ONCOLOGY
MRI-guided radiotherapy:
A real time, personalised
treatment method
Using data from online MR guidance, we will be able to image biomarkers during treatment and
thus be able to adapt the plan or even change the treatment intent based upon real-time data
Enis Ozyar MD
Banu Atalar MD
Gorkem Gungor MSc
Acibadem MA Aydinlar
University, School of
Medicine, Department of
Radiation Oncology
Acibadem Maslak
Hospital, Department
of Radiation Oncology,
Istanbul, Turkey
The main purpose of radiotherapy is to destroy
tumour cells while minimising the damage to
healthy cells around the tumours. While this is
mainly achieved through precise dose distribution
methods such as intensity modulated
radiotherapy (IMRT) or volumetric intensity
modulated arc treatments (VMAT), correct
administration of these techniques necessitates
the process of precise imaging of the patient
immediately before and during the course of the
radiation treatment. This process is called image
guidance or image-guided radiotherapy (IGRT).
IGRT enables precision in radiotherapy by
identifying the exact tumour position during
treatment, ensuring that the planned
tumourocidal dose reaches the target and better
sparing the normal tissues surrounding the
tumour from radiation. While IGRT is a relatively
easy process for cranial targets where there is
almost no movement during treatment, it is
highly complex and critical for the treatment of
anatomical sites such as the thorax, abdomen and
pelvis where respiratory movement affects the
position of targets and normal organs up to 3cm.
Modern linear accelerators (Linac, a common
radiotherapy device) are usually equipped with
X-ray based IGRT systems where 2- or
3-Dimensional kilovoltage or megavoltage
imaging is frequently used. These systems are
used to detect systematic errors, position the
patient correctly, enable the real-time
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modification of the treatment plan and detect
changes in patient or tumour size. However,
X-ray-based systems lack soft-tissue contrast,
especially at the abdomen and pelvis, and this can
necessitate insertion of invasive and high-cost
fiducial gold markers in or around the tumours
in order to track them before and during
treatment. Recently, integration of magnetic
resonance (MR) imaging systems with linear
accelerators has enabled us to improve targeting
accuracy without requiring the aforementioned
fiducial markers. These systems can now perform
real-time assessment of soft tissue anatomy and
motion, using continuous cine mode imaging
before and during radiotherapy. This has allowed
for the correction of intrafractional errors with
geometric accuracy of 1–2mm.
Several MR-guided Linac systems are being
developed, using different magnetic field
strengths and orientations of the static magnetic
field to the treatment beam, yet only two have
been successful and are currently being used.
There are several obstacles while integrating
Linac and MR into the same treatment system.
Firstly, an extensive magnetic shielding of the
Linac components is needed to circumvent
interference between the radiofrequency and
magnetic fields. These critical components are
mainly magnetron and port circulator, which
cannot function properly in the presence of a
magnetic field. The high-power radiofrequency