Proton Beam production and properties

1. PROTON BEAM THERAPY
DR.CHAITRA DESHPANDE
IIIYR RESIDENT, RADIATION ONCOLOGY, AJIMS,
MANGALORE

3. NATURE OF THE PARTICLE
• According to the Big Bang Theory, hydrogen was the first element to form in the universe
(∼100 seconds after the creation of the universe about 13.7 billion years ago).
• Proton is the nucleus of the hydrogen atom.
• It carries a unit positive charge (1.6 × 10−19 C) and has a mass of 1.6 × 10−27 kg (∼1,840
times the mass of electron).
• proton has a substructure, It consists of three quarks (two up and one down) held together
by gluons.
• Proton is the most stable particle (half-life of >1032 years) and decays into a neutron, a
positron, and a neutrino.

4. THE EXISTENCE OF PROTON WAS FIRST DEMONSTRATED BY
ERNEST RUTHERFORD IN 1919.

5. ROBERT WILSON AT HARVARD UNIVERSITY MADE THE FIRST
PROPOSAL IN 1946 THAT ACCELERATED PROTONS SHOULD BE
CONSIDERED FOR RADIATION THERAPY.
•
Fermilab,
Chicago.

6. TOBIAS AND HIS COLLEAGUES AT LAWRENCE BERKELEY LABORATORY FIRST
TREATED PATIENTS WITH PITUITARY TUMOURS WITH PROTONS IN 1955.
1958 : First use of protons as a neurosurgical tool
1990: First hospital based proton therapy facility was opened at the Loma Linda University
Medical Center (LLUMC) in California.

7. INTERACTIONS
• As protons travel through a medium, they interact with atomic electrons and atomic nuclei of the
medium through Coulomb force.
• Interactions mediated by Coulomb force are
(a) inelastic collisions with atomic electrons in which protons lose part of their kinetic energy to produce
ionization and excitation of atoms, thereby resulting in absorbed dose; bremsstrahlung interactions
with nuclei are possible but negligible; and
(b) elastic scattering without loss of energy. Nuclear scattering is the main contributor to multiple
Coulomb scattering of protons.
• Compared to the electron beams, the proton beams (because of having heavier charged particles)
scatter through much smaller angles. As a result, proton beams have a sharper lateral distribution
than the electron or photon beams.

9. MASS STOPPING POWER
• Mass stopping power is the energy loss per unit path length in g/cm2.
• Mass stopping power for protons is greater in low atomic-number (Z)
materials than in high-Z materials.
• low-Z materials are more effective in slowing down protons and High-Z
materials scatter protons through larger angles than the low-Z materials.
• Thus, if we want to scatter a beam with minimum loss of energy (principle of
scattering foils), we should use high-Z materials, and if we want to decrease
proton energy with minimum scattering, we should use low-Z materials.

10. BRAGG PEAK
• The average rate of energy loss of a particle per unit path length in a medium is called the stopping power.
• The linear stopping power (–dE/dx) is measured in units of MeV/cm,also referred to as the linear energy
transfer of the particle.
• These basic parameters, namely stopping power and LET, are closely related to dose deposition in a
medium and with the biologic effectiveness of radiation.
• The rate of energy loss due to ionization and excitation caused by a charged particle traveling in a medium
is proportional to the square of the particle charge and inversely proportional to the square of its velocity.
• As the particle loses energy, it slows down and the rate of energy loss per unit path length increases.
• As the particle velocity approaches zero near the end of its range, the rate of energy loss becomes
maximum.

11. • The depth dose distribution follows the rate
of energy loss in the medium.
• For a monoenergetic proton beam, there is a
slow increase in dose with depth initially,
followed by a sharp increase near the end of
range.
• This sharp increase or peak in dose
deposition at the end of particle range is
called the Bragg peak
BRAGG PEAK

12. SOBP
• The Bragg peak of a monoenergetic
proton beam is too narrow to cover the
extent of most target volumes
• These beams are called the spread-out
Bragg peak (SOBP) beams.
• The SOBP beams are generated by
employing a monoenergetic beam of
sufficiently high energy and range to
cover the distal end of the target volume
and adding beams of decreasing energy
and intensity to cover the proximal
portion.

13. RBE AND LET OF PROTONS
• Relative biologic effectiveness (RBE) of any radiation is
the ratio of the dose of 250-kVp x-rays to produce a
specified biologic effect to the dose of the given
radiation to produce the same effect.
• The greater the LET, the greater is the RBE.
• Because charged particles, in general, have greater LET
than the megavoltage x-rays, the RBE of charged
particles is greater than or equal to 1.0.
• It is seen that the slope of the survival curve is greater
for the higher LET radiations, thus giving rise to higher
RBE.
• Because the LET of charged particles increases as the
particles slow down near the end of their range, so does
their RBE.

14. PROTON ACCELERATORS
• Protons can be accelerated to high energies by using
• (a) a linear accelerator,
• (b) a cyclotron, or
• (c) a synchrotron.
• (d) High gradient EletrostaticAccelerator
• (e)Laser Plasma particle Accelerator
• A linear accelerator would require a large amount of space to generate proton beams in the clinically useful
range of energies. Therefore, cyclotrons and synchrotrons are currently the main accelerators for proton beam
therapy

15. CYCLOTRON
• It is a fixed energy
machine which
produces continuous
beam of monoenergitic
(250Mev Range)
protons.
• Cyclotrons can
produce a large proton
beam current of up to
300 nAand thus deliver
proton therapy at a
high dose rate.

16. • It is a short metallic cylinder divided into two sections,
usually referred to as dees (for their resemblance to the
letter D).
• The dees are highly evacuated and subjected to a constant
strength magnetic field applied perpendicular to the plane
of the dees.
• A square wave of electric field is applied across the gap
between the two dees. Protons are injected at the center
of the cyclotron and accelerated each time they cross the
gap.
• The polarity of the electric field is switched at the exact
time the beam re-enters the gap from the opposite
direction.
• The constant magnetic field confines the beam in ever-
increasing orbits within the dees until the maximum
energy is achieved and extracted.

17. ENERGY DEGRADERS
• A cyclotron used in radiotherapy is a fixed-energy machine, designed to generate proton
beams of a maximum energy of about 250 MeV (range ∼38 cm in water).
• This energy would be sufficient to treat tumors at any depth by modulating the range and
intensity of the beam with energy degraders.
• The energy degraders consist of plastic materials of variable thickness and widths to
appropriately reduce the range of protons as well as achieve differential weighting of the
shifted Bragg peaks in order to create SOBP beams suitable for treating tumors at any
depth.
• in the IBA cyclotron, the energy degrader consists of a variable-thickness polycarbonate
wheel located in the beam line.
• It is rotated into position to insert appropriate degrader thickness in the beam to reduce
the proton range down to the desired depth.

18. SYNCHROTRON
• In the synchrotron, a proton beam of 3 to 7 MeV, typically from a linear accelerator, is injected and
circulated in a narrow vacuum tube ring by the action of magnets located along the circular path of the
beam .
• The proton beam is accelerated repeatedly through the radiofrequency (RF) cavity (or cavities),
powered by a sinusoidal voltage with a frequency that matches the frequency of the circulating protons.
• Protons are kept within the tube ring by the bending action of the magnets.
• The strength of the magnetic field and the RF frequency are increased in synchrony with the increase in
beam energy, hence the name synchrotron.
• When the beam reaches the desired energy, it is extracted.
• synchrotron is operated to produce the SOBP beams at any desired depth without the use of energy
degraders.

20. BEAM DELIVERY SYSTEMS
• A single accelerator can provide proton beam in several treatment rooms .
• Beam transport to a particular room is controlled by bending magnets, which can
be selectively energized to switch the beam to the desired room.
• The particle beam diameter is as small as possible during transport.
• Just before the patient enters the treatment room, the beam is spread out to its
required field cross section in the treatment head—the nozzle.
• This beam spreading is done in two ways:
• (a) passive scattering, in which the beam is scattered using thin sheets of high-
atomic-number materials (e.g., lead, to provide maximum scattering and minimum
energy loss); or
• (b) scanning, in which magnets are used to scan the beam over the volume to be
treated.

21. SCATTERING BEAM TECHNIQUE
• It aims to produce a dose distribution with a flat lateral profile.
• The depth-dose curve with a plateau of adequate width is produced by
summing a number of Bragg peaks
• Range modulation wheels consisting of variable thicknesses of acrylic glass or
graphite steps are traditionally used for this purpose.

23. SCANNING BEAM TECHNIQUE
• As the pencil beam exits the transport system, it is magnetically
steered in the lateral directions to deliver dose to a large
treatment field.
• The proton beam intensity may be modulated as the beam is
moved across the field, resulting in the modulated scanning beam
technique or IMPT

24. TREATMENT PLANNING
• Treatment planning for proton therapy requires a volumetric patient CT scan
dataset
• Marking the intended SOBP with a distal margin beyond the target and a
proximal margin before the target in the range calculation of each treatment
field.
• The concept of PTV does not strictly apply to proton therapy.

26. POTENTIAL APPLICATIONS
• Many publications have reported significant differences in dose distribution
• Reduction in the volume of non targeted receiving low- to medium-range
radiation doses.
• In some cases, there is also a reduction in the volume of non targeted tissue
receiving moderate- to high-dose irradiation.

27. THANK YOU