UCLA Journal of Radiation Oncology APRIL 2023 - Flipbook - Page 14
UCLA RADIATION ONCOLOGY JOURNAL
daily doses are delivered with high precision in
generally five or fewer treatments, as a curative
option for most men with localized prostate
cancer.7 As suggested by the term “stereotactic,”
SBRT relies on advanced imaging and treatment
delivery techniques to ensure precise delivery
of radiation to the PTV while minimizing dose
outside the PTV.
CT-guided LINAC, an SBRT plan could be
delivered in roughly 3-4 minutes. This margin
formulation is well within the parameters of the
standard-of-care margins used internationally.
For instance, the large PACE-B and NRG-005
randomized trials recommend margins of 5 mm
in all directions, with an option to narrow these
to 3 mm in the posterior dimension.
What, precisely, contributes to the PTV?
Recently, two MRI-guided LINACs have
become commercially available: the ViewRay
MRIdian and the Elekta Unity. Both units have
solved the aforementioned long-standing
technical dilemma of integrating an MRI
with a LINAC. MRI-guidance offers several
theoretical advantages in the context of prostate
radiotherapy.10
The major sources of uncertainty that are
considered when deciding on the required PTV
margin are uncertainties related to prostate
delineation and prostate motion.8 The prostate
is best delineated using magnetic resonance
imaging (MRI), but due to concerns related
to electromagnetic interference, most linear
accelerators (LINACs) delivering radiotherapy
have only been equipped with computedtomography (CT) or X-ray based imaging tools.
First, MRI-guided LINACs can monitor prostate
motion directly, rather than relying on fiducial
markers that are proxies for prostatic motion
and require an invasive procedure to place.
As such, prostate MRIs are often fused to CTbased imaging to help design the target for
radiation, which imbues a small uncertainty
into the prostate. To account for motion between
individual doses of radiation, implanted markers
are placed inside the prostate to provide a
proxy for prostate position. Typically, these
are radiopaque metallic markers that can
be visualized on CT or X-ray. Motion during
treatment can be managed either by shifting
treatment based on frequent X-ray images
focused on the fiducials, or limiting the
treatment time as much as possible.
Second, and perhaps most importantly, the
frequency of monitoring is extremely high, and
a “cine” MRI can be used to “gate” treatments
on prostatic position. For instance, the MRIdian
obtains an MRI four times per second. If a set
percentage of the prostate target (say, 10%) is
outside of a preset margin around the prostate
(say, 3 mm), the radiation beam can be held
automatically.
Third, the improved soft tissue contrast from
an on-board MRI also improved the accuracy of
alignment prior to radiation.
Having performed CT-guided SBRT for over a
decade, we were able to query our institutional
database to develop a margin formulation for
the PTV that was based not just on theory, but
on our own empirical observations.9 We found
that a 3 mm margin around the prostate would
be necessary to account for motion alone, and
another 1 mm should be added for other residual
uncertainties—this would translate a 4 mm total
margin around the prostate.
Fourth, residual errors resulting from fusing an
MRI to a CT scan for accurate target delineation
can be minimized if the on-board MRI is used
directly for target delineation.
Given these enhanced imaging capabilities, we
estimated that we could now use a PTV margin
of only 2 mm – essentially 1 mm for motion,
and 1 mm for residual uncertainty. But a salient
question remained: would reducing these
margins actually help patients?
By using volumetric arc therapy on an advanced
14