A comprehensive presentation on treatment with Proton Therapy

1. PROTON THERAPY
Presenter : Dr. Moumita Paul
PGT-3rd Year
Moderator : Dr. M.
Bhattacharyya
Professor
Dept. of Radiation Oncology

2. Introduction
• Proton is the nucleus of hydrogen atom and
has a positive charge of 1.6 x 10ˉ¹⁹ C
• Its mass is 1.6x10ˉ²⁷kg(1840 times of electron)
• It consists of 3 Quarks(two up and one down)
• It is the most stable particle in universe with
half life of >10³² years
• It decays into a neutron, a positron and a
neutrino.

3. History
• 1919 - The Existence of proton was first demonstrated
by Ernest Rutherford
• 1930 - E.O. Lawrence built the first cyclotron
• 1946 - Robert Wilson at Harvard University first
proposed that accelerated protons should be
considered for radiation therpy
• 1955 - Tobias and his colleagues at Lawrence Berkeley
Laboratory first treated patients with proton
• 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.

4. Proton Interactions
• It interacts with electrons and atomic nuclei in
the medium through coulomb force
a. Inelastic collisions
-with atomic electrons(ionisation and
excitation) – predominant contributor of
absorbed dose
- with nucleus (bremsstrahlung) – negligibly
small
b. Elastic scattering - primarily by nuclei,
without loss of energy

5. • Protons scatter through smaller angles so they
have sharper lateral distribution than photons
• Mass Stopping Power : The average rate of
energy loss of a particle per unit length in a
medium
• The mass stopping power is given by (S/ρ) ,
ρ=density of the medium
• It is more with low atomic number materials and
low with high atomic number materials
• High Z materials= Scattering
• Low Z materials= Absorption of energy and
slowing down Protons

6. Radiobiology
• The greater the LET, the greater is the RBE
• Because charged particles have greater LET than
the megavoltage X-rays, the RBE of charged
particles is ≥ 1
• Because the LET of charged particles increases as
the particles slow down near the end of their
range, so does their RBE
• So, RBE of protons is greatest in the region of
their Bragg peak
• RBE for proton has been universally adopted to
be 1.1

8. Proton dose distribution
• Depends on the concept of Linear energy transfer (LET)
• LET is defined as dE/dx, where dE is the mean energy
deposited over a distance dx in media.
• Mass stopping power is proportional to the square of
the particle charge and inversely proportional to the
square of its velocity
• As the particle velocity approaches zero near the end
of its range, the rate of energy loss becomes maximum.
• The sharp increase or peak in dose deposition at the
end of particle range is called the Bragg peak.

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

10. Bragg Peak
• Characteristics :
•Low entrance dose (plateau)
•Maximum dose at depth(Bragg peak)
•Rapid distal dose fall-off

11. What is SOBP?
• SOBP(Spread-out Bragg peak) beams are
beams of different energies used to provide
wider depth coverage
• Generated by using monoenergetic beams of
sufficiently high energy and range to cover the
distal end of the target volume and adding to
it beams of decreasing energy and intensity to
cover the proximal portion

12. Need of SOBP
•The Bragg peak of a monoenergetic particulate beam is too narrow to cover
the extent of most target volumes.
•In order to provide wider coverage, the Bragg peak can be spread out by
superim-position of several beams of different energies as spread-out Bragg
peak (SOBP).

14. Why Proton Beam Therapy?
• To Reduce dose to non target regions
• Dose escalation
• To Reduce probable second malignancies
• Better constraints to Organ at Risk

15. Proton Generators
• Protons are produced from hydrogen gas
1.Either obtained from electrolysis of
deionized water or
2. Commercially available high-purity hydrogen
gas.
• Application of a high-voltage electric current
to the hydrogen gas strips the electrons off
the hydrogen atoms, leaving positively
charged protons

16. Proton Accelerators
• Protons can be accelerated to high energies
using –
a) A linear accelerator
b) A cyclotron
c) A synchrotron
• Cyclotrons and synchrotrons are currently
the main accelerators for proton therapy
• High-gradient electrostatic accelerators and
Laser-plasma particle accelerators are on the
horizon.

17. Cyclotron
• It is a fixed energy machine which produces
continuous beam of monoenergitic (250Mev
Range ~ 38 cm in water) protons.
• This energy is sufficient to treat tumours at
any depth by modulating the range and
intensity of the beam with energy degraders.
• Cyclotrons can produce a large proton beam
current of up to 300 nA and thus deliver
proton therapy at a high dose rate.

18. *Energy degraders are 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 tumours
at any depth.
• Energy selection system (ESS) consist of energy
slits, bending magnets, and focusing magnets, is
then used to eliminate protons with excessive
energy or deviations in angular direction.

19. •Two short metallic cylinders, called Dees
•Placed between poles of direct magnetic field
•An alternating potential is applied between Dees
•Frequency is adjusted of alternating potential to accelerate
the particle as it passes from one Dee to another
•With each pass, the energy of the particle and the radius of
the orbit increases.

20. Synchrotron
• Produce proton beams of selectable energy,
thereby eliminating the need for the energy
degrader and energy selection devices.
• Beam currents are typically much lower than
with cyclotrons, thus limiting the maximum
dose rates that can be used for patient
treatment, especially for larger field sizes.

21. •Proton pulse exiting a pre-accelerator, with energy typically 3-
7 MeV is injected into ring shaped accelerator.
•Each complete circuit of the proton pulse through the
accelerator increases the proton energy.
•When the desired energy is reached, the proton pulse is
extracted from the applicator.

22. Cyclotrons vs Synchrotrons
Cyclotron Synchrotron
Needs energy degraders No need of energy degradors
Has energy selection systems No need of energy selection
system
Higher beam currents
produced (upto 300 nA)
Low beam currents
Delivery of high dose rate Due to low beam currents–the
dose rate is limited

23. Advantage of Synchrotron over Cyclotron
• Synchrotrons accelerate the charged particles
to precise energies needed for therapy
• Lower radiation exposure because of
elimination of energy degraders
• Less shielding required

24. Beam line/ transport system
• The proton beam has to be transported to the
treatment room(s) via the beam transport system.
• Consists of bending and focusing magnets and beam
profile monitors to check and modify beam quality as it
is transported through the beam transport system.
• Gantries are usually large because of 2 reasons
–protons with therapeutic energies can only be bent
with large radii and
–Beam monitoring and beam shaping devices have to
be positioned inside the treatment head affecting the
size of the nozzle
• Nozzle has a snout for mounting and positioning of
field specific aperture and compensator

25. A modern nozzle consists of many
components for creating and monitoring a
clinically useful beam—
• Rotating range-modulator wheel
• Range-shifter plates to bring the SOBP dose
distribution to the desired location
• Scattering filters to spread and flatten the
beam in lateral dimensions
• Dose-monitoring ion chambers
• An assembly to mount patient-specific field
aperture and range compensator

27. Beam delivery system
• The proton beam exiting the transport system is a
pencil-shaped beam with minimal energy and
direction spread.
• The beam has a small spot size in it’s lateral
direction and a narrow Bragg peak dose in its
depth direction.
• This dose distribution is not suitable for practical
size of tumors.
• Pencil beam is modified either by
1.Scattering BeamTechnique
2.Scanning BeamTechnique

28. 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
• The width and thickness of the modulation wheels are calibrated to
achieve SOBP.
• The width of SOBP is controlled by turning the beam off when a
prescribed width is reached.
• Small fields: single scattering foil (made out of Lead)
• Larger field sizes: double-scattering system (bi- material: High and
low z material) to ensure a uniform, flat lateral dose profile

29. Passive scattering

31. • Magnets are used to scan the beam over the
volume to be treated
• Uniform fields are produced without loss of range
by magnetically scanning a narrow beam of
proton
Eg. (i) Spot Scanning : In which the beam spot is
moved to a location within the target and the
prescribed dose delivered to the spot, before it
moves to another spot
(ii) Raster Scanning : In which the pencil beam
scans the field in a raster
Scanning beam technique

32. Scanning beam technique
• The proton beam intensity may be modulated
as the beam is moved across the field,
resulting in the modulated scanning beam
technique or IMPT.
• Current implementation of IMPT uses so
called spot scanning technique.

34. Advantage of scanning
• In contrast to broad beam technique, arbitrary
shapes of uniform high dose regions can be
achieved with a single beam
• No first and second scatterers, less nuclear
interactions and therefore the neutron
contamination is smaller
• Great flexibility, which can be fully utilized in
intensity-modulated proton therapy (IMPT)
Disadvantage
• Technically difficult and more sensitive to organ
motion than passive scattering

35. Clinically used range of Proton
• 70-250 MeV

36. Treatment planning
• Treatment planning for proton therapy requires a
volumetric patient CT scan dataset.
• The CT HU numbers are converted to proton
stopping power values for calculating the proton
range required for the treatment field.
• Delineation of target volumes and OARs;
selection of beam angles and energies, design of
field aperture, optimisation of treatment
parameters, plan evaluation are similar.
• Uncertainties in the conversion of CT numbers to
proton stopping power in proton dose calculation
translate into range calculation uncertainties and
errors.

37. • 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.
• Other consideration in determining the margins
include target motion, daily set up errors, beam
delivery uncertainties and uncertainties in the
anatomy and physiologic changes in the patient.
• In contrast to x-ray planning, the PTV for proton
therapy is specific for each treatment field.
• Lateral margins are identical to traditional
definitions, but the distal and proximal margins
along the beam axis are calculated to account for
proton specific uncertainties.

38. Dose calculation algorithms
• Pencil beam
• Convolution/superposition
• Monte Carlo

39. Photon vs Proton Therapy
Photon Therapy Proton therapy
Has a significant exit dose Has no exit dose
More integral dose Less integral dose- preferred modality in
pediatric tumours
Dose escalation not possible beyond a
limit
Dose escalation is possible
Surrounding normal tissues are exposed
to high doses comparatively
Significant reduction in the exposure of
normal tissues beyond the target
Not suitable for tumours where nearby
critical organs are to be spared
Suitable for tumours situated near critical
structures like ocular malignancies,
tumours of brain, spine , lung
At the point of entrance, higher dose is
deposited
Lower dose at point of entrance

40. Clinical Applications
• Pediatric malignancies:
-- Craniospinal Axis Irradiation: Medulloblastoma
-- Craniopharyngioma
• Prostate cancers
• Skull base tumors
• Paranasal sinus tumors, Lymphomas, Lung Cancers
• GI Malignancy: HCC, Pancreatic cancers
• Recurrent ,radioresistant or unresectable head
and neck cancers like ACC, Malignant melanoma
• Sarcoma

41. When Should We Use Protons?
• Better organ sparing (Skull base tumours)
• Better local control needed (Ca Prostate)
• Late morbidity (Pediatric malignancies)
• Complex geometry (Ocular melanoma)
• Large target volume (Childhood
Medulloblastoma)

42. CSI

43. CSI
• The exit dose from photon therapy exposes the
thyroid, heart, lung, gut, and gonads to functional
and neoplastic risks that can be avoided with
proton therapy.
• 3DCRT compared with PROTON THERAPY
• The total-body :V10 37.2% and 28.7%
• Total-body integral dose : 0.223 Gy-m3 and 0.185
Gy-m3
*Krejcarek SC, Grant PE, Henson JW, et al.. Int J
RadiatOncol Biol Phys 2007;68:646–649.

44. Lung Cancer
• Lung cancers typically are diagnosed at an
advanced stage and occur in patients with
underlying lung damage.
• Consequently, concern for protection of
unaffected lung tissue often mandates
compromise in the tumour dose.
• A smaller volume of non targeted lung tissue,
spinal cord, esophagus, and heart is exposed
to radiation with proton therapy.

45. Lung Cancer

46. Lung Cancer
• The proton plan lowers the risk of
-- Acute (potentially fatal) pneumonitis
-- Acute esophagitis
• Has impact on the delivery of chemotherapy,
as well as the cardiac exposure, likely
correlating with greater chance of survival.
*Chang JY, Zhang X,Wang X, et al. Int J Radiat
Oncol Biol Phys 2006;65:1087–1096

47. Prostate Cancer
• Prostate cancer results with IMRT are
generally excellent, but dose-escalation trials
are significantly associated with the incidence
of gastrointestinal toxicity.
• Dosimetry studies show that the low to
moderate doses delivered to the rectum with
proton therapy are less than with IMRT

48. Prostate Cancer

49. Prostate Cancer
• Rectal wall V30, V40, and V50 :29%, 23%, and
17% with IMRT
• Rectal wall V30, V40, and V50 : 18%, 16%, and
14% with proton therapy
*Vargas C, Fryer A, MahajanC, et al. Dose-
volume comparison of proton therapy and
intensity-modulated radiotherapy for prostate
cancer. Int J RadiatOncol Biol Phys
2008;70:744–751.

50. DISADVANTAGES OF PROTON THERAPY
Patient related
• Patient set up
• Organ motion
• Patient movement
Physics related
• CT number conversion
• Dosimetry
Machine related
• Cumbersome- large area requirement
• Cost

51. CONCLUSION
• Currently, proton therapy is a rare medical resource.
• Best used in situations where outcomes with
commonly available radiation strategies present
opportunities for improvement in the therapeutic ratio
via improvements in dose distributions.
• Protons give less integral dose than photons by a factor
of 3.
• Sharper dose drop-off beyond the Bragg peak is a
double-edged sword – better dose conformity but
greater chances of geometric miss in depth.
• At this stage in the development of proton therapy,
there are no clear class solutions to treatment
planning.

52. CONCLUSION
• In addition, the full potential for dose distribution
improvements with protons has not been realized
because of uncertainties in both treatment-
planning algorithms and delivery modes.
• Strategies for motion management and quality
assurance are not fully developed.
• Finally, the clinical impact of some patterns of
dose distribution improvements achievable with
proton therapy may require time, careful trial
design, and special assessments to define.

53. Thank
You