Hadron therapy

Full text: Historically discoveries and technological advances in radiation and accelerator physics have been rapidly applied in the healing arts. X rays, discovered by Roentgen in 1895, and radioisotopes, discovered by Becquerel in 1896, were both applied to the treatment of cancer by the first year of the twentieth century. By the time that Chadwick discovered the neutron in 1932 radiotherapy was a well-established modality for the treatment of cancer. The potential use of neutrons in radiotherapy was immediately recognized by L.H. Gray (a recent Ph.D. graduate from the Cavendish laboratories in Cambridge). He built a neutron generator for radiobiology research at Mount Vernon Hospital in London, it's design was based on a similar device built at Cambridge by Mark Oliphant. In parallel with these developments Earnest Lawrence, in collaboration with his brother John Lawrence, a physician, were also studying the radiobiology of neutrons. They obtained some funding from the US National Cancer Institute to construct a 60'' cyclotron for neutron radiation therapy. These clinical trials, started in 1938, were the first application of heavy particles for cancer therapy. Immediately after the second World War advances in accelerator technology lead to the building of the first synchrocyclotrons, which produced proton beams with ranges approaching 30 cm in water. At the Harvard synchrocyclotron Robert R. Wilson realized that proton beams might have considerable advantages over other radiation beams in the treatment of deep-seated tumors. This advantage derives from the increase in stopping power at the end of the proton range, first observed by W.H.Bragg at the University of Adelaide; a phenomenon now known as the Bragg peak. Conventional X-ray beams are exponentially attenuated by matter and, hence, delivering large radiation doses to tumors at depth while not causing excessive damage to the overlying normal tissues can be problematical. The Bragg peak and the well-defined range of proton beams offer a solution to these problems through their superior dose distribution properties. In recent years the term 'Hadron Therapy' has been coined to describe cancer treatments involving heavy particles including neutrons, protons and heavy charged particles such as 12C, 20Ne, 28Si, etc. While protons offer a dose distribution advantage, neutrons, which are exponentially attenuated, offer therapeutic advantages derived from differences in cellular response to the different types of radiation depending on cell kinetics and physiology. These differences are attributed to the high stopping power (or 'linear energy transfer') of the secondary particles set in motion in tissue by the primary neutron beam. Heavy charged particle beams (12C, etc) combine the dose distribution advantages of the Bragg peak with the biological advantages of high linear energy transfer (LET). Research in neutron therapy has shown that the biological advantages of this therapy can only be realized for a limited number of disease sites and histologies. The dose distribution advantages of proton therapy, however, are more widely applicable and realizable. It can be argued that, because of the basic physical differences in the dose distribution mechanisms for x ray and proton beams, proton beams can always give a better dose distribution; i.e. more uniform dose in the tumor volume and less dose to the surrounding normal tissues. In spite of the large capital costs of proton facilities, several large facilities, which incorporate a single accelerator and multiple treatment rooms have been built or are planned. It is argued that such large facilities operated over 20-25 years can provide a cost effective and superior cancer treatment in comparison with x ray therapy. The rationales for hadron therapy and the modern accelerator, computing and imaging technologies necessary for it's successful application to cancer therapy will be presented and discussed in detail