Seite 27 - Einblicke 57

57 EINBLICKE 29
Die Autoren des Artikels (v.l.): Hui
Khee Looe, Kay C. Willborn, Ndimofor
Chofor, Björn Poppe im Oldenburger
Pius-Hospital.
The authors of the article (from left):
Hui Khee Looe, Kay C. Willborn,
Ndimofor Chofor, Björn Poppe at the
Pius-Hospital in Oldenburg.
tion in the body. Furthermore, when x-ray technology was
digitalised, conventional x-ray films and the machines used
for developing them became obsolete. Consequently it be-
came necessary to develop new detectors that reflected the
advances in radiation research. The digital detectors initially
used in radiology are also of limited use. On the one hand the
level of radiation energy is so high that most devices working
on a CCD basis are destroyed relatively quickly. Moreover the
physical characteristics of these detectors mean that precise
dose measurement is extremely complicated.
As an alternative, together with researchers at Physikalisch-
Technischen Werkstätten (PTW) in Freiburg, the Oldenburg
physicists have developed two-dimensional detectors on the
basis of ionization chambers. The ionization chambers of these
detectors are arranged in a single layer, similar to the pixels in a
digital camera. These detectors are considerably larger than the
individual pixels of a CCD chip, for example. But how large can
thedetectors be, andhowmany are needed for precise compa-
risons between thepre-calculatedandactual dosedistribution?
The physicists found the answer in multidimensional signal-
processing. To put this method into practice they first had to
adjust the mathematical description of the dose deposition
and apply it to themeasuring procedure. This was the onlyway
to estimate the necessary and optimal number of chambers
and their size. For practical purposes they first built a detector
with around 1.000 measuring chambers. This configuration
enables measurements of the dose distribution that are pre-
cise enough for most clinical applications.
After initial scepticism among both physicists and medical
practitioners, this chamber array construction has now be-
come the global standard. In recent years other work groups
have confirmed the results of the Oldenburg and Freiburg
scientists and developed detectors according to this principle.
Today this type of detector array is likely to be found in all
institutions providing radiation therapy.
But this was just the beginning. Theoretical analyses of radi-
ation transport within the human body were able to prove
that there is a minimum value for detector size and distance
from the patient – approximately 2.5 millimetres. Below that
value it is not possible, even in theory, to improve accuracy
owing to the interaction between radiation and matter that
occurs with the photon beams typically used. In practice,
the inaccuracies increase to somewhere in the range of five
millimetres. Ion-beams could make it possible to target the
radiation evenmore precisely, but the research in this field is at
a very early stage, and it will be a long time before ion-beams
are put into routine clinical use.
The Oldenburg scientists and their partners are therefore
concentrating on a detector array that works on the minimum
resolution limit. Owing to the low volume of the chambers, air
is no longer a viable detection medium. It is replaced by non-
conductive liquids, such as isooctane. But the diverse physical
properties of these liquids pose new challenges for medical
physics. The devices of the future will have to comprise consi-
derablymore than 1000 chambers – and this in turnwill require
optimisation in terms of the signal processing on the array.
Under the auspices of their collaboration with partners from
Ashland Inc., inWayne, New Jersey (USA), theOldenburg scien-
tists are also researchingmonomers. After absorbing radiation
themonomers can bind to formpolymers with different light
absorption properties. When these monomers are applied
to a thin film-like base, the altered light absorption produ-
ces a kind of
"blackening".
Because these
monomers are
just a fewmicrometers in size, it would in principle be possible
to increase the resolution of the measurements to an almost
infinite degree. The physical properties of these processes and
their application in dosimetry are currently the subject of a
variety of studies worldwide, in which Oldenburg's medical
physicists are also involved.
Ultimately, the goal of all these efforts is to optimise the
concordance between the calculated dose and the admini-
stered dose to ensure further advances in radiation therapy
techniques. In order to achieve this, physicists and physicians
probablywork together more closely in radiation therapy than
in any other area of modern medicine.
Two-dimensional detectors developed
on the basis of ionization chambers