Advances in Image-guided Radiotherapy - The Future is in Motion
Peter H Cossmann Head of Medical Physics, Hirslanden Klinik Aarau, Switzerland
Introduction
External beam radiotherapy is the most common
form of radiation treatment offered to cancer
patients. Currently, different types of external
beam therapy techniques are used. The goal of
three-dimensional conformal radiotherapy (3-D
CRT) is to deliver a full dose of irradiation to the
target structure with as little radiation as possible to
the surrounding normal tissue. Intensity
modulation radiotherapy (IMRT) is a further
refinement of conformal radiotherapy, which
allows the dose within a target to be modified so as
to spare specific tissue and organs. In order to take
respiratory motion of the target into account,
another approach is 4-D CRT, which can also be
combined using intensity modulated fields.
Historical Overview
Different approaches towards image-guided
radiotherapy (IGRT) have been followed over the
years. In the 1990s, the first electronic portal
imaging devices (EPID) for linear accelerators
(linacs) were developed, initially with chargecoupled
device (CCD) camera optics, later using
liquid ion chamber technology and now mostly
based on amorphous silicon flat panels. The next
step has been the introduction of room or gantrybased
kilovoltage (KV) radiograph and fluoroscopy
devices, also allowing localisation by means of
bony structures or fiducial markers.
Initially, obtaining 3-D information was achieved
by placing a conventional computed tomography
(CT) scanner in the treatment room in a known
geometric relationship with the linear accelerator’s
isocentre. Now, CT functionality has been
integrated in the linac in order to eliminate the
need for a separate scanner. These cone beam CT
options are based on either an additional kV system
or by using megavolt radiation from the therapy
beam source. Most recent publications indicate that
on-board imaging devices with a separate kV
system offer the largest flexibility with regard to
different modes such as radiography, fluoroscopy
and CT.
4-D Imaging
Most 3-D treatment planning systems utilise CT
images based on a diagnostic CT scanner. These
scanners limit how the patient can be positioned
because of their relatively small opening; in order to
overcome these issues, dedicated oncology scanners
with a large bore have been developed. The new
radiotherapy department at the Hirslanden Klinik
Aarau was one of the first clinics in Europe to be
equipped with a special radiotherapy scanner (see
Figure 1). Such multi-slice systems with up to 20
detector rows now allow the acquisition of highspeed
scans. These scanners additionally offer 4-D
functionality, which means the scans are acquired
with co-registered respiratory signals. This
technique entails the creation of multiple CT slices
at each relevant table position for at least the
duration of one full respiratory cycle, while
simultaneously recording signals from a respiratory
motion monitoring system.
As a consequence, 4-D imaging is part of IGRT
because, in order to function effectively, like with
gating, 4-D imaging capability is required. The
crucial element of the gating system is that, unlike
some others, it is a non-invasive form of gating that
works via room-mounted cameras and a detector
device that is placed on the patient’s torso. It records
the patient’s breathing pattern during the scan,
resulting in a 4-D-CT data set. An analysis of the
data with regard to target motion during the different
phases of the breathing cycle by the radiotherapist
and medical physicist determines the minimum
movement and leads to the therapeutic window
defined by a lower and upper threshold.
On-board Imaging
Aarau was only the second European hospital to
install an on-board imaging system for IGRT, as
shown in Figure 2. Last July, the new radiotherapy
department at the Hirslanden Klinik Aarau was
equipped with one of the most advanced and
sophisticated radiotherapy systems in the world
today. In addition to the on-board imager (OBI),
which is used to fine-tune patient positioning at the time of treatment, the system incorporates the
realtime position management (RPM) respiratory
gating system, which tracks tumour motion and turns
the treatment beam on and off as the tumour moves
in and out of range. A gated treatment is illustrated
in Figure 3, showing the patient with the detector
device (two reflector spots) in the upper left corner
and the breathing curve with inhale and exhale
period information in the lower right corner. By
‘gating’ the treatment beam, doctors are able to
deliver a more precise dose to the tumour and avoid
more of the surrounding healthy tissues.
