Proton therapyat the Paul Scherrer Institute
PSI proton therapy for tumours of the eye (OPTIS). The patient’s head is fi xed 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.
33
The aim of the radiotherapy system provided at
the Paul Scherrer Institute (PSI) is to use charged
particles, called protons, to destroy cancerous
tissue. Protons are particularly suited to this task
because they exert their greatest impact deep
within a patient’s body, inside the tumour itself.
Thanks to an irradiation technique that is the only
one of its kind world-wide, the innovative proton
therapy facility at PSI is able to adapt the radia-
tion dose extremely accurately to the shape of a
tumour (which is usually irregular). As a result,
this technique is able to safeguard healthy tissue
better than the conventional modern radiotherapy
techniques.
Tumours of the eye were treated with radiation at
PSI for the fi rst time in 1984. This was the fi rst
installation of this type anywhere in Europe. The
fi rst proton gantry for the irradiation of deep-
seated tumours was taken into service at PSI in
1996, and was also the fi rst in Europe. With the
ongoing development of this innovative irradiation
technique, it shall be possible in future to irradiate
also tumours which move during treatment (e.g.
breast and lung cancer) with a high degree of
precision. PSI is a leader in the technological
development of proton therapy, setting world-wide
trends in radiotherapy for cancerous tumours.
Proton therapyat the Paul Scherrer Institute
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 benefi ted from this therapy at PSI.
4 P R O T O N T H E R A P Y AT P S I
Improved radiation therapy
means
• more precise match between
the radiation dose and the
shape of the tumour
• higher radiation dose in the
target volumes (tumour plus
safety zone)
• lower radiation stress for
healthy structures in the
body
• better, more sustainable
odds on recovery
• fewer side effects
• better quality of life
• justifi able treatment costs
This can signifi cantly reduce, or even prevent, short
and long term side effects.
Radiation therapy, or radiotherapy, is a local
method of treatment (like surgery), and it is there-
fore used to fi ght against tumours that are limited
to a specifi c location. It is not interchangeable with
therapies that have to act on the whole body (sys-
temic therapies), such as chemotherapy and immu-
notherapy (especially for the treatment of metas-
tases).
In radiation therapy, the tumour cells are
destroyed by x-ray or gamma radiation (photon
therapy) or by particle radiation (e.g. proton ther-
apy). The aim of each additional stage in the
development of radiotherapy is to destroy the
tumour completely, while being even better at
safeguarding healthy tissue.
Great progress has been made in conventional
radiotherapy during the past 20 years. Neverthe-
less, proton therapy can help to achieve signifi -
cantly better results for certain tumour indications
and tumour localisations. The developments at PSI
also demonstrate that the potential for improve-
ment is still far from exhausted.
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
have any chance to repair themselves, so that they
all die off, without any exception, but that the
healthy cells suffer as little damage as possible,
and are able to recover without any diffi culty.
The radiation dose is a measure of the energy
absorbed in a material, e.g. in tissue. However, the
biological effect of radiation is not just dependent
on how much energy is deposited in the cells, but
also on the way in which it is deposited. The energy
dose is measured in Gray (Gy). A typical therapy
dose used to destroy a tumour would be about 60
to 70 Gy. It is delivered in individual fractional
doses on several consecutive days of radiotherapy
(about 30 to 40 fractional doses in total).
Radiotherapy and its signifi cance
It is anticipated that one in every three people in
Europe will suffer from cancer at some point in
their lives. In Switzerland alone, about 30,000
people discover that they have cancer every year.
Around 70 % of these will require radiotherapy
during their illness. A little more than 45 % of all
the tumours diagnosed today are curable, where
«curable» is taken to mean that the patient lives
without suffering any new outbreak of cancerous
disease for more than fi ve years after treatment.
About 22 % owe their recovery to surgery, about
12 % to radiotherapy, about 6 % to a combination
of both methods and about 5% (metastasised and
non-localised tumours) to other treatments and
combinations, including chemotherapy.
Radiotherapy is therefore an important form
of treatment, and is often the only possibility in
the case of non-operable tumours. In the case of
treatment for primary tumours, the odds are
improving for recovery, and therefore for life
expectancy. It is therefore all the more important
that radiotherapy should be administered as pre-
cisely as possible, and that the healthy areas of
the body should be irradiated as little as possible.
PSI proton therapy for tumours of the eye, using a special
proton beam with a low penetration depth (OPTIS).
These photographs through the pupil show the interior of
the eye; above before proton therapy, below one year
later – the tumour has shrunk.
5P R O T O N T H E R A P Y AT P S I
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 fi rst time at the Lawrence Berkeley Laboratory
in California (USA) in 1954, and the fi rst proton
therapy programme in Europe ran in Uppsala
(Sweden) between 1957 and 1976. In 1961, the
Harvard Cyclotron Laboratory and the Massachu-
setts General Hospital in Boston, USA, started a
proton therapy project. Melanoma of the eye was
treated with protons for the fi rst time in Europe in
1984, at the OPTIS facility developed especially for
this purpose at PSI.
The fi rst proton therapy facility to be used at
a hospital went into operation at the Loma Linda
University Medical Center, California, in 1990. Fol-
lowing a development and testing phase of almost
10 years, up to 1500 patients have routinely ben-
efi ted from proton therapy there since 1999. Today
there are more than 35 centers in operation world-
wide, and already more than 80,000 patients have
been treated with Proton therapy, nearly 10 % of
them at PSI.
The technique known as Spot-Scanning, used
to treat deep-seated tumours with protons, was
developed at PSI at the beginning of the 1990s.
This PSI technology is superior to the proton
radiation methods used in other centres, and
provides better protection for healthy tissue. This
extremely precise method has been used to treat
patients with tumours that are particularly hard to
treat at PSI since 1996. As well as PSI, there are
now six other operational proton therapy facilities
in Europe, three of these are only able to treat
tumours of the eye. World-wide, there are more
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 world-
wide. 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 demon-
strated that the spatial precision of the irradiation
is often crucial to the successful result of the
therapy. Because the technique developed at PSI
provides a particularly high level of accuracy in the
irradiation, it has become the world-wide trendset-
ter for further developments in proton therapy.
Almost all facilities in planning or under construc-
tion today rely on the scanning technique, fi rst
used at PSI. As well as having the appropriate
accelerators and experienced staff, this success
has also been built on the interdisciplinary environ-
ment at PSI, and the particular background of
experience resulting from basic physical research.
The PSI team now has more than 25 years of
experience of proton therapy. By the middle of
2011, almost 6000 tumours of the eye and over
750 deep-seated tumours have been treated at
PSI. A therapeutic success rate of over 98 % cures
for irradiated melanoma of the eye is particularly
impressive. The results for the patients treated at
the proton gantry, about one third of them children
and young people, are also very encouraging, with
over 80 % tumour control in most cases.
Proton treatment of
deep-seated tumours
at Gantry 1.
A glimpse inside the COMET cyclotron (archive image taken during construction). Protons are accelerated to 180,000 kilometres per second along spiral-
shaped tracks from the inside to the outside of this machine.
7
Protonp+
Hydrogen atom
e– Electron
Positively-charged protons are building blocks of matter.
Hydrogen atoms have a nucleus containing one proton,
and free protons are achieved by ionising these atoms (the
electron is stripped away from the atomic shell).
P R O T O N T H E R A P Y AT P S I
The physics and engineering of proton therapy
Protons are elementary particles that carry a pos-
itive charge. As a result, they can be defl ected
within magnetic fi elds, bundled together and
formed into a beam as required. Unlike the photons
currently used in radiotherapy, protons are associ-
ated with a very defi nite, precisely limited depth
of penetration within the body. Photons emit their
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
low dose is absorbed in the material between the
surface of the body and the stopping point, and
the protons lose speed continuously as they travel.
At the end of their range, they stop and emit their
maximum dose, the Bragg peak. Behind this point,
the dose falls to zero within millimetres.
Protons therefore deposit their highest dose
of radiation directly inside the tumour, in the form
of a patch or a spot, and have a signifi cantly weaker
effect than photons on the healthy tissue between
the surface of the body and the tumour.
The diagram below shows the dose progression
for a single thin pencil beam of protons. The lower
part of the diagram also demonstrates that protons
emit a signifi cantly weaker dose than photons in
front of the target volume. Tissues behind the
target volume are signifi cantly irradiated by pho-
tons, while they are not affected at all by pro-
tons.
Photon
γ
Proton
p+
stops
Photons (electromagnetic waves) and protons (charged
particles) behave very differently from each other.
Target volume
Protons
Photons
100%
50%
10%
Depthcm0 10 20 30 40
Dose
Spot
Body surface
Individualprotonpencil beam
Bragg peak (spot)
The radiation dose of a proton pencil beam along its
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 pene tration 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
specifi ed 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.
9
The PSI Spot-Scanning technique
Protons are accelerated in the COMET cyclotron
and focussed into a beam of approximately 5 to
7mm width (the spot). The protons are then
directed by magnets to the irradiation equipment,
known as the gantry, where they are guided
towards the patient and the tumour. These high-
dose spots cover the tumour in all three spatial
dimensions (the scanning). At Gantry 1, the pen-
etration depth of the proton spot is controlled by
a system of plastic plates that slide into the path
of the beam, and the movements only last for a
few milliseconds. Individual lines are irradiated in
the tumour, layer by layer, and the patient is moved
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 defl ected 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
for a precisely pre-set time. By superimposing a
large number of individual spots – approximately
10,000 for a volume of 1 litre – the tumour can be
covered evenly by the required radiation dose,
while the dose is monitored individually for each
individual spot. This produces extremely precise,
homogenous radiation, with an optimum match to
the shape of the tumour (which is usually irregu-
lar). We call this dynamic, three-dimensional form
of radiotherapy the «Spot-Scanning technique».
It has been used to treat cancer patients at PSI
since 1996, is unique world-wide, and enables
tumours to be irradiated extremely accurately
while affecting the healthy surrounding tissue less
than conventional photon therapy.
D I E P R O T O N E N T H E R A P I E A M P S I
The principle of the spot-scan-
ning technique developed at PSI.
Dose distributions of any shape
can be produced by shifting and
superimposing the dose spot of
a proton pencil beam, and the
dose can be matched extremely
accurately in three dimensions
to the shape of the tumour.
This treatment plan demonstrates the particular precision
of the spot-scanning technique, using the example
of a brain tumour. The dose is matched individually in
each plane of the relevant boundary (yellow). The
tissue outside the tumour remains largely unaffected.
1
2
3
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 defl ection magnet to defl ect 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).
11D I E P R O T O N E N T H E R A P I E A M P S I
Gantry 2 for the irradiation of movable tumours
Gantry 2 will enable this scanning technique to be
used to irradiate tumours extremely accurately,
even if they move during irradiation (e.g. lung or
breast tumours). In this gantry, the proton beam
is guided by defl ecting magnets in two dimensions
at a pre-set energy level into the tumour, and a
slice of the tumour is irradiated. The energy can
be changed in a fraction of a second to irradiate
the next layer of the tumour. The tumour is there-
fore «scanned» in three dimensions. Because of
the high speed at which the beam is defl ected and
the energy changed, the dose can be applied to
the tumour several times very quickly, and the
overall radiation time stays short. This repeated
«scanning» of the tumour volume allows the dose
to be distributed very evenly, even if the tumour
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.
Gantry 1
Gantry 2
Optis 2
COMET Cyclotron
Beamline
13D I E P R O T O N E N T H E R A P I E A M P S I
The proton therapy process at PSI
Proton therapy is administered in individual daily
fractions, just like conventional photon therapy,
and a course of treatment usually lasts six to eight
weeks (approx. 30 to 40 sessions). Most of the
patients are referred through university hospitals
and other hospitals in Switzerland and abroad,
and are then looked after by a qualifi ed team of
radio-oncologists, medical physicists and other
specialists at PSI. After producing an individual
mould to support the patient’s body, computer
tomography images are taken slice by slice. The
PSI medical team then establishes the dose bound-
ary for each plane of the tumour, i.e. the three-
dimensional target volume with a safety zone. This
forms the basis of the treatment plan, in which
computer programs developed specially for this
purpose at PSI pre-calculate, optimise and store
every setting of the radiation equipment in a data
set, together with the resulting dose distribu-
tion.
X-ray images are used to check the location of
the tumour and the patient’s position within their
individual moulded support at each treatment
session. Patients undergo regular follow-up checks
for several years after the course of treatment is
over.
The majority of patients are treated as out-
patients, though a few are accommodated in one
of the hospitals near to PSI. Infants are anaesthe-
tised during the individual treatment sessions, and
an anaesthetics team from the children’s hospital
in Zurich regularly attends infant treatment ses-
sions at PSI.
Patients are selected by the medical team at
PSI on the basis of the added medical value that
might be expected from the proton therapy from
experience. In Switzerland, the cost of treatment
for the following indications is currently paid by
the compulsory health insurance scheme:
• Intraocular melanomas (radiation for tumours
of the eye in the OPTIS facility)
• Meningiomas (benign and malignant), low-grade
gliomas
• Tumours in the area around the base of the skull
and in the ear, nose and throat region (ENT
tumours)
• Sarcomas, chordomas and chondrosarcomas
• Tumours in infants (including anaesthesia), chil-
dren and young people
Other indications are being investigated in trials
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 90° defl ecting magnet and radiation head (the person shown on the photo is not a patient).
15
Infants are anaesthetised for irradiation, so that the tumour position remains precisely fi xed. Proton therapy offers
particular advantages in their case, since an infant’s organism reacts particularly sensitively to radiation.
The medical team will need all the available
information, including previous investigations,
medical history and radiological documentation,
in order to prepare and implement a course of
proton therapy. Direct contact with the doctor
referring the patient is also very important to
ensure that good care is provided before and after
the therapy at PSI.
D I E P R O T O N E N T H E R A P I E A M P S I
Exact positioning of the patient is particularly important for proton therapy. This is ensured by a number of measures:
by providing individual moulded supports, fi tted 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.
By mid-2011, more than 750 patients with
deep-seated tumours located near to critical
organs have been treated at Gantry 1. Almost 6000
patients with tumours of the eye have been suc-
cessfully irradiated at the OPTIS facility since 1984.
Since 2010, a new OPTIS facility (OPTIS 2) is avail-
able. Once Gantry 2 will go into operation (from
2012 on), about 500 patients with tumours will be
able to benefi t from proton therapy at PSI per year.
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
www.psi.ch
www.protontherapy.ch
Villigen PSI, September 2011
Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
Tel. +41 56 310 21 11, Fax +41 56 310 21 99www.psi.ch, www.protontherapy.ch
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 fi 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 experi-
ments that are not possible anywhere else.
ContactsCentre for Proton Therapy
Administration
Tel. +41 56 310 35 24
Contact person for journalists:
Dagmar Baroke
Tel. +41 56 310 29 16, Fax +41 56 310 27 17
Pro
ton
enth
erap
ie_e
, 10/
2011
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