Post by Kämpfer on Nov 2, 2006 14:19:12 GMT -5
Radiation therapy is one of the mainstays of cancer treatment. In addition to highly toxic chemotherapy drugs, this regimen is used to treat highly aggressive tumors. However, radiation therapy is not a very specific treatment. At its most basic form, the patient is exposed to high energy radiation, which can ionize atoms within cells. These ions wreak havoc, destroying the cells. At its most extreme this treatment could kill the patient and it certainly causes side effects which include the possibility of causing more cancer. In an effort to maximize the benefit of radiation therapy and minimize the potential side effects, there is continuous research effort to improve radiation therapy. Focusing the radiation into the region of the tumor was a first step on this journey. Although this maximizes damage at the center of the beam focus, there is still damage along the beam line throughout the body. A further step, was to recognize that charged particles of a particular energy have a characteristic depth at which most of the particles collide with an atom and do their damage. This has led to the development of several facilities dedicated to radiation treatment based on ionized heavy atoms or protons.
With this discovery another possibility has arisen—the use of antimatter. In particular, the antiproton presents itself as a good candidate because can be placed in a storage ring to build up a high intensity beam. It is also charged, which means that it will do most of its damage at a depth specified by the energy of the particles. However, in addition to ionizing the atoms, they also release a huge amount of energy (1.88GeV) as they collide with and annihilate with a proton. This energy consists of gamma rays, neutrons and high-energy pions. Most of this radiation passes straight out of the body; however, about 30MeV of it does additional damage in the focus of the beam. Which, considering a typical proton radiation therapy beam is 40-50MeV, represents a substantial increase in energy deposited in the tumor.
Now some researchers have used the antiproton beam at CERN to see how anti-protons compare with protons in tissue damage. They put live cells in a gel to immobilize them. Then the gel was either irradiated with antiprotons, protons or cobalt. The gels were then sliced into thin sections and the cells from the gel used to grow cultures. After incubation, the cells were counted to estimate the effectiveness of each form of radiation. Although the cell cultures show that the antiprotons are qualitatively more effective, quantitative results were more difficult to obtain. Normally the results would be represented on a "relative biological effectiveness" scale, but this relies on knowing the dose delivered, which in the case of antiprotons is uncertain due to the annihilation process. To compensate for this, the researchers invented a new scale that uses the differences in radiation flux at different places along the path. Using this scale the researchers estimate that an antiproton beam that causes the same amount of damage at the entry of the sample will cause about 3.8 times the damage at the target volume.
Antiproton beams are usually associated with very high energy facilities such as CERN because they require proton beams of 20-30GeV to create them. However, this can now be achieved in a synchrotron of 100-200m in diameter, which is not incompatible with regional "facility" style operations. These facilities would help where radiation treatment is limited by the amount of radiation a body can stand or where treatment efficacy can increase disproportionately with a small increase in effective dose. This makes it clear that the widespread adoption of antiproton radiation treatment (should it make it through trials) will not be an easy decision.
