https://en.wikipedia.org/wiki/Particle_therapy
Particle therapy is a form of external beam radiotherapy using beams of energetic neutrons, protons, or other heavier positive ions for cancer treatment. The most common type of particle therapy as of 2012 is proton therapy.
In contrast to X-rays (photon beams) used in older radiotherapy, particle beams exhibit a Bragg peak in energy loss through the body, delivering their maximum radiation dose at or near the tumor and minimizing damage to surrounding normal tissues.
Particle therapy is also referred to more technically as hadron therapy, excluding photon and electron therapy. Neutron capture therapy, which depends on a secondary nuclear reaction, is also not considered here. Muon therapy, a rare type of particle therapy not within the categories above, has also been attempted.
Particle therapy is a form of external beam radiotherapy using beams of energetic neutrons, protons, or other heavier positive ions for cancer treatment. The most common type of particle therapy as of 2012 is proton therapy.
In contrast to X-rays (photon beams) used in older radiotherapy, particle beams exhibit a Bragg peak in energy loss through the body, delivering their maximum radiation dose at or near the tumor and minimizing damage to surrounding normal tissues.
Particle therapy is also referred to more technically as hadron therapy, excluding photon and electron therapy. Neutron capture therapy, which depends on a secondary nuclear reaction, is also not considered here. Muon therapy, a rare type of particle therapy not within the categories above, has also been attempted.
Method
Unlike
electrons or X-rays, the dose from protons to tissue is maximum just
over the last few millimeters of the particle’s range.
Particle therapy works by aiming energetic ionizing particles at the target tumor. These particles damage the DNA
of tissue cells, ultimately causing their death. Because of their
reduced ability to repair DNA, cancerous cells are particularly
vulnerable to such damage.
The figure shows how beams of electrons, X-rays or protons of different energies (expressed in MeV) penetrate human tissue. Electrons have a short range and are therefore only of interest close to the skin. Bremsstrahlung X-rays penetrate more deeply, but the dose absorbed by the tissue then shows the typical exponential decay
with increasing thickness. For protons and heavier ions, on the other
hand, the dose increases while the particle penetrates the tissue and loses energy continuously. Hence the dose increases with increasing thickness up to the Bragg peak that occurs near the end of the particle's range. Beyond the Bragg peak, the dose drops to zero (for protons) or almost zero (for heavier ions).
The advantage of this energy deposition profile is that less
energy is deposited into the healthy tissue surrounding the target
tissue. This enables higher dose prescription to the tumor,
theoretically leading to a higher local control rate, as well as
achieving a low toxicity rate.
The ions are first accelerated by means of a cyclotron or synchrotron.
The final energy of the emerging particle beam defines the depth of
penetration, and hence, the location of the maximum energy deposition.
Since it is easy to deflect the beam by means of electro-magnets in a
transverse direction, it is possible to employ a raster scan
method, i.e., to scan the target area quickly like the electron beam
scans a TV tube. If, in addition, the beam energy and hence, the depth
of penetration is varied, an entire target volume can be covered in
three dimensions, providing an irradiation exactly following the shape
of the tumor. This is one of the great advantages compared to
conventional X-ray therapy.
At the end of 2008, 28 treatment facilities were in operation worldwide and over 70,000 patients had been treated by means of pions, protons and heavier ions. Most of this therapy has been conducted using protons.
At the end of 2013, 105,000 patients had been treated with proton beams, and approximately 13,000 patients had received carbon-ion therapy.
As of April 1, 2015, for proton beam therapy, there are 49
facilities in the world, including 14 in the USA. with another 29
facilities under construction. For Carbon-ion therapy, there are eight
centers operating and four under construction.
Carbon-ion therapy centers exist in Japan, Germany, Italy, and China.
Two USA federal agencies are hoping to stimulate the establishment of at
least one US heavy-ion therapy center.
Proton therapy
Fast-neutron therapy
Carbon-ion radiotherapy
Carbon ion
therapy (CIRT) uses particles more massive than protons or neutrons.
Carbon-ion radiotherapy has increasingly garnered scientific attention
as technological delivery options have improved and clinical studies
have demonstrated its treatment advantages for many cancers such as
prostate, head and neck, lung, and liver cancers, bone and soft tissue
sarcomas, locally recurrent rectal cancer, and pancreatic cancer,
including locally advanced disease. It also has clear advantages to
treat otherwise intractable hypoxic and radio-resistant cancers while
opening the door for substantially hypo-fractionated treatment of normal
and radio-sensitive disease.
By mid 2017, more than 15,000 patients have been treated
worldwide in over 8 operational centers. Japan has been a conspicuous
leader in this field. There are five heavy-ion radiotherapy facilities
in operation and plans exist to construct several more facilities in the
near future. In Germany this type of treatment is available at the
Heidelberg Ion-Beam Therapy Center (HIT) and at the Marburg Ion-Beam
Therapy Center (MIT). In Italy the National Centre of Oncological
Hadrontherapy (CNAO) provides this treatment. Austria will open a CIRT
center in 2017, with centers in South Korea, Taiwan, and China soon to
open. No CIRT facility now operates in the United States but several are
in various states of development.
Biological advantages of heavy-ion radiotherapy
From
a radiation biology standpoint, there is considerable rationale to
support use of heavy-ion beams in treating cancer patients. All proton
and other heavy ion beam therapies exhibit a defined Bragg peak in the
body so they deliver their maximum lethal dosage at or near the tumor.
This minimizes harmful radiation to the surrounding normal tissues.
However, carbon-ions are heavier than protons and so provide a higher
relative biological effectiveness (RBE), which increases with depth to
reach the maximum at the end of the beam's range. Thus the RBE of a
carbon ion beam increases as the ions advance deeper into the
tumor-lying region. CIRT provides the highest linear energy transfer (LET) of any currently available form of clinical radiation.
This high energy delivery to the tumor results in many double-strand
DNA breaks which are very difficult for the tumor to repair.
Conventional radiation produces principally single strand DNA breaks
which can allow many of the tumor cells to survive. The higher outright
cell mortality produced by CIRT may also provide a clearer antigen
signature to stimulate the patient's immune system.
Particle therapy of moving targets
The
precision of particle therapy of tumors situated in thorax and
abdominal region is strongly affected by the target motion. The
mitigation of its negative influence requires advanced techniques of
tumor position monitoring (e.g. fluoroscopic imaging of implanted
radio-opaque fiducial markers or electromagnetic detection of inserted
transponders) and irradiation (gating, rescanning, gated rescanning and
tumor tracking).
