energy that is needed to obtain high energy
photons from the Linac might also significantly
deteriorate MRI image quality. Secondly, the MRI’s
magnetic field can cause alterations in planned
dose delivery due the Lorentz force, which can
both broaden the penumbra of a photon beam
and cause an electron-return effect. The magnetic
field can also divert both electrons travelling
within the beam transport system and secondary
electrons generated inside patients. In short, MR
and Linac may severely disrupt each other’s
functions. 1
The first clinical treatment using an MR-guided
Linac system was performed in 2014, using a
Viewray MRIDian system in the US. Since then,
more than 3000 patients have been treated
worldwide. Another commercially available
MR-guided Linac system was developed by Elekta
and was recently approved for use in clinical
treatments. 2
MR-guided radiotherapy is not only a new
technology but also a new treatment algorithm.
Below are a few major differences of MR-guided
radiotherapy when compared to conventional
radiotherapy:
• The patient set-up is made using MR images,
which enables the best soft tissue contrast among
the IGRT methods. This has the greatest advantage
in the thorax, abdomen and pelvis. When
motion-induced blurring occurs with MR imaging,
this can easily be eliminated through several
solutions such as treating during breath hold.
• Rigid immobilisation can be completely
eliminated as adaptive plans can be used during
all fractions.
• Invasive tracking methods that are commonly
used in conventional radiotherapy methods, such
as gold-marker insertion into the tumour or
invasive hydrogel injections to move away normal
structures form tumours, can be eliminated as
superior soft-tissue contrast is available with MR
guided techniques
• Daily real-time replanning of the patient
according to the ‘anatomy of the day’ by
re-countouring the tumour and surrounding
normal organs while patient is on the treatment
bed (namely, adaptive planning)
• Real time continuous tracking of the tumour by
cine MR during beam on. All these steps lead to a
longer treatment time – approximately 2–3-times
longer than conventional radiotherapy as
expected, and this new algorithm requires a
tremendous effort from the treatment team,
which must be available in the control room
during all these steps.
This MR-guided treatment algorithm also has
several clinical advantages over conven-tional
radiotherapy. Using the above methods, fewer
margins are needed around the tumours
compared with conventional radiotherapy, which
leads to lower normal tissue toxicity and better
local control. The other major advantage of
MR-guided systems is the adaptive radiotherapy
process. In conventional radiotherapy, treatment
plans are executed using a first-day anatomy and
an optimal plan is generated to use for treatment
purposes during all fractions by assuming that
the anatomy is always the same. However,
tumours usually shrink during the treatment
process and normal organs deform daily the
surrounding structures depending on their
In the future, by
using MR-guided
Linac, radiation
oncologists will
be better able
to identify and
treat a range
of thoracic,
abdominal, and
pelvic tumours
effectively
References
1 van Herk M et al. Magnetic
resonance imaging guided
radiation therapy: A short
Strengths, Weaknesses, Opportu-
nities, and Threats Analysis.
Int J Radiation Oncol Biol Phys
2018;101:1057–60.
2 Pollard JM et al. The future
of image-guided radiotherapy
will be MR guided. Br J Radiol
2017;90:20160667.
3 Nahum AE. The radiobiology of
hypofractionation. Clin Oncol (R
Coll Radiol) 2015;27(5):260–9.
4 Palma DA et al. Stereotactic
ablative radiotherapy versus
standard of care palliative
treatment in patients with
oligometastatic cancers
(SABR-COMET): a randomised,
phase 2, open-label trial. Lancet
2019;18;393:2051–8.
5 Badiyan SN et al. The role of
radiation therapy for pancreatic
cancer in the adjuvant and
neoadjuvant settings. Surg Oncol
Clin N Am 2017;26(3):431–53.
6 Rudra S et al. Using adaptive
magnetic resonance image
guided radiation therapy
for treatment of inoperable
pancreatic cancer. Cancer Med
2019;00:1–10.
9
HHE 2019 | hospitalhealthcare.com
movement and/or fullness, in daily real life. Thus,
applying an optimal plan of the first-day plan to
all con-sequent fractions may cause under or
overdosing the tumours and normal critical
structures. Adaptive radiotherapy eliminates the
under- and over-dosing of tumours and critical
normal structures respectively, by daily
replanning according to anatomy of the day.
In the last ten years, hypofractionated (higher
dose per fraction with lower number of total
fractions) treatments have replaced the majority
of conventional fractionated treatments in
radiotherapy. 3 Primary and secondary tumours of
the breast, lung, liver, pancreas, prostate, brain,
hepatobiliary, and gynaecological system are
increasingly being treated with hypofractionated
treatments in recent years. MR-Linac has enabled
us to treat the majority of these tumours
effectively by applying the optimal total dose,
with minimal side effects and without using any
invasive markers. Treatment of oligometastatic
diseases (limited number of metastasis) with
stereotactic radiotherapy (SBRT) (total fraction
number of 8 and less) have been associated with
an improvement in overall survival in a recent
study. 4 The findings of this randomised study
represent the strongest clinical evidence available
in support of the management of oligometastatic
state across multiple tumour types. Thus,
MR-guided SBRT is the safest and most effective
radiotherapy method for treatment of several
oligometastatic sites namely, lung, liver, adrenal,
intrabdominal, and pelvic region.
Besides hypofractionated and SBRT
treatments, more clinical data are needed to
define the diseases that migh most benefit from
MR-guided radiotherapy. Hepatopancreatobiliary
system tumours are the most obvious targets as
the current impact of radiotherapy in these
tumours are highly controversial. Several clinical
studies have shown that conventional doses of
radiotherapy do not confer a survival advantage
over chemotherapy alone in locally advanced
inoperable pancreatic cancer. 5 Until now, it has
been common that suboptimal doses are given to
pancreatic tumours, as the pancreas is
surrounded by several critical normal structures
and respiration causes severe movement of both
tumours and normal organs during treatment.
Stereotactic body radiotherapy (SBRT) was
investigated as an alternative radiotherapy
technique in order to apply effective doses to the
pancreatic cancer. Local control rates exceeding
80% were reported by using SBRT in the
management of pancreatic cancer in a
retrospective series. 6 MR-guided SBRT seems to
be a potential new treatment method to improve
outcomes for such patients, thanks to adaptive
radiotherapy and precise dose management of
locally advanced pancreatic cancer.
In the future, by using MR-guided Linac,
radiation oncologists will be better able to
identify and treat a range of thoracic, abdominal,
and pelvic tumours effectively. 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. Considering
all the aforementioned advantages of MR-guided
radiotherapy, it seems that the future of IGRT
will be MR-guided.