Introduction
Charged particle therapy (CPT) is a form of radiotherapy which uses ions, such as protons and carbon, to treat cancer instead of the conventional choice of x-rays. Radiotherapy is highly effective, contributing to 40% of successful treatments, but like all cancer treatments it has side effects. CPT is a relatively new technique which offers the possibility of reducing these side effects by delivering less radiation to the healthy tissue than conventional radiotherapy.
Clinical dose distributions for x-ray and charged particle radiotherapy.
X-ray radiotherapy passes through the entire body, with dose decreasing only slowly as it progresses through. The peak of the dose delivered is near the surface, rarely where the cancer is. These effects both cause damage to healthy tissues. In order to get a much higher dose to the cancer, multiple beams are often used, intersecting at the tumour site. This gives a dose to the surrounding tissue in most directions.
Graph of radiation dose with respect to depth in tissue for an x-ray beam.
X-ray treatments also have side effects which can be both long term and severe. The treatment has to be given in small doses to allow tissues to recover from the radiation. A typical radiotherapy treatment takes 30-40 sessions.
Protons have an energy curve with a very specific peak known as a Bragg peak, where they deposit most of their energy. This can be very small - just a few cubic millimetres. So the Bragg peak seen in CPT leads to only a small radiation dose in front of the targeted area and none behind, making it much safer in front of vital organs. There is much less tissue damage per beam, so the cancer can be more intensely radiated with fewer beams intersecting, and there is less risk of complications from non-cancerous tissues receiving high radiation doses.
Graph of radiation dose with respect to depth into tissue for a proton beam.
Carbon ions have a higher Bragg peak than protons so there is even less tissue irradiation in front of the targeted area for the same dose on the cancer. However the tissue behind the cancer gets a small radiation dose from fragmentation of the ions.
The first hospital-based proton therapy facility in the UK was Clatterbridge, near Liverpool, which opened in the 50's. Since then there have been no more centres built, so the UK has gone from the leader in the field to seriously lagging behind, with cancer patients having to be sent abroad. This is expensive to the NHS and patients as some require up to a two month stay overseas.
The advantages of charged particle therapy are clear but no clinically robust meta-analyses have been done, so putting a convincing case to the NHS to invest in it is currently very difficult. So far, few patients with more common forms of cancer (e.g. lung, liver) have been treated in this way and it is usually only used as a last resort which heavily biases the sample against more easily treatable cases. Facilities cost £25M-100M and the UK needs 2-4 carbon centres and 6-10 proton centres to treat several thousand patients a year. We at the Particle Therapy Cancer Research Institute (PTCRi) must therefore put together a very strong case to justify this expense.
