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SMJ Mortazavi, Ph.D
Professor of Medical Physics
Radiation Biology &
Radiation Therapy for
Medical Students2nd Semester 1393-1394
1st & 2nd Sessions
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
LET & RBE
SMJ Mortazavi, Ph.D
LET• Linear energy transfer (LET) represents the amount of energy transferred from radiation
to a medium (for example, tissues) per unit length of the path traveled by the radiation
(sometimes referred to as 'track'). The commonly used unit is keV/µm. LET is defined
as:
The linear energy transfer (LET) of a medium for charged particles is the quotient of
dE/dl, where dE is the energy lost by a charged particle due to electronic collisions in
traversing a distance dl.
Since energy transfer to the medium is principally via ionization produced, LET is
related to the density of ionization along the track. LET gives an indication of the
‗radiation quality‘.
Radiation LET (keV/µm)
Cobalt-60 gamma radiation 0.2
250 keV X-radiation 2.0
10 MeV protons 4.7
150 MeV protons 0.5
2.5 MeV α particles 166
SMJ Mortazavi, Ph.D
low LET (, x, ~)
high LET ()
air tissue
incid
ent ra
dia
tio
n
greater radiotoxicity
dispersion of energy
LET = linear energy transfer
Linear Energy Transfer
SMJ Mortazavi, Ph.D
Radiation quality
SMJ Mortazavi, Ph.D
Typical Linear Energy Transfer Values
SMJ Mortazavi, Ph.D
Relative Biologic Effectiveness
• The RBE of some test radiation (r)
compared with x-rays is defined by the
ratio D250kVp/Dr ,where D250kVp and Dr are,
respectively, the doses of x-rays and the
test radiation required for equal biological
effect.
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
Relative Biologic Effectiveness
The optimal LET
LET of about 100 keV/μm is optimal
in terms of producing a biologic
effect.
At this density of ionization, the
average separation in ionizing events
is equal to the diameter of DNA
double helix which causes significant
DSBs. DSBs are the basis of most
biologic effects.
The probability of causing DSBs is
low in sparsely ionizing radiation
such as x-rays that has a low RBE.SMJ Mortazavi, Ph.D
LET
SMJ Mortazavi, Ph.D
The Ionization Process
• Radiation causes ionizations of atoms, which
will affect molecules, which may affect cells,
which may affect tissues, which may affect
organs, which may affect the whole body.
SMJ Mortazavi, Ph.D
Absorption Mechanisms
• Radiation can be classified as
directly ionizing or indirectly ionizing
– Direct absorption - by primary radiation
• charged particles
– Indirect absorption - by secondary
radiation
• photons, neutrons
SMJ Mortazavi, Ph.D
Ionization
SMJ Mortazavi, Ph.D
Cellular Effects
SMJ Mortazavi, Ph.D
Ionizing radiation burn
• Large red patches of skin
on the back and arm from
multiple prolonged
fluoroscopy procedures.
SMJ Mortazavi, Ph.D
Energy Absorption
• For Humans LD50/60 = 4 Gy (4 J/kg)
• If a 70 kg person receives a dose of 4 Gy,
they‘ve absorbed an equivalent of 280 J.
• What is the caloric equivalent of 280 J?
– 4.186 J = 1 cal, thus 280/4.186 = 67 calories
• What would be the temperature rise in the
body from this energy deposition?
• Would we expect this to be fatal?
SMJ Mortazavi, Ph.D
Direct and Indirect Ionization
• Direct Ionization: charged particles
–direct disruption of atomic and molecular
structures
– charged particles are directly ionizing (if
sufficiently energetic)
• Indirect Ionization: Gamma-rays, x-rays,
uncharged particles
–Gamma- and x-rays 1st transfer energy to
electrons
– Neutrons first transfer energy to recoil protons
(H1) or nuclear fragments SMJ Mortazavi, Ph.D
Water
Molecule
SMJ Mortazavi, Ph.D
Indirect Action Chain of
Events• Ion Formation:
H2OH2O+ + e-
H2O+ is an Ion Radical
• Radical Formation:
H2O+ H+ + OH•• Hydroxyl Radical
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
Time Scale of Events
• Initial ionization: 10-15s• Ion radical lifetime: 10-10s• Free radical lifetime: 10-5s• Breakage of bonds and expression of
biological effects: hours, days, months, years– cell killing: hours to days– oncogenic (cancer): years– mutation in germ cell: generations
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
Direct Action vs Indirect Action
Direct vs Indirect Action• Indirect Action: Electrons
interact with water to form
OH radicals (2/3 of x-ray
biological damage).
can be modified by sensitizers or protectors
• Direct Action: Electron
directly interacts with
target molecule (High LET
damages by direct action).
• cannot be modified by sensitizers or protectors
SMJ Mortazavi, Ph.D
Chromatid breaks
SMJ Mortazavi, Ph.D
Generally, chromatid
breaks and chromatid
exchanges can be induced
by radiation in the S and G-
2 phases of the cell cycle,
when the chromosome has
split into 2 chromatids.
Chromosome breaks
• Chromosome breaks can
be induced by radiation in
the G-1 phase of the cell
cycle, before the
chromosome splits into 2
chromatids.
SMJ Mortazavi, Ph.D
Factors Affecting Radiation
Effects on Cells• Total energy absorbed by the cell
• LET, RBE, WR
• Dose Rate
• Oxygen
• Chemical Modifiers
– Radioprotectors
– Radiosensitizers
• Rate of Cell Division
• Cell Differentiation
• Age
• Sex
• Different Species
SMJ Mortazavi, Ph.D
Bergonie and Tribondeau‘s law
• By 1906 Bergonie and
Tribondeau realized
that cells were most
sensitive to radiation
when they are:
• Rapidly dividing
• Undifferentiated
• Have a long mitotic
future
SMJ Mortazavi, Ph.D
LD50/30 for various species
Species LD50/30 Gy (rad)
Pig 2.5 (250)
Dog 2.75 (275)
Guinea pig 3 (300)
Monkey 4.25 (425)
Opossum 5.1 (510)
Mouse 6.2 (620)
Goldfish 7 (700)
Hamster 7 (700)
Rat 7.1 (710)
Rabbit 7.25 (725)
Gerbil 10.5 (1050)
Turtle 15 (1500)
Newt 30 (3000)
SMJ Mortazavi, Ph.D
Comparative Radiosensitivity of Living
OrganismsOrganism LD50 in Gy
(X-rays)
Guinea Pig
Pig
Dog
Goat
Monkey Man
Mouse
Rat
Rabbit
Fish
Frog
Yeast (haploid)
Bacteria
Yeast (dipliod)
M. radiodurans (air)
2.5
3.5
3.5
3.5
5.0
6.0
7.0
7.0
7.0
7.0
50
60
300
7000
Deinococcus radiodurans
15,000 Gy with 37% viability
SMJ Mortazavi, Ph.D
Factors that Influence LD50
Biological and Physical
Rapair
• Lethal Damage
• Sublethal Damage
• Potentially Lethal Damage
SMJ Mortazavi, Ph.D
Cells are undamaged.
Cells are damaged, repair damage and operate normally.
Cells are damaged, repair damage and operate abnormally.
Cells die as a result of damage.
Biological Effects of Radiation
SM
J M
orta
za
vi, P
h.D
Whatever the source and amount of ionizing radiation, it will have some biological effect on living organisms. Atoms become ionized when the radiation displaces electrons. These
altered atoms will affect the molecules to which they belong and therefore the biological cells to which the molecules belong.
The biological effect on the cell may be direct or indirect. If the radiation interacts with the cell DNA, the cell is considered to be directly affected. If the radiation interacts with the water
within the cell to create radicals, which have the capability to form toxic substances such as hydrogen peroxide the cell is said to be indirectly affected. The results of these
interactions depend on the sensitivity of the cell type and on the amount and type of radiation the cell receives.
Living cells are not equally sensitive to radiation. Rapidly reproducing cells, such as those of a fetus, are more sensitive than those cells which have a longer time to repair damage
before reproducing. Cells damaged by radiation respond one of four ways:
(1) The cells are not damaged
(2) Less active cells that receive small amounts of radiation are able to complete the normal repair of damage
(3) Incomplete or incorrect repair of damage may cause the cell to operate abnormally or causes future generations of cells to have mutations
(4) Large amounts of damage cause the cell to die
Cell Death
and Survival Curves
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
Colony = >50 cells
Small colonies?From: Hall
Radiobiology for the Radiologist
SMJ Mortazavi, Ph.D
There are a number of assays used for in vitro survival curves. Some look at cell
metabolism or membrane integrity as a measure of viability. The most
quantitative are those that are based directly on plating or cloning efficiencies.
Essentially, a known number of single cells are plated onto dishes and left for 2-4
weeks, depending on how fast the cells grow, until they can form a visible colony.
As mentioned in the previous slide, we define a colony as one containing at least
50 cells. The fraction of cells plated that form colonies define cloning efficiency.
Radiation exposure reduces that number, and dividing the cloning efficiency seen
after irradiation by that of the unirradiated, yields a surviving fraction.
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
Effect of LET on cell survival
Survival curves for cultured cells of human origin exposed to 250-kV X-rays,
15-MeV neutrons, and 4-MeV alpha-particles. As the LET of the radiation
increases, the survival curve changes: the slope of the survival curves gets steeper
and the size of the initial shoulder gets smaller.
A more common way to represent these data is on the next slide.
SMJ Mortazavi, Ph.D
RBE for different cells and tissues
Even for a given total
dose or dose per
fraction, the RBE varies
greatly according to the
tissue or endpoint
studied.
•Survival curves for
various types of
clonogenic mammalian
cells irradiated with 300
kV X-rays or 15-MeV
neutrons.
•Note that the variations
in radiosensitivity among
different cell lines is
markedly less for
neutrons than for X-rays.SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
LD 50/30
SMJ Mortazavi, Ph.D
3-4 Gy: LD 50/30
for Adult Humans
without Medical
Intervention
Stochastic Effect
• That Occurs On A Random Basis,
Independent of the Size of Dose.
• The Effect Typically Has No
Threshold and is Based on
Probabilities, With The Chances Of
Seeing The Effect Increasing With
Dose.
• If it Occurs, The Severity Of A
Stochastic Effect Is Independent Of
The Dose Received.
• Cancer Is A Stochastic Effect.
SMJ Mortazavi, Ph.D
Non-stochastic Effects
• Effects That Can Be Related Directly
To The Radiation Dose Received.
• The Effect Is More Severe With A
Higher Dose.
• It Typically Has A Threshold, Below
Which The Effect Will Not Occur.
• These Are Sometimes Called
Deterministic Effects.
• For Example, A Skin Burn From
Radiation Is A Non-stochastic Effect
That Worsens As The Radiation Dose
Increases. SMJ Mortazavi, Ph.D
The image shows severe radiation burns
on the back of a man. The man was one
of three woodsmen who found a pair of
canisters in the mountains of the country
of Georgia (formally part of the USSR).
The men did not know the canisters were
intensely radioactive relics that were
once used to power remote generators.
Since the canisters gave off heat, the
men carried them back to their campsite
to warm themselves on a cold winter
night.
Stochastic vs Non-Stochastic Effects
SMJ Mortazavi, Ph.D
Factors which may Influence Radiation
Somatic & Genetic Damages
– Total Absorbed Dose
– Potential of Ionizing Tissues
– Area (Volume) Irradiated
– Type of Tissue irradiated
SMJ Mortazavi, Ph.D
Dose Response Models
LNT
SMJ Mortazavi, Ph.D
Typical dose-survival curves for mammalian
cells exposed to x rays and fast neutrons
SMJ Mortazavi, Ph.D
Three periods of gestation
1. Preimplantation → 0 - 9 days (in humans)
2. Organogenesis → 10 days - 6 weeks
3. Fetal Period → 6 weeks to term
SMJ Mortazavi, Ph.D
Acute Radiation
Syndrome
SMJ Mortazavi, Ph.D
Acute Radiation Syndrome
• Signs and symptoms experienced by individuals
exposed to acute whole body irradiation.
• Data collected largely through Japanese atomic
bomb survivors at Hiroshima and Nagasaki.
• Limited number of accidents at nuclear
installations.
• Clinical radiotherapy.
• Well-characterized animal data base.
• LD50/60 dose of human is ~4.5 Gy.
• Lethal Dose (LD 100) is ≥8 Gy (~10 Gy).
SMJ Mortazavi, Ph.D
Prodromal Radiation Syndrome
• Early symptoms that appear after
exposure to whole body radiation:
– gastrointestinal: nausea, vomiting,
diarrhea, anorexia
– neuromuscular: easy fatigability
• Effect is dose dependent:
– Varies in time of onset
– Severity
– Duration
SMJ Mortazavi, Ph.D
Hematopoietic syndrome
• Cause of death at doses 2-10 Gy.
• Peak incidence of death occurs at about 30
days post-irradiation, and continues for up to 60
days.
• Suppresses normal bone marrow and spleen
functions.
• Symptoms associated with hematopoietic
syndrome are: chill, fatigue, hemorrhages,
ulceration, infection and anemia.
• Death is possible unless receive medical
interventions such as bone marrow transplant.SMJ Mortazavi, Ph.D
Gastrointestinal syndrome
• Occurs at dose 10-50 Gy of gamma-rays or its
equivalence.
• Death usually occurs within 3 to 10 days.
• Symptoms due largely to depopulation of the
epithelial lining of the GI tract by radiation.
• No human has survived radiation dose >10 Gy.
• Clinical symptoms include nausea, vomiting,
and prolong diarrhea, dehydration, loss of
weight, complete exhaustion, and
eventuallydeath.
SMJ Mortazavi, Ph.D
Cerebrovascular syndrome
• Identified at doses >50 Gy of gamma-rays.
• Death occurs within hours from cardiovascular
and neuromuscular complications.
• Clinical manifestations include severe nausea,
vomiting within minutes of exposure,
disorientation, loss of muscular co-ordination,
respiratory distress, seizures, coma and
death.
SMJ Mortazavi, Ph.D
Radiation-induced Mutagenesis
• Radiation DOES NOTproduce new, unique
mutations, but increases the incidence of the
same mutations that occur spontaneously.
• Mutation incidence in humans is DOSE and
DOSE-RATE dependent.
• A dose of 1 rem (10 mSv) per generation
increases background mutation rate by 1%.
• Information on the genetic effects of radiation
comes almost entirely from animal and IN VITRO
studies.
• Children of A-bomb survivors from Hiroshima and
Nagasaki fail to show any significant genetic
effects of radiation.
SMJ Mortazavi, Ph.D
Radiation Carcinogenesis
• Cancer is a stochastic late effect.
• No threshold, an all or none effect.
• Severity is not dose related.
• Probability of carcinogenesis is dose dependent.
• Leukemia has the shortest latency period of ~5
years.
• Solid tumors have a latency period of ~20 to 30
years.
• Total cancer risk for whole body irradiation is
one death per 104 individuals exposed to 1 rem.
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
Cancer Treatment
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
Radiation Therapy
Radiation Therapy
1. Cells can be ―killed‖ by ionizing radiation.
2. Most important target appears to be nuclear
DNA.
3. Radiation damage to DNA results in non-
viable offspring.
4. Rapidly dividing cell populations are the
most sensitive to ionizing radiation (e.g.
tumors, epithelial cells, hemopoietic cells.
SMJ Mortazavi, Ph.D
TCP & NTCP Curves
At a high enough
dose we would have
a high probability of
curing every tumor.
Unfortunately, we
must also irradiate
some normal tissue
and its response
usually limits the
dose that can be
used
SMJ Mortazavi, Ph.D
The physical goal of radiation
therapy
• ―Deliver a high dose to all parts of the
tumor while minimizing the dose to
surrounding normal tissue.‖
SMJ Mortazavi, Ph.D
What is achievable?
• This ideal dose distribution is not
physically achievable, but we attempt
to satisfy it through two general
strategies:
– Brachytherapy and
– Teletherapy
SMJ Mortazavi, Ph.D
Treatment of Cancer
• Radiation is very effective in the treatment
of certain cancers.
• The choice is basically do you administer
the radiotherapy externally or internally.
SMJ Mortazavi, Ph.D
Some cancers are considered more
responsive to radiation therapy
• Certain types of cancer are considered more responsive to
radiation therapy. In these cancers radiation can sometimes
successfully stop growth without permanently damaging the
surrounding normal tissue. If these tumors can be treated early,
before metastasis the cure rate is high.
• Cancers in this category:
– skin and lip
– head and neck
– breast
– cervical and endometrium
– prostate
– Hodgkin's disease and local extranodal lymphoma
– Seminoma of testis and dysgerminoma of ovary
– Medulloblastoma, pineal germinoma,and ependymoma
– Retinoblastoma
– Choroidal melanoma SMJ Mortazavi, Ph.D
Some cancers are considered limited
responsive to radiation therapy
• Other tumors with limited response to radiation that may be curable with
combined therapies include:
– Wilms tumor
– Rhabdomyosarcoma
– colorectal cancer
– soft tissue carcinoma
– embryonal carcinoma of testis
• Most other malignant cancers are not considered curable with radiation
because they are difficult to detect early enough and/or they have a much
higher growth rate.
• Tumors found in especially sensitive tissue cannot be treated with the large
dose of radiation necessary to kill the tumor. Also, radiation alone is not
usually successful against highly metastatic tumors. In some instances, a
limited number of cures are obtained following surgery, radiation, or a
combination of the two. SMJ Mortazavi, Ph.D
External Beam Therapy
• External beam therapy (EBT) is a method for delivering a beam of high-energy photons to the location of the patient's tumor.
• The beam is generated outside the patient and is targeted at the tumor site.
• These high-energy photons can destroy the cancer cells and careful treatment planning allows the surrounding normal tissues to be spared.
SMJ Mortazavi, Ph.D
Teletherapy
• In teletherapy an
external source at a
distance of about one
meter from the patient
is used to irradiate the
tumor.
• A series of daily
fractions, each about 2
Gy, is used.
• It takes about one
minute to deliver the
actual treatment.SMJ Mortazavi, Ph.D
Teletherapy1. Any anatomical site can be treated.
2. Large fields (even the whole body!) can be
accommodated.
3. Treatment is quick and convenient.
4. Usually done as an outpatient procedure.
5. Noninvasive.
6. Can be performed on patients who are not well.
7. No significant radiation dose to staff, No
radiation to family members, etc.
8. The physical disadvantages can be largely
overcome. SMJ Mortazavi, Ph.D
Dose in Teletherapy
• To understand the dose distribution from
an external photon beam, we need to
consider:
1. Dose is due mainly to electrons.
2. Electrons have finite range.
3. Attenuation of primary photons.
4. Inverse square law.
5. Compton scattered photons.
SMJ Mortazavi, Ph.D
Simple Model for Dose Near the Surface
SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
Surface DoseDose at Beam
Exit
Maximum Dose
Buildup region
• The dose region between the surface (depth z
= 0) and depth z = zmax in megavoltage photon
beams is referred to as the dose buildup region
and results from the relatively long range of
energetic secondary charged particles
(electrons and positrons) that are first released
in the patient by photon interactions
(photoelectric effect, Compton effect, pair
production) and then deposit their kinetic
energy in the patient.SMJ Mortazavi, Ph.D
SMJ Mortazavi, Ph.D
Modifying dose at the skin surface
and at depth• In radiation therapy, bolus is a material which has properties
equivalent to tissue when irradiated.
• It is widely used in practice for modifying dose at the skin
surface and at depth
• A specific thickness of bolus can be applied to the skin to alter
the dose received at depth in the tissue and on the skin surface.
• A typical example of this is the application of a defined thickness
of bolus to a chest wall for post-mastectomy chest wall
treatment, to increase the skin dose.
• When a full bolus is applied, bolus thickness equal to the depth
of the build-up region removes the skin-sparing effect of a
megavoltage x-ray beam.
SMJ Mortazavi, Ph.D
Dose distribution
SMJ Mortazavi, Ph.D
Multi Beams
• Consider what happens if you use two
beams entering the patient from opposite
directions.
• The resulting dose distribution will be the
sum of the contributions from the two
fields.
• Plotting the dose along the central axis of
this opposing pair of fields we get
something that looks likeSMJ Mortazavi, Ph.D
SINGLE BEAMS
A single beam may be used to treat a tumour which is
near enough to the body surface for sufficient dose to be
received without overdosing overlying and underlying
tissues within the treatment beam
Most tumours require a different method...
SMJ Mortazavi, Ph.D
Multi-beam treatments
• Used if a high dose is required to kill tumours deeper in the body
• Several beams used• Beams only overlap in the
tumour area• Tumour receives fatal dose
but healthy cells receive a lower, safer dose.
Each of these beams delivers 1/3 of the required dose.
SMJ Mortazavi, Ph.D
Before treatment can begin, scansare taken to accurately locate the tumor.
SMJ Mortazavi, Ph.D
Then computers are used to help plan the best route for each beam
SMJ Mortazavi, Ph.D
Sensitive areas such as the eyesand spinal cord must be avoided.
brain
eyes
tumour
SMJ Mortazavi, Ph.D
Two beams from opposite directions
SMJ Mortazavi, Ph.D
Now we can get more
dose in a deep-seated
tumor than in the
overlying normal
tissue.
This idea can be
extended to more
complex
arrangements ranging
from standard 3, 4 or
more field geometries
to quite complex
individualized plans
that incorporate beam
modifiers.
SMJ Mortazavi, Ph.D
Fractionation
Sterilization Endpoint Experiments
1920‘s-30‘s
• Single Fraction Severe Skin
Effects
• Multiple Smaller Fractions Less
Severe Skin Effects
Isoeffect Curves
• Each isoeffect curve represents a different clinical
acute toxicity endpoint.
• Examples A = skin necrosis, E = skin erythema.
Brachytherapy
• Brachytherapy is not a new treatment method.
• Throughout this century, several types and routes of implantation of radioactive seeds have been used to treat cancer.
• Radioactive Iodine seeds were widely used during the 1970s and 1980s.
SMJ Mortazavi, Ph.D
• Brachytherapy sources can be divided into
permanent and temporary groups.
• Permanent sources tend to have lower energy
and shorter half-lives.
• The advantage of these lower energies is
enhanced safety.
• The disadvantage is that anatomical
adjustments cannot be made once the sources
are placed.
Brachytherapy
SMJ Mortazavi, Ph.D
Brachytherapy
• In brachytherapy radiation sources are
placed adjacent to or within the target
volume.
• The sources may be implanted
permanently or temporarily.
• Temporary implants may be performed at
high dose rate (HDR, treatment time of
minutes) or low dose rate (LDR, treatment
time of days).
SMJ Mortazavi, Ph.D
• Temporary seed placement is shown here.
SMJ Mortazavi, Ph.D
Brachytherapy
• The isotopes used most commonly in
brachytherapy are Cs-137, Ir-192, and
I-125.
SMJ Mortazavi, Ph.D
Radionuclide Half Life Energy
Brachytherapy
• In brachytherapy radioactive seeds or sources are placed in or near the tumor itself, giving a high radiation dose to the tumor while reducingthe radiation exposure in the surrounding healthy tissues.
• The term "brachy" is Greek for short distance.
SMJ Mortazavi, Ph.D
Brachytherapy
SMJ Mortazavi, Ph.D
• Currently, temporary
implants consist
primarily of 192Ir and
137Cs.
Temporary Brachytherapy
SMJ Mortazavi, Ph.D
• 67 year old male with metastatic prostate cancer.
SMJ Mortazavi, Ph.D
• Iridium 192 is used for high–
dose rate treatment of
prostate cancer.
• During the implantation,
hollow needles are inserted
transperineally.
• The needles are then
connected to an automated
remote-controlled loading
machine.
• The total irradiation time is
usually only 5-10 minutes.
HDR Brachytherapy
SMJ Mortazavi, Ph.D
Linear accelerator (Linac)
• An electron linear accelerator uses microwaves propagating
in a special waveguide to accelerate the electrons.
• The largest linac accelerates electrons to 2 GeV, but
medical accelerators operate in the 4 - 25 MeV range.
SMJ Mortazavi, Ph.D
Linear Accelerator
SMJ Mortazavi, Ph.D
•Linacs consist of four major components—a
modulator, an electron gun, a radio-frequency
(RF) power source, and an accelerator guide.
•The electron beam produced by a linac can be
used for treatment or can be directed toward a
metallic target to produce x-rays.
•The modulator amplifies the AC power supply,
rectifies it to DC power, and produces high-
voltage DC pulses that are used to power the
electron gun and RF power source.
•The electron gun injects electrons into the
accelerator guide in pulses of the appropriate
duration, velocity, and position to maximize
acceleration.
•The RF power source, either a magnetron or a
klystron, supplies high-frequency
electromagnetic waves (3,000 MHz), which
accelerate the electrons injected from the
electron gun down the accelerator guide.
SMJ Mortazavi, Ph.D
Linear Accelerator
• The electron beam is
focused onto a metal
target (usually
tungsten).
• An ionization chamber
measures the radiation
output in real time and
is the means by which
the dose to the patient
is controlled.
SMJ Mortazavi, Ph.D
Linear Accelerator
SMJ Mortazavi, Ph.D
At high energies the
bremsstrahlung beam is
forward peaked, so a
metal ―flattening filter‖ is
used to produce a more
uniform beam
A set of moveable
collimators allow the
user to define
rectangular beams of
dimensions from 4 to 40
cm.
Linear Accelerator
Proton Beam Radiotherapy
• This form of external
beam irradiation involves
directing radiation
through the front of the
eye in order to reach the
intraocular tumor.
• When compared to low-
energy eye-plaque
radiation therapy, it is
easier to treat tumors that
are surrounding the optic
nerve with protons.
SMJ Mortazavi, Ph.D
Proton Therapy
• Protons can produce
excellent dose
distributions with a single
field.
• However, these
installations are an order
of magnitude more
expensive than photon
facilities and it is
questionable whether they
are justified when modern
conformal photon
techniques can produce
competitive results.
SMJ Mortazavi, Ph.D
Thank you for your attention!
SMJ Mortazavi, Ph.D