PROTON THERAPY

Types of Radiations

• X-rays• γ-rays

• α-particles• β-particles• β+-particles•Protons

• neutrons

Carry enough energy which if deposited in matter can produce ions

Radiation therapy idea

Selective cell destruction (cancer)

How it can be done?

By destroying the cell using Energy

High energy particles damage a cell by altering it’s atom

Cause the atom’s electron to become excited and then ionized Enzymes repair this damage

• But cancer cell slower than healthy cell

So, the end results (during radiation exposure )

More cancer cell end up dying more than healthy cell

Reminder

• Absorbed dose D is the energy (joules) deposited per unit mass (kg) of target material, D = dE/dm.

• The special unit of absorbed dose D is the Gray (Gy) ≡ 1 Joule/kg

• In biological systems• Radiation Biologic effects dependent on “the

spatial distribution of energy deposition” (LET)

Linear Energy Transfer is energy deposited per unit path length = dE/dx with units ev/cm

Overview of presentation

• Photon therapy (briefly) • Proton therapy (in detailed)• How it works ?• The remarkable phenomenon of physics “Bragg peak”• Delivery of the beam (how it can be useful )• How it can be produced ? (synchrotron) • RBE of protons .• Proton therapy Vs Photon therapy .(summary)

The desirable goal

In order to treat cancer :

The main goal is to delivers a defined dose distribution within the target volume and none out side it.

Now

Let’s see what type of radiation would be the

Best??

Treatment options

1)Photon therapy.

2) Proton therapy.

Interactions of Photons

There are 3 modes:

• Photo-electric effect. Entire energy transfer from

photon to an atomic electron .

• Compton effect.Fraction of energy transferred to

Compton electrons.

• Pair production.

What happen when a beam of photon entering

a tissue ?

Exponential behaviour

• It falls exponentially

• Number of photon gets attenuated

as depth increases .• As their number decreases,

the dose that they deposit decreases also (proportionately ).

o enE E exp( x)

Photon’s therapy failure

• Based on “how radiation interacts with matter”

The failure is :

Most of the radiation is deposited on healthy tissue.

Cause of failure !!• They are not easy to control

Why ?(low mass & high energy)

“Low LET”

Treatment options

1) Photon therapy

2)Proton therapy.

Did Proton therapy has the solution ?

What can Proton therapy provide ?

Short story

• “A man with a vision “

In 1946 Harvard physicist ,

Robert Wilson suggested:

• Protons can be used clinically .• Maximum radiation dose can

be placed into the tumor .• Proton therapy provides

sparing of healthy tissues .

Characteristics of protons

• Subatomic particle . • Stable , positively charged .• Heavy particle with mass 1800

that of electron.• Very little scattered as they

travel through tissue . • Travel in straight lines.

Which leads to very different modes of interactions

with matter .

Let’s see!!!!!!

Mp=1.672621636(83)×10−27 kg9.10938215(45×)10−31 kg Me=

Interactions of Protons

• Coulomb interactions with atomic electrons .

Electronic (ionization ,excitation)

• Coulomb interactions with atomic nuclei .

“multiple Coulomb scattering.”

• Nuclear interactions with atomic nuclei .

Elastic nuclear collision Non elastic nuclear collision

Key fact

Different modes of interactions

Means Different dose distributions

The shape of dose distribution

It means that :• Low entrance dose

(plateau)• Maximum dose at depth

(Bragg peak)• Rapid distal dose fall-off

But Why this shape of

distribution ?

Let’s see

Remarkable phenomena “Bragg peak”

Protons have the ability of loosing little energy when entering tissue .

But depositing more and more as they slow down…..

Finally, depositing a heavy dose of radiation just before they stop ,

giving rise to theso-called Bragg peak

”“dE/dx profiles Energy loss

• a proton’s linear rate of energy loss “linear energy transfer” (LET)

• is given by the Bethe-Block formula:

Bragg Peak like (skiing)

NOTE THE SIMILARITY

Bragg peak dependence on energy

• The range is( the depth of penetration

from the front surface to the distal point on the Bragg peak)

• Bragg peak

depends on the initial

energy of the protons so

the greater the energy, the greater the range

There is a problem

Is the current shape of Bragg peak could provide the tumor with uniform dose ?

No, it can’t. Because

The Bragg peak is too narrow to fit the shape & depth of

the tumor

Is there a solution?

So, how to make the beam of proton useful for treatment?

Is it possible to shape the beam to fit the

shape of the tumor ?

Let’s see!!!!!

Smart Idea

• The spread-out Bragg peak (SOBP):

• Extending the dose in depth

means

An extension in depth can be achieved by proton beams of successively

delivering not just one, but many Bragg peaks each with different range (energy)

Superposition of Bragg-peaks by energy variation

energy variation

(Nozzle ) Beam delivery system

There are two main approaches ( techniques) for shaping the beam : (both laterally and in

depth)

1) passive scattering.

2)Scanned beam.

passive scattering.

Shaping the beam Laterally

The beam is spread laterally to clinically useful size by

double – scatterer

and compensator

Click icon to add clip art

Tailoring the beam in depth:((fan like the range modulator

The modulator spins around in front of the proton beam pulling the beam back and forward causing a flat topped dose distribution

providing the tumor with a uniform dose.

Scanned beam

• Expand the lateral dimensions of a proton beam by using the electromagnetic

technique to scan the beam laterally & in shape .

The engine))Synchrotrons

• What is Synchrotron mission ?

• They produce the proton beam .

• It is a modified Cyclotrons.

synchrotron provides energy variation by extracting the

protons when they have reached the desired energy.

Hardware components

• Proton accelerator• Beam transport

system• Treatment Rooms• Gantry• Standard table

A word about Treatment plane

How do you know what to include and what to exclude in treating deep –seated tumors with radiation?

By using number of imaging tools (CT,MRI,PET….) Gives ability to see To image To map

Relative Biological Effectiveness of proton

RBE is the ratio of the dose of reference radiation beam (e.g., photons) to that of test beam (e.g., protons) required to produce a defined biological response .

• Is used to compare the biologic effects of various radiation sources .

Protons has exactly the same biologic effects as X-rays!!

Because the calculated RBE is 1.1

The bottom line is that the only difference between protons and standard X-rays lies in the physical properties of the beam and not in the biologic effects in tissue.

SUMMARY

• Photon therapy Proton therapy

the interactions are stochastic . they are deterministic events .

they not easy to control . They easier to control .

At point of entrance,It receive large amount of dose. It receive very small dose .

As they reached the tumor,Continue to pass through tissue a sharp burst of energy released

at tumor and none beyond it.

Used for treat superficial tumors. ideal for tumors in or near

critical structures (brain, heart,

eye) pediatric cancers.

References and sites

• Radiation Oncology A Physicist's-Eye View: Michael Goitein .

• Radiation therapy physics: William R Hendee & Geoffrey .

• Sites:• www.wikipedia.org • Loma Linda University Medical Center

www.llu.edu• www.mpri.org • www.proton-therapy.org• http://www.varian.com/

THANKS FOR LISTENING

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