Why protons
Protons deposit most of their energy at a depth that depends on their energy and on the material they pass through. By choosing the energy, the beam can be made to act at the depth of a tumour, sparing healthy tissue in front of and behind it.
At PSI the protons are accelerated in a dedicated medical cyclotron, COMET, to 250 MeV. A device called a degrader then slows the beam so that its energy matches the depth of the tumour, and a vacuum beam line carries it to a treatment station in less than a thousandth of a second.
Eye tumours since 1984
The story began with particle physics. In the 1970s researchers at PSI’s predecessor, the Swiss Institute for Nuclear Research, experimented with pions for radiotherapy; in the 1980s interest shifted to protons. In March 1984 patients were irradiated with protons at PSI for the first time, against eye tumours, the first such treatment in Western Europe.
The eye facility, OPTIS, was created by PSI physicist Charles Perret and the ocular oncologist Leonidas Zografos of the Jules-Gonin Eye Hospital in Lausanne, adapting a method first tested in Boston. Its successor, OPTIS2, has been in operation since 2010. PSI reports more than 8,400 eye tumour treatments since 1984.
Spot scanning
On 25 November 1996 a patient was treated at PSI with spot scanning, a technique developed at PSI, which PSI describes as a world first. Instead of shaping a broad beam, a narrow beam is moved point by point through the tumour. Before it was used on people, the method had been tried in veterinary medicine; PSI’s first patient treated with this technique, in 1994, was a dog.
Since 1996, more than 2,600 patients with deep-seated tumours have been treated at PSI, among them more than 900 children and adolescents.
The treatment rooms
Deep-seated tumours are treated at Gantry 2 and Gantry 3; at these treatment stations a tumour can be irradiated from any direction. Gantry 2 is PSI’s own development and can also treat tumours that move as the patient breathes. Gantry 3, built in scientific collaboration with Varian Medical Systems, opened in 2018. The older Gantry 1 is now used for research. Eye tumours are treated at OPTIS2.
Villigen.science describes the facility and its history from PSI’s own publications. It gives no medical advice; questions about diagnosis or treatment belong with a doctor.
Sources
- PSI, Center for Proton Therapy: Therapy facilities
- PSI, Center for Proton Therapy: Additional information
- PSI, Center for Proton Therapy: Patient numbers
- PSI, Physics against Cancer: How PSI pioneered modern proton therapy (2023)
- PSI, A 40-year success story: Protons against eye cancer (2024)
- PSI, The history of PSI
Sources checked on: 3 November 2026