Particle therapy is a form of external beam radiotherapy using beams of energetic protons, neutrons, or positive ions for cancer treatment. The most common type of particle therapy as of 2012 is proton therapy. Although a photon, used in x-ray or gamma ray therapy, can also be considered a particle, photon therapy is not considered here. Additionally, electron therapy is generally put into its own category. Because of this, particle therapy is sometimes referred to, more correctly, as hadron therapy (that is, therapy with particles that are made of quarks).
Neutron capture therapy might be considered a type of particle therapy, but it is not discussed here, as the damage it does to tumors is mostly from energetic ions produced by the secondary nuclear reaction after the neutrons in the external beam are absorbed into boron-10 (or occasionally some other nuclide), and not due primarily to the neutrons themselves. It is therefore a type of secondary particle therapy.
Muon therapy, a rare type of particle therapy not within the categories above, has occasionally been attempted.
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 damaged DNA, cancerous cells are particularly vulnerable to attack.
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 (see electron therapy). 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.
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.
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.
Heavy-ion therapy is the use of particles more massive than protons or neutrons, such as carbon ions. Compared to protons, carbon ions have an advantage: due to the higher density of ionization at the end of their range, correlated damages of the DNA structure within one cell occur more often so that it becomes more difficult for the cancerous cell to repair the damage. This increases the biological efficiency of the dose by a factor between 1.5 and 3. Compared to protons, carbon ions have the disadvantage that beyond the Bragg peak, the dose does not decrease to zero, since nuclear reactions between the carbon ions and the atoms of the tissue lead to production of lighter ions which have a higher range. Therefore, some damage occurs also beyond the Bragg peak.
By the end of 2008, more than 5,000 patients had been treated using carbon ions.
At the end of 2013, around 13 000 patients had received carbon-ion therapy
Particle beams offer benefits over conventional photon radiation for the treatment of many tumors. Currently, 49 facilities worldwide— including 14 in the US—are producing proton beams, and another 29 are under construction. But carbon-ion therapy, which can benefit patients with deepseated or radiation-resistant tumors, remains in relative infancy, with eight centers operating and four under construction as of 1 April.
Carbon-ion treatment centers in operation
The Particle Therapy Co-Operative Group lists treatment centers in operation or in the planning or construction stage. At least five centers using carbon ions are in operation, four in Japan: the HIMAC at Chiba, the HIBMC at Hyogo, and Gunma University's Heavy Ion Medical Center in Maebashi, and SAGA-HIMAT, Tosu. A fifth in Japan is currently under construction, tentatively named "i-ROCK". In Germany, treatment at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, which is primarily a physics laboratory, has been discontinued in 2008, but the new HIT in Heidelberg, which is a dedicated facility, started in November 2009. The HIT facility is using robotic technology with sub millimeter precision to position the patients. Moreover, Heidelberg has developed and took into clinical operation in 2011 the first gantry worldwide for proton and ion beams. The rotating part of this structure has a weight of 600 tons. The CNAO in Pavia, Italy opened in 2011 and will be one of the most advanced centers[clarification needed] for particle therapy with hadrons. CNAO will combine precise dose delivery with highly accurate patient alignment based on stereoscopic X-ray imaging. Sophisticated approaches in image-guided particle therapy (IGPT) augments the radiotherapy machines with imaging capabilities and the latest computer vision technology to increase accuracy of target localization and enable patient alignment accuracies of 0.5 mm and better. In January 2015 the Shanghai Proton and Heavy Ion Centre opened after successfully completing the clinical trials. The Marburg ion treatment facility, MIT (Marburger Ionenstrahl Therapie) treated their first patient in October 2015.
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