Particle therapy is a form of external beam radiotherapy using beams of energetic neutrons, protons, or other heavier positive ions for cancer treatment. In conventional external beam radiotherapy photons (X-rays) are most commonly used.
For protons and heavier ions the radiation dose increases while the particle penetrates the tissue, continuously losing its energy.
The advantage of particle therapy is that less energy is deposited into the healthy tissue surrounding the tumour. This enables higher doses to be delivered to the tumour, theoretically leading to a higher local control rate, as well as achieving a low toxicity rate.
However, it is difficult to evaluate the superiority of particle beam radiotherapy over photon beam radiotherapy as no clinical trials have directly compared the outcomes between the two types of therapy due to the limited number of facilities using particle beam therapy.
The most common particle beam therapy in clinical use are Protons.
How does proton beam therapy work?
Proton beam therapy uses beams of protons to shrink tumours. Protons are small particles of an atom, a ‘particle accelerator’ (cyclotron) is used to speed up the protons to produce the treatment beam.
With proton therapy, there is less radiation dose outside of the tumour. In conventional radiation therapy, x-rays (photons) continue to deliver radiation doses as they pass through the body. This means that radiation damages nearby healthy tissues, possibly causing side effects.
One of the main advantages of proton therapy is that it has a lower chance of causing severe side effects than other forms of therapy. This is because protons are larger and heavier than the particles used in other types of radiation therapy. The larger mass means the protons don’t scatter into other parts of the body as much, reducing collateral damage to surrounding tissue. It also is easier to target protons into tissue at a certain depth. If other types of radiation are used, such as X-rays, it is difficult to get the radiation to stop at the precise depth of the cancer. This means the radiation continues to deliver dose and can therefore damage other parts of the body it interacts with. The energy of protons can be adjusted so most do not penetrate any further than required.
The proton beam’s ability to be specifically targeted during treatment allows for harder to reach tumours to be treated, as well as tumours that are in sensitive locations such as near the spine, in the brain or affecting vital organs. The proton beam can be specifically targeted to a particular treatment site and the beam made to ‘stop’ before affecting surrounding healthy cells.
The individual sessions for proton beam therapy, referred to as ‘fractions’, can take between 15 to 45 minutes
Proton beam therapy is not always a standalone treatment and can be used alongside other cancer treatments such as surgery, chemotherapy or immunotherapy. Not all cancers can be treated with protons however, and there are currently no studies that appropriately compare proton therapy to traditional therapy options. More research needs to be carried out to establish the full benefits. The lack of data with this treatment option means there are few best practices available with proton therapy simply because optimal dosing must come through a trial-and-error process.