Medical Radiation – Rationale
Neutrons and protons are both nuclear particles of approximately the same mass. However, the fact that neutrons are uncharged and protons are charged particles results in their having vastly different physical properties and biological effects. Using conventional photon radiation as the standard, the dose distributions of fast neutrons are very similar, but their biological effects offer advantages for the treatment of certain types of tumours, while the advantages of protons lie in their physical dose distributions.
Many large tumours have central cores, which lack oxygen because the blood supply has been reduced by the proliferating tumour cells. Other tumours are slow-growing and the cells spend a relatively short time in the dividing phase of the cell cycle, where they are most sensitive to radiation. These tumours are resistant to conventional photon radiation but are far less resistant to neutron irradiation, which therefore in principle has a better chance of effecting a cure. Examples of tumours that are effectively treated by neutrons include salivary gland tumours, large breast tumours and certain tumours of other soft tissues. However, because radiation causes damage to the normal tissue in front of a deep-seated tumour, a so-called isocentric beam delivery system (one which rotates about the patient) is essential so that the patient can be irradiated from several different angles. This concentrates the dose at the tumour and limits the dose to normal tissue.
High energy protons are particularly suitable for treating cancer or other (benign) abnormalities near sensitive structures such as the optic nerve, spinal cord, kidneys, etc., where other forms of radiation would do too much damage to healthy tissue. This feature results from the fact that protons can be steered and focused very accurately and can also be given exactly the right energy to stop at any particular point within the body, thus completely protecting any organs beyond this range. They also allow better protection of normal tissue, situated in front and at the sides of the target, than other types of radiation. This allows the application of higher doses to a tumour which means a better chance of cure. The high precision required in proton therapy demands the ability to accurately set the patient up in the proton beam. Although not as important as in the case of neutron therapy, the ability to irradiate the patient from different angles is desirable.