With the new treatment technique at PSI a pencil beam
(7mm diameter) of protons is regulated by computer
controlled magnets in such a way that the high-dose
spot can be positioned very precisely, for an
exactly specified period of time and at any desired
location within a tumor. By superimposing many
individual spots – more than 9000 within
a volume of one liter– we can impose the
desired radiation dose uniformly within a tumor,
with the dose being individually monitored for
each single spot. This enables extremely precise
and homogeneous irradiation, ideally adapted to
the shape of the tumor, which is in most cases
irregular.
We refer to this dynamic, three-dimensionally
conforming radiation therapy procedure as the
Spot-Scanning technique. Made a reality for the
first time at PSI, it is an international breakthrough,
making it possible for a higher radiation dose
to be applied to a tumor, with reduced damage
to the surrounding healthy tissue for most of
the irradiated tumors.
The
magnet controlled proton beam can be applied from
almost any direction. With the Spot-Scanning technique
and the rotatable beamline system, we achieve
the optimum conformation of the dosage to the
shape of the tumor.
By
using large kicker-magnets, we can direct the
protons to the Gantry and to the patient, into
the tumor. Step by step, the high-dose spot scans
the tumor in all three spatial dimensions. The
penetration depth of the proton-spots is controlled
by a system of moving plastic plates. These movements
last only a few milliseconds. They are repeated,
layer by layer, corresponding to the shape of
the tumor, by slowly moving the patient in an
individually moulded couch in the beam.
The
Gantry 1, which weighs more than 100 tonnes, can
be rotated as one unit about its central axis,
and therefore enables irradiation to be carried
out from any desired direction. For all its complexity
and enormous weight, the unit still needs to be
extremely precise and stable. Depositing exactly
the right dose in the tumor very homogeneously
calls for the utmost precision in delivering every
single spot, with regard to both, its location
and duration. The extremely short reaction times
of the facility which this requires proved to
be an extraordinary technical challenge. It takes
less than four minutes to irradiate a volume of
1 liter, but during this time more than 9000 spots
are deposited, every single one with a dose precision
of 1%.
In
order to treat moving tumors (i.e. in the chest and
lung area) also with such a high accuracy, the
treatment technique is being developed further
(see research and development
and the project PROSCAN).
