Proton therapy

Proton therapy is a kind of external beam radiotherapy where protons are directed to a tumor site.

Indications

Proton therapy is of interest because of its ability to accurately target and kill tumors, both near the surface and deep seated within the body, while minimizing damage to the surrounding tissues. For this reason, it is favored for treating certain kinds of tumors where conventional X-ray radiotherapy would damage surrounding radio-sensitive tissues to an unacceptable level . This is of particular importance in the case of pediatric patients where long term side effects such as residual occurrence of secondary tumors resulting from the overall radiation dose to the body are of great concern. Because of the lower dose to healthy tissue protons have less severe collateral side-effects than conventional radiation therapy.

The logic for treating common cancers (for example lung, head/neck, etc) with proton therapy is the same as saying that surgery alone should cure most cancers, as surgery is the Definitive Local Treatment. Of course, surgery does not - because most cancers spread microscopically very early beyond the tumor ('local') site.

Historically, one area where proton therapy had considerable early successful application was in treating choroidal malignant melanomas, a type of eye cancer for which the only known treatment was enucleation (removal of the eye). Today, proton therapy is one of the techniques that are capable of treating this tumor without mutilation. (see article in French)

How it works

Proton therapy, like all forms of radiotherapy, works by aiming energetic ionizing particles (in this case, protons) onto the target tumor. These particles damage the DNA of cells and thus ultimately cause their death. Because of their high rate of division, and their reduced ability to repair damaged DNA, cancerous cells are particularly vulnerable to this attack on their DNA.

As protons scatter less easily in the tissue there is very little lateral dispersion; the beam stays focused on the tumor shape without much lateral damage to surrounding tissue. All protons of a given energy have a certain range; no proton penetrates beyond that distance. Furthermore, the dosage to tissue is maximum just over the last few millimeters of the particle’s range, this maximum is called the Bragg Peak. This depth depends on the energy to which the particles were accelerated by the proton accelerator, which can be adjusted to the maximum rating of the accelerator. It is therefore possible to focus the cell damage due to the proton beam at the very depth in the tissues where the tumor is situated; tissues situated before the Bragg peak receive some reduced dose, and tissues situated after the peak receive none.

Proton therapy, however, needs heavy equipment. For instance, the Orsay proton therapy center, in France, uses a synchrocyclotron weighing 900 tons in total. Such equipment was formerly only available within centers studying particle physics; and in the case of the Orsay installation, the treatment machine was converted from particle research usage to medical usage.

However, there are now several dedicated proton therapy centers in operation or under construction in North America, Europe, and Asia. Proton beam radiation therapy has had remarkable success in the treatment of many types of cancer, including brain and spinal tumors, as well as prostate cancer. Some researchers have suggested that antiprotons may be even more effective at killing cancer cells than their proton opposites, but as of now this highly speculative claim remains unverified and untested.

History of Proton Therapy in the United States

The first suggestion that energetic protons could be an effective treatment method was made by Robert R. Wilson in a paper published in 1946 while he was involved in the design of the Harvard Cyclotron Laboratory (HCL). The first treatments were performed at Particle accelerators built for physics research, notably Berkeley Radiation Laboratory in 1954 and at Uppsala in Sweden in 1957. In 1961, a collaboration began between HCL and the Massachusetts General Hospital (MGH) to pursue proton therapy. Over the next 41 years this program refined and expanded these techniques while treating 9,116 patients before the Cyclotron was shut down in 2002. Following this pioneering work, the first hospital based proton treatment center in the United States was built in 1990 at Loma Linda University Medical Center in Loma Linda, California (LLUMC) (recently renamed the James M. Slater Proton Therapy Center). This was followed by The Northeast Proton Therapy Center at Massachusetts General Hospital (recently renamed the Francis H. Burr Proton Therapy Center), to which the HCl treatment program was transferred in 2001-2002. Proton therapy for ocular tumors is also available in Sacramento at the UC Davis Proton Facility, a facility operated exclusively by the UC San Francisco Department of Radiation Oncology. It is estimated that over 44,000 patients have been effectively treated with proton therapy. Now on line as well is the [1] Midwest Proton Radiotherapy Institute at Indiana University. In the summer of 2006 treatment started at two new facilities: the for-profit University of Texas M. D. Anderson Cancer Center [2] in Houston, Texas, and the University of Florida Proton Therapy Institute [3] in Jacksonville, Fla. The University of Pennsylvania is slated to open the biggest proton therapy institute in the world (in the Center for Advanced Medicine) in 2009.

Therapy equipment suppliers

Following firms are currently supplying or developing proton therapy equipment:Optivus Proton Therapy USA, IBA (Belgium), Hitachi (Japan), ACCEL (Germany, now acquired by Varian, USA), Siemens (Germany) [4]

References

"Radiological Use of Fast Protons", R. R. Wilson, Radiology, 47:487-491 (1946)
"Use of Protons for Radiotherapy", A.M. Koehler, Proc. of the Symposium on Pion and Proton Radiotherapy, Nat. Accelerator Lab., (1971)
"Protons in Radiation Therapy: comparative Dose Distributions for Protons, Photons and Electrons, A.M. Koehler, W.M. Preston, Radiology, 104(1):191-195 (1972)
"Bragg Peak Proton Radiosurgery for Arteriovenous Malformation of the Brain" R.N. Kjelberg, presented at First Int. Seminar on the Use of Proton Beams in Radiation Therapy, Moskow (1977)
"Fractionated Proton Radiation Therapy of Cranial and Intracrainial Tumors" Austin-Seymor, M.J. Munzenrider, et al. Am.J.of Clinical Oncology 13(4):327-330 (1990)
"Proton Radiotherapy", Hartford, Zietmean, et al. in Radiotheraputic Management of Carcinoma of the Prostate, A. D'Amico and G.E. Hanks. London,UK, Arnold Publishers: 61-72 (1999)

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