Proton Therapy
The facility for proton therapy
Further details on the:
Proton source
Protons are the core of hydrogen atoms, where a bound electron surrounds a proton like a cloud, as seen from a physics point of view. Protons are electrically positive, while electrons are negatively charged. These charge carriers are separated from a hydrogen gas in a hot plasma inside the source and the protons are extracted through electric fields. The in-house developed source dedicated for the beam operation at HZB is unique in the world. It reliably provides a beam of remarkable stability as well as high proton intensity.
Pre-accelerator
The protons which come out of the plasma at the exit of the source are being gradually brought to the energy which offers them a 3 cm penetration depth in soft tissue, as required for the eye tumor therapy. A Van de Graaff generator with a maximum voltage of 5.5 million volts serves as a pre-accelerator, delivering a proton beam of excellent energy spread.
First acceleration step. The ion source can be seen at the tip of the Van de Graaff generator inside its red enclosure, which has been opened here for maintance.
Main accelerator
In this stage the protons are being accelerated by alternating electric fields up to an energy of 68 million electronvolts, corresponding to approximately half of the speed of light. The main accelerator is a so-called sector cyclotron with two high-frequency systems, responsible for gradually increasing the proton energy, and four magnets which steer the beam, keeping the protons in a trajectory around the center of the machine. For this reason, a cyclotrons is also mentioned as a circular accelerator, although the proton trajectory is in fact spiral.
The protons from the Van de Graaf pre-accelerator are being injected very close to the cyclotron's center, at the initial narrow orbit. Each magnet bends the beam by 90 degrees, in order to create a loop that always traverses the two high-frequency systems. As a result of the acceleration by the alternating electric fields, the radius of the beam's orbit constantly increases until it reaches the extraction position. At that location, the beam is deflected outside the cyclotron and guided towards the irradiation room.
Second acceleration step. The protons coming from the pre-accelerator are being brought to almost half of the speed of light in the sector cyclotron. The four bending magnets (brown blocks) and the two high-frequency systems (yellow) ensure that the proton beam stays in its design trajectory.
Beam steering
The size of the proton beam, which is roughly that of a pinhead after extraction from the cyclotron, has to be widened in the accelerator vault in order to reach a diameter of up to 35 millimeters, following the proton therapy requirements.
The enlarged beam is then delivered by HZB to Charité's irradiation room, where the protons exit through a beam tube. This is the last component of the beam line.
In the tube's opening, an individually shaped brass aperture confines the beam to the tumour's edges. Sophisticated beam optics ensure that the protons deposit their maximum dose to the tumour tissue.
