The depth at which the peak occurs can be controlled by the amount of energy the protons are given by the accelerator. Accelerated proton beams can be very precisely controlled. The upper plot shows the sharp Bragg peak of 70 MeV protons stopping in water. This peak can be spread out over the depth of the tumour using a rotating modulator wheel. The lower plot displays a lateral scan of the proton beam defined by a 20 mm collimator, showing the sharp fall-off dose at the edges.
Protons deposit their energy near the end of their path and have little lateral scatter.
Therefore the beam energy can be precisely delivered to the tumour volume without seriously harming surrounding tissues or adjacent critical organs. Proton beam therapy has demonstrated success for the treatment of selected tumours. More than 20,000 patients have been treated with protons or light ions in research laboratories or hospitals around the world. Patients who require proton therapy for other tumours have to travel to other centres outside of Canada for their treatment. Referrals to these centres are made through the BC Cancer Agency .