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Washington and Leaver Chapter 16 Radiation Therapy Notes
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Radiation Therapy PhysicsChapter 16 Notes
Particle therapy has the ability to reduce the exposure to normal tissue because maximum energy
can be focused to a specific range
Particle therapy is EBRT that uses particles accelerated to high energies
Most commonly used therapeutically is carbon and proton particles
Maximum energy can be spread out – 3D treatment possible (unlike in photon therapy)
Uncertainty’s in…
-Range
-CT density conversions
-RBE (relative biologic effectiveness) at the distal range of the particle beam
Proton therapy represents 88% of particle therapies (100% in the U.S)
Proton therapy has been traditionally treated with passively scattered particle therapy (PSPT)
which delivers monoenergetic protons, and apertures and compensators are used to shape the
dose uniformly throughout the tumor
Advancements include being able to treat with pencil beam scanning (PBS) aka intensity-
modulated proton therapy (IMPT) which delivers protons by scanning multi-energy protons as
discrete or continuous spots with magnets throughout the tumor
IMPT can now match the high-dose conformality of IMRT with the added benefit of low-dose
sparing to normal tissue
Protons were discovered by Ernest Rutherford in the early nineteenth century – named after
Greek word “protos” meaning first
He concluded that protons have a definable range
Bragg peak – (discovered by William Bragg) most of the proton dose (determined by energy) is
deposited at a certain depth, no dose after this specified depth
Ernest O. Lawrence discovered first cyclotron in 1930 – it could accelerate protons with
sufficiently high energy to treat cancer
-accelerates protons by alternating electric fields in a circular magnetic field
Dr. Robert Rathburn Wilson (Harvard physicist) was the first to propose proton therapy for the
treatment of cancer
-emphasized Bragg peak and range modulator wheel to spread maximum energy
-modulator wheel made of absorbers of multiple thicknesses that rotate in the path of the
beam to change the range of the maximum energy needed to reach the most distal end of the
target volume
Most early facilities weren’t used for patient care, but primary tumors treated were intracranial
and ocular tumors
, Worlds first hospital-based proton therapy facility – 1990 – California – passively scattered
synchrotron with three rotating gantries and one fixed beam
-synchrotron accelerates charged particles within an electric field and has an increasing
magnetic field for increasing energy
Both cyclotrons and synchrotrons are used to treat cancer worldwide, and they both produce
about 1-2 Gy per minute for a 30cmx30cm field size
-synchrotrons achieve a continuously variable energy through a clean methodology
-ability to generate energies needed to treat patients
-complex system needed to extract correct energies
-longer treatment times due to lesser current
-cyclotrons create a higher dose rate with a uniform intensity
-need energy degraders
-produce secondary neutrons
-metals then need “cool down time”
Therapeutic proton range 50-250 MeV
Protons are transported in a vacuum through a series of strong magnets
Nozzle – houses devices used to focus and shape the beam
Passively scattered particle therapy (PSPT):
-most commonly used proton beam therapy (PBS growing rapidly)
-single or double scattering devices within the nozzle spread out the focused proton pencil beam
to the lateral edges of the field
-range modulator wheel is used for the spread-out of Bragg peak (SOBP) so that maximum
energy can treat the whole tumor volume
-distal dose is conformed to the tumor shape using an acrylic or wax compensator, TPS creates
the compensator based on WET (water equivalent thickness) between the patient’s skin surface
and tumor volume, the compensator is then smeared (a process that modifies the compensator
design to consider the internal motion of the tumor and setup uncertainties)
-smearing ensures that the full tumor is still encompassed even with uncertainties
Pencil beam scanning (PBS) particle therapy:
-uses the charged protons and two magnets to focus and direct the protons throughout the tumor
in a scanning fashion
-cyclotron delivers scanning beam therapy through an energy selection system after beam
extraction and before the nozzle because only the maximum energy can be extracted in a
cyclotron
-synchrotron achieves scanning beam therapy by extracting the required energy at each range,
and then the protons are spread laterally by a lateral spot beam scan (beam shaping devices are
not needed)
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