Medical linear accelerator - proton therapy

The role of vacuum in tumor, proton and heavy ion therapy

Let's explore the different uses of vacuum technology for those therapies

June 23, 2020

Vacuum technology for tumor therapy

Several devices requiring vacuum are used in tumor therapy. An important device is the cyclotron.

Cyclotrons accelerate electrically charged particles to extreme high speed (= energy). These particles are used in medical therapy to destroy tumors to avoid surgery. 

Cyclotrons accelerate particles (electrons, protons (= nuclei of hydrogen) or heavier ions) generated in the center by passing through a strong magnetic field in spiral trajectories receiving their energy by radio frequency. The complete trajectory (from center to patient >> 10 m) must be in high vacuum. The mean free path of the particles must be longer than the distance of travel otherwise they would be scattered and deflected to the wall. As a result, pressure has to be < 1 x 10 -06 mbar.

The vacuum system of a cyclotron may be exposed to radiation and magnetic fields. Thus, a classic composition is oil diffusion pumps backed by rotary vane pumps. Alternatively, turbomolecular pumps backed by rotary vane pumps or scroll pumps are used. Very large cyclotrons also use cryopumps.

Vacuum technology for proton therapy

Proton Therapy is a new technology for cancer treatment that became qualified in the 1990s.

Irradiation of human tissue by X-ray photons destroys healthy tissue on its path to the tumor and behind. In the case of proton beams, the protons decelerate only slowly until they reach the point at which the beam is stopped. Tissue in the beam path absorbs only a very small dose relative to photon beams. At the end of the range, the protons are stopped at the 'Bragg Peak' (see image 2) and transfer a large amount of energy to the tumor - and almost zero behind.

As a result, proton therapy is a powerful but gentle tool where damage to healthy tissue is minimized. It can avoid many surgeries and can be used for complicated treatments such as tumors in eyes to avoid complete removal!

Comparison X-Ray photon therapy to proton therapy

The protons (nuclei of hydrogen) are generated and accelerated in a cyclotron. Since the protons are electrically charged, they can be deflected and transported in beamlines to several rooms where patients are medicated in the gantries.

The vacuum system is of extreme importance for the proton therapy and underlies high medical standards.

The cyclotron is generally pumped by mid-size turbomolecular pumps. They are exposed to magnetic fields and radiation, often shielded by iron.

The beamlines have small diameters (approx. 60 mm) and require a small TMP every few meters. Since the protons may not be scattered by residual gas, they require a mean free path of more than 100 m, thus a pressure of 10-07 mbar. Baking of vacuum parts is not necessary. Since the quality of vacuum is significant for the success of the therapy status monitoring, documentation and safety are crucial.

Vacuum technology for heavy ion therapy

Heavy Ion Therapy challenges vacuum technology with Ultra High Vacuum requirements.

It is an enhancement of proton therapy and uses heavier ions like C+ instead of protons. Destruction of cancer cells is even more efficient.

The technology applied in heavy ion therapy is more complex: The ions are generated in an ion source, accelerated in a linear accelerator and then fed into a synchrotron. In the synchrotron, ions are further accelerated and guided by magnets in circular tracks before guided into the beamlines. The synchrotrons have circumferences of up to 100 m. A pressure well below 1 x  10-09 mbar is required to ensure ions travel such long distances without interaction. This leads to UHV technology, using ion pumps, dry roughing systems and even a bakeable beam guiding system. Since it is common today to use superconducting magnets in the synchrotron, the cold walls support the pumping.

Setting up a proton therapy center demands an extraordinary investment and the heavy ion therapy even more so. But this is justified by the outstanding prospect of healing success.

In this blog we have outlined the importance of vacuum technology in different medical procedures as well as the role various vacuum pumps play in their performance. 

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