Elekta’s latest linacs deliver higher doses over shorter courses
Visit the radiation department of a major hospital and, down winding corridors, there will be a host of machines whirling and gurgling, hidden behind radiation shelters. As well as traditional imaging devices such as X-ray- machines and scanners, you may also find sophisticated accelerators that are used to bombard cancer cells with radiation.
Linear accelerators, or linacs, have been used to treat cancer for decades. The machines accelerate electrons to almost the speed of light and direct them to hit a tungsten target, where their energy is converted into X-rays. These then pass through a series of filters before they reach the patient and irradiate the cancerous tumour.
The UK is home to one of the largest linac manufacturers worldwide, Elekta. The firm employs 800 staff at its site in Crawley, Sussex, where the machines have been manufactured for more than 50 years – under the name initially of Mullard Equipment and then under Philips Radiotherapy Systems. Swedish-owned Elekta acquired the latter in 1997, and since then has ramped production up from 60 to around 400 machines a year.
Around 98% of the £1 million to £2.5 million machines are shipped globally. Kevin Brown, the company’s global vice-president of scientific research, says that radiotherapy today is “totally different” from when he started in the industry 32 years ago. “I remember my first visit to a hospital, and it just seemed barbaric. Today the outcomes are better, and the patients are treated with more respect and with better technology.”
The latest linacs can deliver higher doses of radiation over shorter treatment courses, and are incredibly complicated machines that use a host of different technologies. Engineers have to grapple with high vacuums, dielectric gas, mechanical hazards, high and low voltage, water, and radiation shielding.
Advances in technology mean the modern linac can shape the X-ray beam to match the shape of the tumour more precisely than has been possible before. This accuracy helps to spare healthy and critical structures, such as the heart or the spinal cord, from getting bombarded with radiation during treatment.
Elekta’s latest X-ray beam shaper technology uses 160 leaves of tungsten, each powered by a small DC gear motor. The leaves move independently of each other, and together shape the X-ray beam coming out of the device.
Both 80-strong halves of the leaf bank can also move as one on a secondary system. Combining the two systems is useful for tracking a tumour that may move during treatment, such as one on the lung. “We can form a shape with the leaves and then move the whole thing very quickly,” says Duncan Bourne, principal engineer of mechanical systems at Elekta.
At the tip of each of the tungsten leaves sits a tiny synthetic ruby, which is illuminated by ultraviolet light. A radiation-resistant camera picks up the infrared fluorescence to create a live image of each leaf’s position on the unit’s desktop control system.
This feature is a development over previous beam shapers, which used potentiometers or encoders on each leaf and could run into reliability issues with such high numbers of leaves. The technology has a nice failure mode as well, says Per Bergfjord, principal mechanical engineer of hardware engineering. “It either doesn’t read anything or it gives you the right image.”
Elekta is working on the next generation of the linac, which will combine a magnetic resonance imaging (MRI) device with a linac. The combination will allow better visualisation of the patient’s tumour and internal organs than current systems, which use computerised tomography (CT) scanners.
CT scanners take up to a minute to acquire the images, so shots must be taken before treatment. But MRI provides clear images of abdominal organs and can take shots in real time, so the tumour can be monitored during treatment.
The company is working with Philips, a prototype machine has been built at the University Medical Centre Utrecht in the Netherlands, and tests are under way. But the new approach poses challenges.
“Fundamentally, MRI and linacs are two technologies that you wouldn’t put together,” says Bourne. “We have whole new challenges that we never had to worry about before.”
MRI uses huge magnetic fields to send out radiofrequency waves to produce detailed images of inside the body. It is listening out for incredibly small radiofrequency signals coming back from the water molecules in a patient’s body. A linac, on the other hand, makes massive amounts of radiofrequency noise as it guides the electron beam through the machine to strike the target and generate X-rays. “We need an electron beam going through very small holes in a nice straight line, so you don’t want magnetic fields,” says Bourne.
So far, Elekta has invested “a huge amount of money” engineering solutions to these problems, says Brown, but the machine will not be available for years.
Electrons are not the only particles that can be whipped up into a high-energy frenzy and used to treat cancer. “The big thing that is happening at the moment is proton therapy,” says Hywel Owen, a lecturer in physics in the accelerator group at Manchester University.
Proton therapy uses a beam of high-energy protons, instead of high-energy X-rays, to irradiate cancer cells. Treating cancer in this way can be more effective because proton therapy directs the radiation to precisely where it is needed, with less spilling over and causing damage to the surrounding tissue.
This technique involves accelerating the protons generated from an ion source or reserve of hydrogen gas. “Protons are about 2,000 times heavier than electrons, so they are much harder to accelerate and the equipment needs to be a lot larger,” says Owen.
Typically, protons are accelerated in a cyclotron or a synchrotron. These are large circular accelerators with a diameter of between two and 10m, respectively. They use voltages and an overall magnetic field to accelerate the protons around the ring.
Several manufacturers are working to develop small and reliable high-energy proton therapy devices, and some medical facilities around the world already offer proton therapy. The most advanced of these is the Heidelberg Ion Beam Therapy Centre in Germany.
This 5,000m2 facility opened in 2012 at a cost of €120 million. Protons, or other heavy ions, are accelerated first by a two-stage linac and then by a synchrotron. The latter has six 60-degree magnets that bend the protons into a circular path. Over the course of one million cycles, the protons reach 75% the speed of light. Magnets then guide the proton beam through vacuum tubes to each of the three treatment rooms.
Heidelberg has an advanced facility for this type of therapy
Proton power: Treatment room at Heidelberg
One of the treatment rooms contains a 25m long, 360-degree-rotating gantry, that can direct the proton beam towards the patient at different angles. It has a diameter of 13m and weighs 670t, 600t of which can be rotated with submillimeter precision. This rotation gives the option to irradiate tumours that cannot be reached with the horizontal beams, which are available in the other two rooms.
The UK has only limited facilities for proton therapy, and since 2008 the NHS has been sending patients abroad for treatment, to facilities in Switzerland and the US. But that is about to change.
The government is investing £250 million to build two proton therapy centres, at Christies hospital in Manchester and at University College London Hospital. The facilities will be up and running by 2017, and Owen is involved in procuring the equipment.
Owen is also working on new ways to accelerate particles, using the EMMA test accelerator at the Science and Technology Facilities Council’s Daresbury laboratory near Warrington in Cheshire. The set-up allows him to test concepts of particle physics and apply them to proton therapy applications.
EMMA test accelerator at Daresbury laboratory
ALICE, also at Daresbury
EMMA is not the only accelerator at Daresbury. The lab also has the first prototype energy recovery linac to be built in Europe, ALICE. ALICE’s energy recovery system involves electrons that have been accelerated giving up their energy to the next set of electrons to be accelerated. This means that scientists can get higher currents at bigger voltages. “We can get massive amounts of terahertz power – this is a region of the spectrum that we haven’t been able to use,” says Professor Peter Weightman from the University of Liverpool’s School of Physics.
In the accelerator, bunches of electrons are accelerated through superconductive radiofrequency cavities to around 30MV, and are then compressed to give out light. If the timing is right, the process creates an infrared-free electron laser. This intense form of light can be used to probe objects at the atomic level.
Weightman is using the laser to develop tests that can spot the hallmarks of cancer within cells. He does this by adding to the laser a scanning near-field electron microscope, which can distinguish different components within a cell.
“We can make images at different wavelengths, and the different wavelengths respond to different biological molecules,” he says. This technique can flag cancerous changes in cells that standard techniques cannot pick up.
Initially, Weightman’s work focused on developing the technology to create a diagnostic test for oesophageal cancer, which is traditionally difficult to diagnose. But he now has funding to work on tests for prostate and cervical cancer, too.
X-rays from electrons: how a linac works
Linear accelerators create a beam of X-ray radiation that is directed onto a patient to irradiate cancer cells. At one end of the machine sits an electron gun that shoots electrons into a 3m-long copper tube known as the waveguide.
Once inside the waveguide, the electrons fly through small holes that connect a series of hollow chambers, that help steer the electrons into a fine beam. The waveguide is surrounded by two sets of quadruple magnets that focus the beam, and a vacuum sucks out other particles that could obstruct it.
The gantry of the linac can rotate around the patient
As the electrons travel through the waveguide, they are pushed to almost the speed of light by radiofrequency waves that are pulsed into the waveguide by a magnetron. The high-energy electrons then pass into a second tube, where three more pairs of magnets focus the electron beam to the diameter of a pinhead, and steer it to hit a small tungsten target.
Once the electrons strike the target, their energy is converted into X-rays, which spray from the target in a variety of directions. A primary collimator gathers the forward-travelling X-rays, and absorbs those travelling in other directions, to minimise the amount of unwanted radiation the patient is exposed to.
Before the X-rays reach the patient, the radiation dose and the quality of the beam are measured in an ion chamber. The gantry of the linac can rotate around the patient, so radiation can be directed to the cancerous cells in a way that reduces the effect on surrounding tissues.