Radiotherapy has been used for many years, but most of the treatments have been done with X-rays (photons). X-rays are successful in destroying tumours but they also destroy healthy tissue around the tumour. This is because the penetrating power of X-rays decreases exponentially with increasing depth. This means the X-rays deposit most of their energy near the surface of the body. If the tumour is deep-seated, this presents a problem
Protons, being particles instead of rays, slow down much faster than X-rays. Their power, however, does not decrease exponentially (as it does for X-rays). Instead, they deposit more energy as they slow down, culminating in a peak (called a Bragg peak [top graph]). 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 plot on the left 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 plot on the right 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 90,000 patients have been treated with protons or light ions in research laboratories or hospitals around the world. At TRIUMF, we only have the necessary equipment available to treat eye tumours. 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.
To date more than 50% of proton beam treatments have been used for malignant melanoma of the eye. The precise dose localization with protons allows a uniform dose to be delivered to the tumour while sparing the nearby structures such as the lens and optic nerve. The alternative therapies are removal of the eye (enucleation) or radioactive plaque therapy. The ophthalmologist, in consultation with the radiation oncologist and the patient, decides on the preferred course of treatment. Proton treatments are delivered in four daily fractions.