Use of radiation to treat cancer
Proton Beam Therapy
Approved by the FDA in 1988, Proton Beam Therapy involves targeting tumours with high energy beams of protons. Using a particle accelerator, protons are accelerated to approximately 2/3 the speed of light, and are then aimed using strong magnetic fields at the target site[17], where they can penetrate the body to around 30cm, depending on the speed produced by the particle accelerator. Using strong, cooled magnets, it is possible to precisely shape the beam, with the low temperature being integral so the metal does not change shape.
Braggs diffraction peak
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The energies of directed protons can be varied to produce a spread out Bragg peak, allowing the whole tumour region to receive treatment.
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High quantities of radiation is released at the peak, delivering a strong dose to the affected area, but unlike standard radiation, there is very little which continues past the tumour site, as almost all is absorbed., so there is no exit dose.
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There is a nuclear build-up before the beam comes into contact with the tumour site, and as the proton beam slows down with distance travelled, the energy increases rapidly at a certain depth until the point of maximum energy, where a high dose of radiation is released.
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To calculate an appropriate dose, the nuclear stopping power is calculated using the relationship between the Bragg peak and the fluence.
Pencil Beam Scanning
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A property of protons is that they are positively charged, and therefore are affected by magnetic fields. Using carefully controlled magnetic fields, the trajectory of the protons can be precisely determined, resulting in much less healthy tissue being damaged. This is particularly important when treating paediatric patients as the long-term risks of radiation lower as radiation exposure of healthy tissue minimises.
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Using imaging techniques, the tumour is ‘sliced’, and each layer is treated separately. Pencil Beam Scanning is when the beam of protons controlled by magnetic fields is traced onto each layer of tumour, conforming to the shape and size of each section[18]. The beam is scanned over the layer, and the proton intensity adapted so that in areas with higher cancer volume, higher intensity can be precisely directed into those areas.
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If multiple scanning fields are used, the dose patterns from each field overlap, allowing complex shaped tumours to be comprehensively treated[19]. This is called Intensity Modulated Proton Therapy.
This diagram shows a 3D tumour being 'sliced' into separate pieces and the orange line represents the scanning of the pencil beam of protons which is conformed to the shape of each layer.