Image-guided Radiotherapy
Dirk Verellen Director, Medical Physics Group, Department of Radiotherapy, Academic Hospital, Vrije Universiteit Brussel
Inaccurate knowledge of a patient’s anatomy and position during the
course of therapy has always been a major source of concern in radiation
therapy, potentially compromising the clinical results by insufficient dose
coverage of the target volume and/or overdose of normal tissues. The
management of target localisation emanates from the concept of
treatment margins to cope with the uncertainty of the true location of
the target volume during irradiation (gross target volume (GTV); clinical
target volume (CTV); set-up margin (SM); internal margin (IM); planning
target volume (PTV); and planning risk volume (PRV)).1,2 It is generally
accepted that two classes of these so-called set-up uncertainties can be
identified: systematic and random. Systematic errors exist because the
imaging performed for treatment planning is typically a snapshot and the
target position determined at that moment may differ from the average
target position at treatment time, or if a certain procedure introduces an
error that is repeated systematically over time. The random error is the
day-to-day deviation from the average target position introduced with
internal organ motion and the repeated treatment set-up that occurs in
fractionated radiation therapy.
The systematic error is generally considered more important, because if
uncorrected it would propagate throughout the treatment course and
lead to a deleterious effect on local control. Day-to-day variations may be
substantial and require safety margins that limit the maximum dose
administered to the tumour volume due to possible toxicity to
surrounding healthy tissue. With the introduction of image-guided
radiation therapy (IGRT), clinical confidence has grown and it is possible
to examine whether the traditional fractionated radiation therapy at 2Gy
per fraction is still the optimum strategy. This introduces treatment
schedules using fewer fractions (so-called hypo-fractionation),3 and the
day-to-day variation in target localisation may no longer be statistically
random. Finally, motion management becomes an issue as tumour
motion interacts with dose delivery, causing a dose spread along the path
of motion in some delivery techniques.4
With the improved imaging modalities to define and delineate tumour
volumes, identifying both morphological as well as functional and
biologic information, and the introduction of treatment modalities that
allow for shaped dose distributions (e.g. intensity-modulated radiation
therapy (IMRT), stereotactic body radiotherapy (SBRT) and chargedparticle
beams), the radiotherapy community is now capable of creating
dose distributions that match the tumour volume tightly.3,5 Conformal
radiation therapy (CRT) aims at shaping the dose distribution to the
delineated target volume, whereas conformal avoidance aims at avoiding
critical structures. These advances have been driven by the dual goals of
maximising radiation dose to tumour volume while minimising the dose
to surrounding healthy tissue. Accurate knowledge of the patient’s
anatomy during the radiation process is of utmost importance, and it can
be argued that novel technologies such as IMRT and shaped-beam
radiosurgery are useless without proper image guidance.
The concept of image guidance is not new in radiotherapy. Aspects of
image guidance have always existed, even with the first use of X-rays for
cancer therapy, probably using the same radiation source for both
imaging and treatment. The concept of IGRT has been introduced to
define the accomplishment of tumour and soft tissue imaging in
‘realtime’ or ‘near-realtime’ for the correction of both systematic and
random errors on a daily basis. It was born out of the need for both
accurate target localisation in IMRT and SBRT and the delivery of boost
doses to sub-volumes identified with functional and biological imaging.
IGRT will be necessary to exploit the possible clinical benefits of the new
treatment procedures. As the capabilities of IGRT improve, it will provide
tools to better understand treatment uncertainties and allow a reexamination
of the present practice regarding treatment margins.
Conceptually, IGRT refers to in-room image guidance just before or
during treatment and is based on the assumption that the tumour
volume has been defined adequately. The imaging modalities applied for
tumour identification and delineation, although they also help to ‘guide
the treatment’, are not part of the IGRT concept in its current definition.
Image-guided Radiation Therapy Solutions
An ideal Mage-guided radiotherapy (IGRT) system should have three
essential elements: three-dimensional (3-D) and, if possible, motion (fourdimensional
(4-D)) assessment of the target volume, preferably 3-D
volumetric information of soft tissue, including tumour volume; efficient
comparison of the image data with reference data; and an efficacious
and fast process for clinically meaningful intervention (preferably fully
automated). The clinical introduction of on-line electronic portal imaging
devices (EPIDs) has led to an improved understanding of treatment uncertainties and a need for strategies to further reduce them.6 As early as the 1990s, strategies had been developed to use EPID for near-realtime
patient set-up.7,8 Although the first requirement (3-D assessment) could
be established by using multiple planar images, this procedure never
became a mainstream solution as it was cumbersome to implement.6 This
development did raise the awareness of the potential benefits of image
guidance, and the concept of IGRT was born. IGRT solutions could be
classified as follows: megavoltage (MV) imaging, kilovoltage (kV) imaging
and solutions using non-ionising radiation.
