Beam production and delivery

Research into beam production and delivery at the PTCRi includes:


PAMELA (Particle Accelerator for MEdicaL Applications) is a new type of accelerator which can accelerate protons and other ions quickly to deliver an intense variable energy beam to treat cancer. This being developed by a consortium of UK universities and presently a conceptual design is ready.

Existing facilities uses either cyclotron or synchrotron to accelerate the protons, and synchrotrons to accelerate heavier ions such as carbon. The cyclotron is a relatively simple accelerator, usually producing particles at a fixed energy, which then has to be appropriately modified to make it suitable for treatment. Synchrotrons, on the other hand, can in principle produce beams of almost any energy up to the maximum, but because they are pulsed machines deliver the dose rather slowly, leading to longer treatment times and increased patient discomfort.

PAMELA aims to combine the best qualities of the cyclotron and the synchrotron by using a unique acceleration scheme known as non scaling fixed field gradient (NS FFAG) acceleration. Because of the fixed magnetic field (like that used in the cyclotron), it can accelerate the particles rapidly. Because of the alternating gradient (as used in a synchrotron), the radial distance moved by the particles is relatively small, so that variable energy extraction is possible.

Diagram of the PAMELA accelerator

The layout of a PAMELA proton and carbon ion accelerator. The protons are pre-accelerated in a low energy cyclotron and injected into the inner ring, accelerated to the desired energy and extracted to the treatment beam line. Carbon ions are pre-accelerated before being injected into the inner ring, where they are accelerated to an intermediate energy, extracted and injected into the outer ring. The latter accelerates such ions to the required energy and they are extracted and ultimately delivered to the patient.

Another advantage of using PAMELA is the possibility of using a compact Gantry. A Gantry transports the beam from the accelerator to the patient, and can deliver it at any desired angle. It must contain the magnets to bend the ion beam by 90º, sets of magnets to focus the beam, and also scanning magnets and detectors to endure that the beam is at the exact position needed for each unique treatment. All of this makes the gantries very heavy, between 150 and 400 metric tonnes, depending on the size of the ions, so new designs are constantly being developed and modified to reduce their weight and make their movement around the patient easier.

Diagram of the Heidelberg carbon ion facility gantry

Heidelberg carbon ion facility gantry

Spot scanning

Modern CPT facilities use a technique called spot scanning to deliver the dose to the patient. In spot scanning a narrow beam (pencil beam) of few mm in width, is used to deposit the dose in small spots. By using varying magnetic fields, which deflects the pencil beam and by changing the energy (which determines the depth of dose deposition) it will be possible to deliver the dose in three dimensions with a high degree of dose conformation to the tumour and almost completely sparing normal tissue.

Spot scanning must be extremely precise and accurate to within ±2%, which is very difficult to do if the cancer is near something such as the respiratory system, which is constantly moving. If the very small, precise dose were delivered to healthy tissue or to cancerous tissue more than once, it could cause a secondary tumour. The procedure is made harder to control due to physical factors such as beam dispersion.

Work at the moment is focussing on collaborating with designers to ensure the accelerator satisfies all clinical requirements, and looking into the possibility of designing a new scanning system to really maximize the rapid energy changes possible with PAMELA.

Repeated delivery

In radiotherapy, cells are killed when radiation ionises atoms in the DNA strands and they break. This effect is augmented by oxygen which is a radiosensitiser. In larger tumours, the cells towards outer edges are far from any blood vessels, so become lacking in oxygen (hypoxic). This means the tumour cells are more radio resistant than healthy cells.

A method being investigated is delivering the dose gradually, in scans lasting ~1μs instead of 1s, with more scans in total, so the whole tumour is treated repeated times in each fraction. The hope is that between each scan healthy cells will help reoxygenate the tumour cells in order for them to repair, then, as the beam returns, the cell will have gained more oxygen, so will be more susceptible to radiation.

The research at the moment is into modelling this problem in order to assess the most effective pattern of delivery, in order to cause the maximum cell damage. The aim is to find the optimum scan length, time between each fraction and the dose required each time.