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Proton therapy at the Paul Scherrer Institute

PSI proton therapy for tumours of the eye (OPTIS). The patient’s head is fixed using a mask and a bite block. Actual irradiation of the eye tumour lasts less than one minute. Four separate irradiations have to be carried out on four consecutive days.


Proton therapy at the Paul Scherrer Institute

The aim of the radiotherapy system provided at

Tumours of the eye were treated with radiation at

the Paul Scherrer Institute (PSI) is to use charged

PSI for the first time in 1984. This was the first

particles, called protons, to destroy cancerous

installation of this type anywhere in Europe. The

tissue. Protons are particularly suited to this task

first proton gantry for the irradiation of deep-

because they exert their greatest impact deep

seated tumours was taken into service at PSI in

within a patient’s body, inside the tumour itself.

1996, and was also the first in Europe. With the

Thanks to an irradiation technique that is the only

ongoing development of this innovative irradiation

one of its kind world-wide, the innovative proton

technique, it shall be possible in future to irradiate

therapy facility at PSI is able to adapt the radia-

also tumours which move during treatment (e.g.

tion dose extremely accurately to the shape of a

breast and lung cancer) with a high degree of

tumour (which is usually irregular). As a result,

precision. PSI is a leader in the technological

this technique is able to safeguard healthy tissue

development of proton therapy, setting world-wide

better than the conventional modern radiotherapy

trends in radiotherapy for cancerous tumours.


OPTIS facility for the irradiation of eye tumours using protons. After the proton beams have been adjusted precisely to the tumour in the eye, the irradiation is carried out. More than 5000 patients have so far benefited from this therapy at PSI.



Radiotherapy and its significance

This can significantly reduce, or even prevent, short and long term side effects.

It is anticipated that one in every three people in

Radiation therapy, or radiotherapy, is a local

Europe will suffer from cancer at some point in

method of treatment (like surgery), and it is there-

their lives. In Switzerland alone, about 30,000

fore used to fight against tumours that are limited

people discover that they have cancer every year.

to a specific location. It is not interchangeable with

Around 70 % of these will require radiotherapy

therapies that have to act on the whole body (sys-

during their illness. A little more than 45 % of all

temic therapies), such as chemotherapy and immu-

the tumours diagnosed today are curable, where

notherapy (especially for the treatment of metas-

«curable» is taken to mean that the patient lives


without suffering any new outbreak of cancerous

In radiation therapy, the tumour cells are

disease for more than five years after treatment.

destroyed by x-ray or gamma radiation (photon

About 22 % owe their recovery to surgery, about

therapy) or by particle radiation (e.g. proton ther-

12 % to radiotherapy, about 6 % to a combination

apy). The aim of each additional stage in the

of both methods and about 5% (metastasised and

development of radiotherapy is to destroy the

non-localised tumours) to other treatments and

tumour completely, while being even better at

combinations, including chemotherapy.

safeguarding healthy tissue.

Radiotherapy is therefore an important form

Great progress has been made in conventional

of treatment, and is often the only possibility in

radiotherapy during the past 20 years. Neverthe-

the case of non-operable tumours. In the case of

less, proton therapy can help to achieve signifi-

treatment for primary tumours, the odds are

cantly better results for certain tumour indications

improving for recovery, and therefore for life

and tumour localisations. The developments at PSI

expectancy. It is therefore all the more important

also demonstrate that the potential for improve-

that radiotherapy should be administered as pre-

ment is still far from exhausted.

cisely as possible, and that the healthy areas of the body should be irradiated as little as possible.

How does radiotherapy work? If a charged particle (e.g. a proton) passes through a cell, or stops within, the energy it deposits (the dose) damages the core of that cell. Under certain circumstances, however, the cell can repair this damage. The art of radiotherapy is to deliver the dose in such a way that the tumour cells do not Improved radiation therapy

have any chance to repair themselves, so that they


all die off, without any exception, but that the

• more precise match between

healthy cells suffer as little damage as possible,

the radiation dose and the

and are able to recover without any difficulty.

shape of the tumour

The radiation dose is a measure of the energy

• higher radiation dose in the target volumes (tumour plus

absorbed in a material, e.g. in tissue. However, the

safety zone)

biological effect of radiation is not just dependent

• lower radiation stress for healthy structures in the

on how much energy is deposited in the cells, but


also on the way in which it is deposited. The energy

• better, more sustainable odds on recovery

dose is measured in Gray (Gy). A typical therapy PSI proton therapy for tumours of the eye, using a special

dose used to destroy a tumour would be about 60

• fewer side effects

proton beam with a low penetration depth (OPTIS).

• better quality of life

These photographs through the pupil show the interior of

• justifiable treatment costs

the eye; above before proton therapy, below one year

doses on several consecutive days of radiotherapy

later – the tumour has shrunk.

(about 30 to 40 fractional doses in total).

to 70 Gy. It is delivered in individual fractional



Proton therapy world-wide and at PSI Proton therapy is based on experience gathered over more than 50 years on the biological effect of proton radiation on diseased and healthy tissue in the body. A patient was treated with protons for the first time at the Lawrence Berkeley Laboratory in California (USA) in 1954, and the first proton therapy programme in Europe ran in Uppsala (Sweden) between 1957 and 1976. In 1961, the Harvard Cyclotron Laboratory and the Massachusetts General Hospital in Boston, USA, started a

Proton treatment of

proton therapy project. Melanoma of the eye was

deep-seated tumours

treated with protons for the first time in Europe in

at Gantry 1.

1984, at the OPTIS facility developed especially for this purpose at PSI. The first proton therapy facility to be used at

is often crucial to the successful result of the

a hospital went into operation at the Loma Linda

therapy. Because the technique developed at PSI

University Medical Center, California, in 1990. Fol-

provides a particularly high level of accuracy in the

lowing a development and testing phase of almost

irradiation, it has become the world-wide trendset-

10 years, up to 1500 patients have routinely ben-

ter for further developments in proton therapy.

efited from proton therapy there since 1999. Today

Almost all facilities in planning or under construc-

there are more than 35 centers in operation world-

tion today rely on the scanning technique, first

wide, and already more than 80,000 patients have

used at PSI. As well as having the appropriate

been treated with Proton therapy, nearly 10 % of

accelerators and experienced staff, this success

them at PSI.

has also been built on the interdisciplinary environ-

The technique known as Spot-Scanning, used

ment at PSI, and the particular background of

to treat deep-seated tumours with protons, was

experience resulting from basic physical research.

developed at PSI at the beginning of the 1990s.

The PSI team now has more than 25 years of

This PSI technology is superior to the proton

experience of proton therapy. By the middle of

radiation methods used in other centres, and

2011, almost 6000 tumours of the eye and over

provides better protection for healthy tissue. This

750 deep-seated tumours have been treated at

extremely precise method has been used to treat

PSI. A therapeutic success rate of over 98 % cures

patients with tumours that are particularly hard to

for irradiated melanoma of the eye is particularly

treat at PSI since 1996. As well as PSI, there are

impressive. The results for the patients treated at

now six other operational proton therapy facilities

the proton gantry, about one third of them children

in Europe, three of these are only able to treat

and young people, are also very encouraging, with

tumours of the eye. World-wide, there are more

over 80 % tumour control in most cases.

than 30 proton therapy projects currently under construction or at a late stage of planning, and approximately 10 of these are located in Europe. Today, more than 10,000 patients per year are treated with protons at about 35 centers worldwide. Most of these suffer from tumours of the eye or brain, or tumours in the head, neck, pelvis and spinal area. Clinical experience with protons has demonstrated that the spatial precision of the irradiation

A glimpse inside the COMET cyclotron (archive image taken during construction). Protons are accelerated to 180,000 kilometres per second along spiralshaped tracks from the inside to the outside of this machine.


The physics and engineering of proton therapy

Hydrogen atom e–

Protons are elementary particles that carry a pos-




itive charge. As a result, they can be deflected within magnetic fields, bundled together and formed into a beam as required. Unlike the photons currently used in radiotherapy, protons are associ-

Positively-charged protons are building blocks of matter. Hydrogen atoms have a nucleus containing one proton,

ated with a very definite, precisely limited depth

and free protons are achieved by ionising these atoms (the

of penetration within the body. Photons emit their

electron is stripped away from the atomic shell).

maximum dose immediately after they have entered the body. This means that healthy tissues are also subjected to powerful radiation. The range of protons depends on their initial speed and on the material in which they stop. Only a relatively

effect than photons on the healthy tissue between

low dose is absorbed in the material between the

the surface of the body and the tumour.

surface of the body and the stopping point, and

The diagram below shows the dose progression

the protons lose speed continuously as they travel.

for a single thin pencil beam of protons. The lower

At the end of their range, they stop and emit their

part of the diagram also demonstrates that protons

maximum dose, the Bragg peak. Behind this point,

emit a significantly weaker dose than photons in

the dose falls to zero within millimetres.

front of the target volume. Tissues behind the

Protons therefore deposit their highest dose

target volume are significantly irradiated by pho-

of radiation directly inside the tumour, in the form

tons, while they are not affected at all by pro-

of a patch or a spot, and have a significantly weaker


Body surface

Target volume

Individual proton pencil beam








Bragg peak (spot)



Proton stops

10% 0




40 cm


Photons (electromagnetic waves) and protons (charged

The radiation dose of a proton pencil beam along its

particles) behave very differently from each other.

penetration depth into the body. The range of these protons is 25 cm. The dose distribution is shown above as a contour, while dose values are shown along the penetration depth below, for comparison with the behaviour of a photon dose.


The new compact COMET proton cyclotron at PSI in construction. This is the most compact proton therapy equipment of this type world-wide, and was specified by physicists at PSI. In the lower part of the picture, the stream of protons is extracted from the cyclotron and transported within a fraction of a thousandth of a second to the treatment locations.


The PSI Spot-Scanning technique


for a precisely pre-set time. By superimposing a large number of individual spots – approximately

Protons are accelerated in the COMET cyclotron

10,000 for a volume of 1 litre – the tumour can be

and focussed into a beam of approximately 5 to

covered evenly by the required radiation dose,

7mm width (the spot). The protons are then

while the dose is monitored individually for each

directed by magnets to the irradiation equipment,

individual spot. This produces extremely precise,

known as the gantry, where they are guided

homogenous radiation, with an optimum match to

towards the patient and the tumour. These high-

the shape of the tumour (which is usually irregu-

dose spots cover the tumour in all three spatial

lar). We call this dynamic, three-dimensional form

dimensions (the scanning). At Gantry 1, the pen-

of radiotherapy the «Spot-Scanning technique».

etration depth of the proton spot is controlled by

It has been used to treat cancer patients at PSI

a system of plastic plates that slide into the path

since 1996, is unique world-wide, and enables

of the beam, and the movements only last for a

tumours to be irradiated extremely accurately

few milliseconds. Individual lines are irradiated in

while affecting the healthy surrounding tissue less

the tumour, layer by layer, and the patient is moved

than conventional photon therapy.

slowly in 5mm steps within the radiation area so that all the spatial dimensions have been covered by the spots. A more advanced scanning technique will be used in the new Gantry 2: there, the beam is simultaneously deflected in two directions within the tumour and the change of energy takes place in the «Degrader» (attenuator), at the exit of the cyclotron, all within a split second. In the case of the treatment technique used at PSI, the pencil beam of protons is controlled by computers so that a high-dose spot is located very accurately at the required position in the tumour

The principle of the spot-scanning technique developed at PSI. Dose distributions of any shape can be produced by shifting and This treatment plan demonstrates the particular precision

superimposing the dose spot of

of the spot-scanning technique, using the example

a proton pencil beam, and the

of a brain tumour. The dose is matched individually in

dose can be matched extremely

each plane of the relevant boundary (yellow). The

accurately in three dimensions

tissue outside the tumour remains largely unaffected.

to the shape of the tumour.

Above: Proton Gantry 1: A view from above onto the magnets in the gantry, which weigh many tons. They bundle and direct the proton beam to the treatment location. The facility weighs over 100 tonnes and can be rotated as a whole precisely to the millimeter. Below: This longitudinal section through Proton Gantry 1 shows the principle behind the way in which the proton beam is steered, and the position of the three controlling elements: a deflection magnet to deflect the beam (1) (scanning), plastic plates to vary the penetration depth of the protons within the body (2), adjustable patient table for layer-by-layer radiation (3).

1 2



Gantry 2 for the irradiation of movable tumours


be changed in a fraction of a second to irradiate the next layer of the tumour. The tumour is therefore «scanned» in three dimensions. Because of

Gantry 2 will enable this scanning technique to be

the high speed at which the beam is deflected and

used to irradiate tumours extremely accurately,

the energy changed, the dose can be applied to

even if they move during irradiation (e.g. lung or

the tumour several times very quickly, and the

breast tumours). In this gantry, the proton beam

overall radiation time stays short. This repeated

is guided by deflecting magnets in two dimensions

«scanning» of the tumour volume allows the dose

at a pre-set energy level into the tumour, and a

to be distributed very evenly, even if the tumour

slice of the tumour is irradiated. The energy can

moves while it is being irradiated.

The Gantry 2 radiation station during construction.

The drawing shows the overall technical facility for proton therapy at PSI. In the case of treatment for deep-seated tumours, the protons are accelerated to approximately 180,000 kilometres per second in the COMET cyclotron accelerator. The accelerated protons are then directed by electromagnets via a beamline in less than a thousandth of a second through a steel pipe that is practically free of air to the treatment stations (Gantry 1, Gantry 2 and OPTIS 2), where they are guided into the patient’s tumour at a precisely pre-set energy and direction of irradiation. Computer control is used to ensure that the proton beam deposits the pre-planned and pre-calculated dose, thus destroying the tumour cells.

COMET Cyclotron


Optis 2

Gantry 1

Gantry 2


The proton therapy process at PSI

The majority of patients are treated as outpatients, though a few are accommodated in one

Proton therapy is administered in individual daily

of the hospitals near to PSI. Infants are anaesthe-

fractions, just like conventional photon therapy,

tised during the individual treatment sessions, and

and a course of treatment usually lasts six to eight

an anaesthetics team from the children’s hospital

weeks (approx. 30 to 40 sessions). Most of the

in Zurich regularly attends infant treatment ses-

patients are referred through university hospitals

sions at PSI.

and other hospitals in Switzerland and abroad,

Patients are selected by the medical team at

and are then looked after by a qualified team of

PSI on the basis of the added medical value that

radio-oncologists, medical physicists and other

might be expected from the proton therapy from

specialists at PSI. After producing an individual

experience. In Switzerland, the cost of treatment

mould to support the patient’s body, computer

for the following indications is currently paid by

tomography images are taken slice by slice. The

the compulsory health insurance scheme:

PSI medical team then establishes the dose bound-

• Intraocular melanomas (radiation for tumours

ary for each plane of the tumour, i.e. the threedimensional target volume with a safety zone. This forms the basis of the treatment plan, in which

of the eye in the OPTIS facility) • Meningiomas (benign and malignant), low-grade gliomas

computer programs developed specially for this

• Tumours in the area around the base of the skull

purpose at PSI pre-calculate, optimise and store

and in the ear, nose and throat region (ENT

every setting of the radiation equipment in a data


set, together with the resulting dose distribu-

• Sarcomas, chordomas and chondrosarcomas


• Tumours in infants (including anaesthesia), chil-

X-ray images are used to check the location of

dren and young people

the tumour and the patient’s position within their individual moulded support at each treatment session. Patients undergo regular follow-up checks

Other indications are being investigated in trials

for several years after the course of treatment is

at PSI and at other centres.


A tumour in the head region of a 7-year-old child irradiated at PSI. Irradiation plan for radiation treatment using modern conventional photon therapy (left) and using proton therapy at PSI (right). Irradiation by photons generates a «dose bath» in a large part of the brain, and also affects the brain stem and optical nerves. This can be avoided by using proton therapy.


Gantry 2 with integral 90째 deflecting magnet and radiation head (the person shown on the photo is not a patient).


The medical team will need all the available

By mid-2011, more than 750 patients with

information, including previous investigations,

deep-seated tumours located near to critical

medical history and radiological documentation,

organs have been treated at Gantry 1. Almost 6000

in order to prepare and implement a course of

patients with tumours of the eye have been suc-

proton therapy. Direct contact with the doctor

cessfully irradiated at the OPTIS facility since 1984.

referring the patient is also very important to

Since 2010, a new OPTIS facility (OPTIS 2) is avail-

ensure that good care is provided before and after

able. Once Gantry 2 will go into operation (from

the therapy at PSI.

2012 on), about 500 patients with tumours will be


able to benefit from proton therapy at PSI per year.

Exact positioning of the patient is particularly important for proton therapy. This is ensured by a number of measures: by providing individual moulded supports, fitted to each patient’s body, by moving the patient table with extreme accuracy, and by checking the position with CT (computer tomography) and X-ray images.

Impressum Concept / Editing Martin Jermann, PSI Dagmar Baroke, PSI Photography Paul Scherrer Institut H.R. Bramaz, Lieli Alain Herzog, Source: ETH Board Layout / Printing Paul Scherrer Institut Reproduction with quotation of the source is permitted. Please send an archive copy to PSI. Order from Paul Scherrer Institut Communication Services 5232 Villigen PSI, Switzerland Tel. +41 56 310 21 11 Internet

Infants are anaesthetised for irradiation, so that the tumour position remains precisely fixed. Proton therapy offers particular advantages in their case, since an infant’s organism reacts particularly sensitively to radiation.

Villigen PSI, September 2011

Protonentherapie_e, 10/2011

PSI in brief The Paul Scherrer Institute PSI is a research center for the natural and engineering sciences. At PSI, cutting-edge research is performed in the ďŹ elds of Matter and Materials, Human Health as well as Energy and Environment. We use fundamental and applied research to work on sustainable solutions for key questions raised in society, science and economy. With the equivalent of about 1400 full-time staff positions, we are the largest research institute in Switzerland. We develop, construct and operate complex large-scale facilities. Every year about 2000 guest scientists from Switzerland and around the world come to us. Just like PSI’s own researchers, they use our unique facilities to carry out experiments that are not possible anywhere else.

Contacts Centre for Proton Therapy

Contact person for journalists:


Dagmar Baroke

Tel. +41 56 310 35 24

Tel. +41 56 310 29 16, Fax +41 56 310 27 17

Paul Scherrer Institut, 5232 Villigen PSI, Switzerland Tel. +41 56 310 21 11, Fax +41 56 310 21 99,

Proton therapy at the Paul Scherrer Institute  
Proton therapy at the Paul Scherrer Institute  

The aim of the radiotherapy system provided at the Paul Scherrer Institute (PSI) is to use charged particles, called protons, to destroy can...