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Particle therapy is a form of external beam radiotherapy using beams of energetic , , or other heavier positive for cancer treatment. The most common type of particle therapy as of August 2021 is proton therapy.
In contrast to (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 studied theoretically; however, muons are still most commonly used for imaging, rather than therapy.
The figure shows how beams of electrons, X-rays or protons of different energies (expressed in electron volt) penetrate human tissue. Electrons have a short range and are therefore only of interest close to the skin (see electron therapy). Bremsstrahlung X-rays penetrate more deeply, but the Absorbed 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, as 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 , protons and heavier ions. Most of this therapy has been conducted using protons. PTCOG: Particle Therapy Co-Operative Group
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 US 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 US federal agencies are hoping to stimulate the establishment of at least one US heavy-ion therapy center.
C-ion RT 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. In China, the Shanghai Proton and Heavy Ion Center (SPHIC) opened for treatments in 2015 and Austria will open a CIRT center in 2017, with centers in South Korea and Taiwan soon to open. No CIRT facility now operates in the United States but several are in various states of development. At Mayo Clinic in Jacksonville, Florida, the first carbon ion radiotherapy in North America is planned to begin in 2028.
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