Particle therapy and cancer treatment
(LET). LET describes the energy deposited by a particle per unit distance. The native proton beam deposits a lower dose to tissues near the surface, and a much higher, more concentrated dose at a depth depending on the energy of the beam. This is known as the Bragg peak. It occurs because, as the particles’ velocities decrease, they are more likely to interact with localized particles such as electrons, so LET increases with depth in the tissue. LET is inversely proportional to the square of the particle’s velocity. There is also no exit dose, meaning the damage done to healthy surrounding cells is theoretically much less than with conventional photon therapy. It has been estimated that proton therapy cuts the dose to healthy tissue by a factor of 2-3. Since the Bragg peak for protons is very narrow, beams of different energies can be used together, with the resulting spread-out Bragg peak covering the entire treatment area while maintaining a lower dose than photon therapy to healthy tissue. 5 According to an in vitro study from the University of Pennsylvania, 6 proton therapy induced a greater number of single-strand breaks in cancer cells, as well as an increased level of ROS. Another study by the same university 7 found that patients treated with proton therapy had similar survival rates to those treated with photons, though they experienced much fewer adverse effects. However, more research is needed to ascertain the effectiveness of proton therapy versus conventional radiotherapy. The precise dose deposition of proton therapy can also be a drawback as an off-target beam could cause serious damage to healthy tissues and fail to treat the cancer. CT scans before therapy, as well as real-time beam tracking using MRI are being researched as a solution to this. Combining proton and photon therapy to treat cancers over a larger area has also been suggested. Another modality which has been investigated in order to treat cancer is antiproton beam therapy. Scientists at CERN involved in the Antiproton Cell Experiment (ACE) used antiprotons to target hamster cells suspended in gelatine and compared the results with a proton beam. They found that the antiproton beam was four times more effective than protons in irradiating the cells. Antiproton beams have a similar Bragg peak to proton beams. However, they work in an entirely different way. Antiprotons annihilate with protons in the nuclei of atoms in the tumour. Their mass is converted into energy which can be found using Einstein’s formula of mass -energy equivalence E=mc 2 . This energy is distributed among a wide variety of particles which can be produced as a result of the annihilation. The most biologically active of these particles tend to be the heavy particles produced which affect the target area. 8 Despite these good results, antiprotons are incredibly difficult and expensive to produce, so this therapy will probably not be viable for several decades.
The use of heavy ion therapy using nuclei of heavier atoms has also been investigated. The most studied of these therapies has been carbon ion radiotherapy, using the nuclei of carbon atoms instead of protons as treatment. Carbon ion radiotherapy (CIRT) seeks to combine the lower side-effect profile of proton therapy with increased effectiveness at damaging DNA of cancerous cells. CIRT works in a similar way to proton therapy, but has certain key advantages. Carbon ions have an even narrower Bragg peak than protons, with less energy deposited outside the tumour region (Fig. 4).
Figure 4: A graph of relative energy against depth, including carbon ions
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