SMJ Mortazavi, Ph.D

Professor of Medical Physics

mmortazavi@sums.ac.ir

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