Abstract

Proton therapy is making the move out of the research laboratory and into the clinic. New hospital based facilities in the US, Asia and Europe testify to the growing interest in this treatment modality. Protons have the advantageous characteristic that the energy from a mono-energetic proton beam is deposited in a small region known as the Bragg peak, beyond which the deposited dose is almost, but not quite, zero. Numerous comparative treatment planning studies have shown the theoretical advantage for protons in a number of indications, and the existing and new facilities are working towards translating this theoretical advantage into a real clinical advantage.

In order to make the essentially mono-energetic, and narrow, pencil beams that are emitted from proton accelerators useful for therapy, the method most widely used is the so-called passive scattering technique. In this, the narrow beam is widened through the use of scattering elements, whilst the narrow Bragg peak is extended in depth through the application of a series of depth shifted and modulated Bragg peaks in order to form a so-called ‘Spread-Out-Bragg Peak’ (SOBP). The final form of the delivered field is defined by the use of field specific collimators and compensators, the latter of which match the distal end of the field to the distal extent of the target volume.

Coupled with the development of the new proton facilities is a growing interest in more sophisticated delivery techniques. One such is active scanning, in which narrow, mono-energetic pencil beams are magnetically scanned throughout the target volume under computer control. This approach has a number of potential advantages over the passive approach. It is very flexible, makes more efficient use of the available protons, is more conformal than passive scattering, results in lower secondary irradiations to the patient (i.e. neutron background) and last, but certainly not least, allows for the delivery of Intensity Modulated Proton Therapy (IMPT), the proton equivalent of IMRT . IMPT provides great flexibility in sculpting doses around complex tumours and in the neighbourhood of multiple critical structures, whilst maintaining the inherent characteristic of proton therapy, a substantial reduction of dose to all non-target normal tissues. In treatment planning comparisons, this has been shown to be reduced by a factor of 2-6, which could have a significant impact on late effects such as secondary tumour induction. In addition, it could also be expected that there may even be (as yet undiscovered) advantages from proton therapy in terms of dose escalation, whereby the relatively steep dose gradients across critical structures could allow higher tolerance doses than are possible with other techniques. In summary, the physical characteristics of protons dictate that for the same total dose to the tumour, the dose to the surrounding normal tissues will be reduced. As there is much evidence from past advances in radiotherapy that improved dose conformation and reduced normal tissue doses improves outcomes, then it is to be expected that proton therapy will bring similar advantages.