Types of Radiations X-rays• α-particles• neutrons• γ-rays• β-particles• β+-particles• Protons• Carry enough energy which if deposited in matter can produce ions
Radiation therapy ideaSelective cell destruction (cancer) How it can be done?By destroying the cell using EnergyHigh energy particles damage a cell by altering it’s atomCause the atom’s electron to become excited and then ionizedEnzymes repair this damage • But cancer cell slower than healthy cellSo, 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 goalIn 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??
Interactions of PhotonsThere 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 E E o exp( en x)• Number of photon gets attenuated as depth increases .• As their number decreases, the dose that they deposit decreases also (proportionately ).
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”
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 veryMp=1.672621636(83)×10−27 kgMe= 9.10938215(45)×10−31 kg different modes of interactions with matter . Let’s see!!!!!!
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 distributionIt 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
Energy loss “dE/dx profiles• a proton’s linear rate of energy loss “linear energy transfer” (LET)• is given by the Bethe- Block formula:
Bragg peak dependence onenergy• The range is( the depth of penetrationfrom the front surface to the distal point on the Bragg peak)• Bragg peakdepends on the initialenergy of the protons sothe greater the energy, the greater the range
There is a problemIs the current shape of Bragg peak could provide the tumor with uniform dose ? No, it can’t. BecauseThe 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 meansAn extension in depth can be Superposition of Bragg-peaks by achieved by proton beams energy variation of successivelydelivering not just one, but many Bragg peaks each with different range (energy) energy variation
Beam delivery system Nozzle There are two main approaches ( techniques) for shaping the beam : (both laterally and in depth) 1) passive scattering. 2)Scanned beam.
Shaping the beam LaterallyThe beam is spread laterally to clinically useful size bydouble – scattererand compensator
Tailoring the beam in depth:the range modulator (fan likeThe modulator spins around in front of the proton beam pulling the beam back and forward causing a flat topped dose distributionproviding the tumor with a uniform dose.
Scanned beam• Expand the lateral dimensions of a proton beam by using the electromagnetictechnique to scan the beam laterally & in shape .
Synchrotrons The engine)• What is Synchrotron mission ?• They produce the proton beam .• It is a modified Cyclotrons.synchrotron provides energy variation by extracting theprotons when they have reached the desired energy.
Hardware components• Proton accelerator• Beam transport system• Treatment Rooms• Gantry• Standard table
A word about Treatment planeHow do you know whatto include and what toexclude in treatingdeep –seated tumorswith radiation?By using number ofimaging tools(CT,MRI,PET….)Gives ability to seeTo imageTo map
Relative Biological Effectivenessof protonRBE 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 1.1The 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 therapythe 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 Physicists-Eye View: Michael Goitein .• Radiation therapy physics: William R Hendee & Geoffrey .• Sites:• www.wikipedia.org• Loma Linda University Medical Centerwww.llu.edu• www.mpri.org• www.proton-therapy.org• http://www.varian.com/