depends on the energy.
3
Isodose Curve
• Isodose curve: a line representing various
points of similar value in a beam along the
central axis and elsewhere
• The depth of an isodose curve increases with
beam quality (tells depth dose for different
energy beams)
– The absorbed dose in the medium outside the
primary beam (physical penumbra) is greater for lowenergy beams than for those of a higher energy.
– Limited scatter outside the field for megavoltage
beams occurs because of predominantly forward
scattering of the beam.
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Radiation Therapy Equipment
• Low-energy machines: Uses x-rays generated at
voltages up to 300 kVp
• Primary application is in the treatment of
superficial lesions.
• Kilovoltage units include:
–
–
–
–
–
Grenz: 10-15 kVp
Contact: 50 kVp
Superficial: 50-150 kVp
Orthovoltage: 150-500 kVp
Super Voltage: 500-999 kVp use???
6
Grenz rays
• 10-15kvp
• Grenz rays are almost entirely absorbed in
the first 2 μm of skin and have a useful
depth dose range of about 0.5 μm.
• Treatment of inflammatory disorders
(langerhans’ cells), bowen’s disease,
patchystage mycosis fungoides, herpes
simplex
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Contact therapy
• Superficial skin lesions, treatment unit comes in contact
with patient.
• Endocavitary treatments for curative intent (rectal).
Advantage- preserves sphincter
– Low to middle third of the rectum
– Confined to bowel wall
– Maximum tumor size of 3 x 5 cm
• Hemangiomas
• 4 treatments of 3000 cGy each, separated by a 2 week
interval
• SSD 4 cm
• 1 μm aluminum filtration
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Superficial equipment
•
•
•
•
•
•
50-150kVp
1-6 μm aluminum filtration
Cones 2-5 cm diameter
Pb cutouts
SSD 15-20 cm
Skin cancer and tumors no deeper than
0.5 cm treated as a result of the rapid
falloff of the radiation.
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Orthovoltage machines
•
•
•
•
150-500kVp
HVL 1-4 mm Cu
SSD 50 cm
Skin, mouth, and cervical carcinoma
treatment last several minutes
• Experience limitation in the treatment of
lesions deeper than 2 to 3 cm.
10
Cobalt-60 Machines
• 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.
11
Cobalt 60 Machines
• Constantly emit radiation.
• The 60Co source must be shielded in a
protective housing (source head).
• The source head is a steel shell filled with lead
(may be up to 2 ft in diameter).
• Uses a counterweight to balance the lead
shielding in the head of the machine housing,
(similar to the beam stopper on linac): absorbs a
significant amount of radiation transmitted
through patient.
12
Linear Accelerators vs. Cobalt 60
• Linear accelerators:
– Provide better isodose distribution (greater
dose to the tumor and less dose to normal
tissue)
– Higher and faster dose rate
– More manageable radiation protection
concerns.
13
Application
• The 60Co unit is commonly used today to
treat cancers of the head and neck area,
breast, spine, and extremities.
– Areas just below the skin surface.
– Ideal in treating lymph nodes.
• A 60Co beam can provide an adequate
distribution of dose by using parallel
opposed fields.
14
Cobalt-60 Production
• Cobalt: produced in nuclear reactors by
the irradiation of neutrons of the common
stable form of 59Co.
• The 59Co nucleus absorbs a neutron in the
reactor and becomes 60Co.
15
Cobalt 60
• Radioactive 60Co produces a useful therapy
beam when it undergoes beta decay.
• 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
60Co
 60Ni+ + B- + neutrino (v) + gamma rays
• 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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60Co
Source
• The overall diameter of a 60Co source is 1 to 3
centimeters.
• Consists of pellets of radioactive 60Co encased
in multiple layers of welded metal to prevent
contamination of the environment and to absorb
β- particles produced by the decay process.
• Or 60Co sources are made with the 60Co fused
into a solid cylinder. Advantages:
– Smaller source with less penumbra for the same
beam intensity
– Less hazard of contamination should a source ever
become exposed to the environment.
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60Co
•
Activity
60Co
activity may be expressed in curies (Ci),
the historical unit of radioactivity
– 3.7 x 1010 Becquerel (Bq)
– 1 Bq = 1 disintegration per second
• May also be 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
• 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.
19
60Co
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.
20
Penumbra
• 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.
• Describes the edge of the field having full radiation
intensity for the beam compared with the area at which
the intensity falls to 0.
• The larger the source size, the larger the penumbra
P = S(SSD-SDD)/SDD
• When depth is given:
P = S(SSD + D -SDD)/SDD
• Larger field sizes are necessary to cover the same
amount of tissue adequately compared to the linac.
21
Penumbra
• Geometric penumbra: the place where a lack
of sharpness or fuzzy area occurs at the edge of
the beam
– Occurs at the skin surface and greater depths in
tissue.
• Transmission penumbra: occurs as the
radiation passes through the edge of the primary
collimators
– Occurs at the edge of the patients shielding blocks
mounted or placed below the collimator
– Correlates with the size of the collimator openinglarger field sizes have more transmission penumbra
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Reducing 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.
– 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
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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.
• Electron equilibrium is another term used to
describe Dmax. As energy increases, so does the
depth of electron equilibrium.
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Beam On/Beam Off
• Turning the beam on requires physically
exposing the source either by moving it into
position or by removing shields around the
source.
• Air pressure: the compressor generates air
pressure by pushing the source horizontally into
position over the collimator opening.
• Rotating wheel: the motor rotates a wheel 180
degrees by placing the source over the
collimator opening.
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Warning Lights
• Red light: Radiation present- do not enter
room
• Green Light: time elapsed
• Malfunction: both red and green lights still
on- means that machine is still in on
position after prescribed dose has been
delivered. Remove patient.
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Timer Error
• Timer error (TE, or travel time correction, shutter error):
is a consequence of the physical motion of the source.
Even if its less than 1 second.
• The difference between the beam on time setting and
the time that the source is in the treatment position, the
time it takes to advance and retract the source
• Timer error is determined at the time of machine beam
calibration and is checked monthly.
TE = (R1t1-R2t2)/R1-R2
• The timer setting necessary for a treatment is the time
calculated plus the timer error.
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Quality Assurance
• Required be the Nuclear Regulatory
Commission (NRC)
• A qualified radiation physicist must
perform full calibration testing annually.
More frequent if:
– The source is replaced
– A 5% deviation is noticed during a spot check
– A major repair requiring the removal or
restoration of major components is done.
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Quality Assurance
• Written directive: the prescription, must be clear,
unambiguous, and signed by a licensed authorized user.
• Recordable event must be reported to the RSCRadiation Safety Committee:
– A delivered dose more than 15% in excess of the weekly
prescription
– No WD
– Lack of daily recording of dose
• Misadministration must be reported immediately to the
NRC
– Treatment of the wrong patient, wrong site
– Weekly dose more than 30% in excess of the prescription
– The delivery of more than 20% (25%???) total dose
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Advisory Agencies
• Analyze and assess data, and then
develop recommendations for dose limits
• National Council on Radiation Protection
and Measurement (NCRP) and ICRP
• Recommendations may be made into law
by state or federal government.
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Regulatory Agencies
• License users of radioactive materials and
radiation producing equipment; inspection;
enforcement of laws.
• Nuclear Regulatory Commission (NRC)
oversees isotopes produces in nuclear
reactors, teletherapy, and brachytherapy.
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Tests
• The integrity of the source must be tested twice
a year to ensure the sealed source continues to
be totally sealed.
– Wipe tests: wipe collimator edges with a filter paper,
background radiation reading done, activity is
determined (acceptable under 0.005 mCi)
•
•
•
•
Radiation and light field coincidence
Timer accuracy
Dose rates
& more.
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Leakage Limits
• Leakage in the off position:
– Cannot exceed 2 mrem/hr at 1 meter
– Maximum of 10 mrem /hr at 1 meter at any
measurable location at any time
• Leakage in the on position:
– Cannot exceed 0.1% of the useful beam at 1
meter from the source.
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Betatrons
• Developed by Kerst in 1941
• Can provide x-ray and electron therapy
beams from less than 6 to more than 40
MeV.
• Applied to industrial radiography- used in
World War II
• Operation is based on the principle that an
electron in a changing magnetic field
experiences acceleration in a circular orbit.
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Betatrons
• Accelerating tube shaped like a hollow doughnut and
placed between the poles of an AC magnet.
• A pulse of electrons is introduced at the moment the AC
cycle begins
• As the magnetic field rises, the electrons accelerate
continuously around the tube.
• Energy depends on the number of revolutions and the
speed.
• By the end of the first quarter cycle, electrons are made
to spiral out of orbit, then strike a target to produce xrays or a scattering foil to produce a broad stream of
electrons.
– By choosing the timing of electron injection and extraction,
continuous control of electron energy is possible
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Betatrons
• Most often used for electron therapy
• Can produce x-ray beams with energies over 40MeV,
though cannot compare with dose rates of linacs.
• Used Lucite cones to treat gynecologic, bladder, and
prostate carcinomas.
• Treatment times 3-5 min.
• Disadvantage: Machine is noisy, bulky- isocentric
design impossible, non-isocentric, slower dose rates
• Makes small field sizes and precise patient setup
cumbersome.
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Van de Graff Generator
• Developed by R.J. Van de Graff while working at
the MIT
• First electrostatic Linear accelerator
• Capable of accelerating either positive or
negative ions.
• Disadvantages:
– Size
– Warm-up took as long as 1 hour.
– Used a front pointer device to measure the distance
to the patient because no ODI was available.
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Van de Graff Generator
• An insulating belt transports electric charge to a
collector screen within a metal dome
• The accumulation of charge produces the high
voltage used to accelerate charged particles
• The maximum possible x-ray energy in MeV is
directly proportional to the voltage on the dome
and on the total length of the machine- limits the
voltage of clinical machines to a few MV
• Smaller source size  smaller penumbra
40
Van de Graff Generator
• Operate at 200cGy/minute.
• Provide a standard SSD of 100 cm and
can approximate much greater treatment
distances- useful in the treatment of
extended fields.
• Used to treat seminoma, whole brain, and
mantle field (used to treat lymph nodes in
the neck and thorax for Hodgkin’s
disease).
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Cyclotrons
• Developed by E.O. Lawrence in 1928.
• Accelerate heavy charged particles (protons,
deuterons, positive nuclei, etc)
• Use large magnets to confine the charged
particles to a circular or spiral path.
• Particles are accelerated by an oscillating
electric field.
• Medical uses of the cyclotron:
– Production of radionuclides applied in nuclear
medicine.
– The use of neutrons and protons in radiation therapy.
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Dees
• Metal half-disks with a hollow evacuated center through
which the particles can travel
• Two dees of opposite polarity are separated by an
alternating voltage produced by a high frequency
generator, pull the particles back and forth in a spiral
pattern.
• Charged particles enter the center of one of the dees
with a moderate velocity and are bent in a circular path
by the magnet.
• When the particles enter the gap between the dees, they
experience the high frequency, high voltage electric field
and are accelerated, reenter the opposite dee, only
magnetic force within the dee- no velocity increase.
• Final energy is equal to the sum of the energy gained at
each gap crossing.
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Theory of Relativity
• Acceleration causes particles to gain
mass.
• The increase in weight causes to particles
to slow down and be out of sync with the
frequency of the alternate potential applied
to the dee.
• Electrons cannot accelerate in a dee.
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PET
• Cyclotrons are used for the production of
radionuclides used in PET.
• PET: scanning technique that involves the
systemic administration of a
radiopharmaceutical agent labeled with a
positron emitting radionuclide.
• PET scanners are used in nuclear medicine to
measure important physiologic and biomedical
processes such as blood flow, oxygen, glucose
and metabolism of free fatty acids, etc.
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PET
• PET scanners use radiation emitted from within the
patient to produce images.
• Anatomy can be evaluated in microscopic detail- patients
receive a small amount of an agent that closely
resembles a substance naturally found in the body.
• PET imaging involves positrons emitted during the
breakdown of the nuclei of certain radioisotopes.
• Pure energy, released as gamma rays, is a result of the
collision and subsequent annihilation of matter and
antimatter.
• Radiation from the positron emitting isotope is detected
be the PET scanner and displays in microscopic detail
the chemical processes occurring.
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Neutrons
• To produce neutron beams, deuterons are
accelerated to high energies and then forced to
strike a beryllium target, producing neutrons via
nuclear reactions.
• Use Teflon flattening filters, polyethylene filters.
• Neutrons: effective in penetrating nuclei and
producing reactions by a process called
stripping.
• Neutron studies abandoned due to the high
concentration of hydrogen in the fatty tissue
(lack of information about its RBE).
50
Neutrons
• Advantages
– Good for radioresistant tumors
• GBM
• Salivary gland
• Soft tissue sarcoma
– No oxygen effect
• Disadvantages:
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–
–
–
Poor depth dose
Poor skin sparing
Extra shielding due to neutron contamination
Fixed horizontal beam
51
Protons
• To produce a usable beam of protons, strip
off hydrogen electrons by subjecting it to
an electric current
• Protons are then subjected to an
oscillating electric field, which accelerates
them to half the speed of light and a strong
magnetic field which keeps them
contained in a spiral configuration.
52
Protons
• Precision controlled, accuracy critical within 1 mm.
• Scattering is minimal compared with that from x-rays,
neutrons and cobalt radiation.
– Filtration: brass cutouts
• They have a characteristic distribution of dose with
depth.
• Most of the energy is deposited near the end of the
range, where the dose peaks to a high value and then
drops rapidly to zero (Bragg peak).
– Lucite compensators used to spread out Bragg peak.
• Treatment of choice for lesions close to sensitive areas
of the body, usually given in a single large dose.
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Brachytherapy
• Brachytherapy: a form of radiotherapy where a
radioactive source is placed inside or next to the
area requiring treatment, usually after the tumor
has been surgically removed.
• Remote After loading: this technique relies on
the use of hollow tubes which are connected to a
safe containing a small radioactive source
welded to a wire that is driven out by a stepping
motor to predetermined positions to deliver
radiation dose.
– Personnel do not touch the source
54
High Dose Rate
• Greater than 1200 cGy per hour or more than 20 cGy
per minute.
• Can be fractionated 4-5 treatments over 2 weeks.
• Most use Iridium-192
• Advantages:
– Treatment can be given on an outpatient basis.
– Treatment time is extremely short compared to LDR- implant
reproducibility more precise
– Radiation protection for staff
– No general anesthesia or bed rest with decreased complications.
– Increased comfort for patient.
– Higher dose rate- tumor gets more of the dose and cannot
repopulate.
55
High Dose Rate
• Disadvantages:
– Labor intensive
– Longer setup time
– Treatment plan changes are difficult to make
before the treatment is completed
56
Low Dose Rate
• 40- 200 cGy per hour.
• Use Cesium-137, requires bed rest for 2
days
• Used for prostate
– Platinum
– Iodine
57