Acceleration of protons
There are large numbers of protons moving around in space, but on Earth, protons first have to be extracted from hydrogen gas. Negatively charged electrons are removed from hydrogen atoms by applying an electric field, leaving positively charged protons behind. This procedure takes place on a minute scale. The quantity of hydrogen gas required to carry out a complete course of therapy is smaller than a single champagne bubble.
In the particle accelerator – known as a cyclotron – a strong electromagnetic field is applied to the protons to accelerate them to 60 % of the speed of light (180,000 km per second) along a spiral-shaped path. The fastest spiral pathway at the edge of the cyclotron is deflected outward by an electric field so that it travels in a straight line out of the device
Beam processing – controlling the penetration depth
At 180,000 km/sec, protons penetrate the body to a depth of approximately 38 cm. If a radiation target, i.e., the tumor, is closer to the surface, the protons have to be degraded (slowed down). This occurs immediately after the protons leave the cyclotron in the Energy Selection System (ESS), which places graphite wedges in the path of the beam to achieve the precise speed required.
Beamline and targeting
Once the protons have been degraded, the beam is guided through a vacuum tube to the therapy site, known as the gantry. The beam can travel up to 92 meters, and magnetic lenses are used to continually focus the beam to prevent the protons from dispersing. The gantry is a steel structure that weighs 150 tons, measures eleven meters in diameter, and can be rotated 360° about its horizontal axis. It contains strong magnets that ensure precise alignment of the proton beam. The patient is fixed within this hollow body on a contoured couch placed on a table made of carbon fiber.
The sweeper magnets of the gantry deflect the emitted beam vertically to the axis of rotation of the gantry. When the gantry is rotated, the beam always targets same “isocenter” to within less than 0.5 mm from any angle required. In combination with the precisely adjustable patient couch, this enables all tumors to be irradiated from the optimal therapeutic angle.
The nozzle is the device connected to the end of the tube emitting the beam of protons; it is used to deliver protons to the patient. It is mounted close to the patient so the proton beam can be transported in a vacuum for as long as possible without scattering. Located behind the nozzle is the heart of the high precision scanning procedure used at the RPTC. Essentially, it comprises the two final small pairs of sweeper magnets that deflect the beam in two dimensions: away from the gantry axis in one direction and parallel to the gantry axis in the other. This provides precise targeting with the scanning system in two of the three dimensions. As explained above, the third dimension is scanned by adjusting the beam energy to alter the depth of penetration. This procedure is the latest form of proton therapy. It allows several tumors to be treated from more than one direction during a single session without any additional setup time, provided the radiation fields are not significantly smaller than 20 mm in diameter.
The nozzle can be fitted with miniature templates to treat very small tumors. These templates match the beam precisely to small tumors such as brain tumors. The nozzle also contains beam detectors which control the radiation intensity, the beam energy and thus the penetration depth, and the deflection of the X and Y dimensions. The detectors also match up the desired data for the patient with beam targeting independently of other control functions. The beam then passes through a Kapton plastic window that acts as a vacuum seal and emitted into the open.
The couch is made of carbon fiber, and a contour mattress for the patient is fixed in place. The couch can be moved in any direction, and it can also roll to a certain extent. If a heavier patient causes the couch overhang to sag a few extra millimeters, the system automatically corrects for it. After the couch has been moved to roughly the correct position in the gantry, an X-ray-assisted precision targeting system makes staged adjustments in increments of millimeters, thus positioning the tumor in the precise target area of the proton beam.
Fixed-beam therapy station
In addition to the four gantries, the RPTC has a fifth treatment room that is still being developed. This station is optimized to deliver treatment in the area of the eye and skull. It has a maximum delivery of just 160 MeV in order to allow the use of smaller magnets, so it can only be used to a penetration depth of 17 cm. The patient does not lie down on a couch but is instead seated in a chair with motors that allow it to be moved in any direction. Each patient is positioned using a maxillary mold. The beam is directed horizontally, and the X-ray devices also cross horizontally. Fixation and control devices are also available for the eye position. <br/> The decision on the type of therapy station to be used for a particular patient is made using medical criteria that this specialization takes into account.
Ultra-high precision at the RPTC with the scanning method
With the exception of one other research institute in Switzerland and one in Germany, the RPTC is the only proton therapy center using the precision scanning method.
The mechanical aspects, quality control and the software of the facility are set up to control the beam three-dimensionally with levels of precision equal to or better than +/- one millimeter. This means, for example, that the 150-ton gantries, which are able to rotate 360°, can always hit what is called an isocenter, the center of the irradiation point, within one millimeter. This requires extremely precise welding of these 150-ton steel machines. This also requires keeping the four devices in three-story, fully climate-controlled rooms to prevent any distortion whatsoever due to different temperature levels. And this also requires the complete equalization of tiny measured deviations in the range of 0.7 mm during turning, which are compensated by the software-control of the beams. This is accomplished with the assistance of a special high-precision X-ray positioning system for both the patient and the patient table he reclines on, a system based on our own proprietary patent that the system manufacturer Varian now uses as a production standard.