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This episode of the Physics World weekly podcast
features the medical physicist,
Alonso Gutierrez,
in conversation
with Physics
World's Tammy Freeman.
The Lexel Gamma Knife is a high precision
radiation treatment system,
but the gamma knife is very different to
conventional X-ray based radiotherapy
devices.
It uses an array of roughly 200 tiny
radioactive sources to focus gamma rays directly onto
the target,
most commonly to destroy brain tumors in one
fraction using a technique known as radiosurgery.
I'm speaking today with Alonzo Gutierrez,
chief physicist at the Miami Cancer Institute,
about his experience of using the Lexile Gamma
Knife for radiosurgery
in a busy radiotherapy department.
Welcome to the podcast, Alonzo.
Well, hi. Good morning, good afternoon, depending on
where in the world you're listening to. So
it's an absolute pleasure to be here and,
look forward to,
to chatting a little bit about the, gamma
knife,
dreaming unit.
Excellent. So first of all, could you briefly
describe how a gamma knife works and how
it's different from the more prevalent external beam
radiotherapy systems?
Sure. So the, gamma knife,
radiosurgery unit, is really
a dedicated platform that is, you know, distinctly
different compared to,
you know, linac based radiosurgery delivery approaches. One
is that it uses different types of sources.
So linear accelerators
tend to use a linear accelerating device that
accelerates electrons and generates photons
based off of production.
The gamma knife unit uses cobalt 60,
has the radio isotope that delivers the gamma
rays through natural decay. The energy is also
slightly different. So
the most linear accelerators tend to use six
megavoltage,
photon beam energy, whereas the average decay product
of cobalt 60 is 1.25
MeV,
so it's a slightly lower energy.
On top of that, in terms of the
localization,
for patients,
both treatment delivery platforms can deliver both
frame based deliveries where patients are immobilized using
a
frame
that is, you know, placed in via pins
to the patient's skull
or with a thermoplastic
mask that is custom molded to the patient,
and it serves as sort of a immobilization
device to prevent the patient's head from moving
during the course of treatment.
This could be done on both delivery platforms.
On top of that, and more more recently
with the latest platform which is the Gamma
Knife Esprit,
you have the addition of onboard imaging
and typically and historically
the way that a lot of radiosurgery was
done was through the frame using lasers
to align the patients to the isocenter of
the treatment delivery approach. Similarly with
Gamma Knife, it was using a frame through
a coordinate system.
Now with the addition of image guided, you're
able to acquire a high resolution
volumetric
cone beam CT
that could be used to localize
the patient
and place the patient in the correct position
before treatment, for each,
for each lesion or isocenter that's being delivered.
Okay. So it's the cobalt cobalt 60 sources
that are emitting the gamma rays that's used
for the,
treatment. So you actually have these a 92
small sources that could be delivering radiation at
the same time. So,
when you deliver and you have these radioactive
sources, you can, have an output of around
three and a half gray, per minute in
terms of at the ISO center,
when you're using the largest columnator size.
Now
the difference between that is, as you deliver
this, the, the radiation, it's been delivered from
all 360 degrees around the patient head.
Typically, with linear accelerators, you deliver it
at specific set angles
or kind of as
a kind of rotating dynamic arc in which
you're able to deliver the radiation as a
function of time.
And so these sources stay in place. They're
calibrated
at the time that the sort the gamma
knife unit is loaded.
And then after that, the dose rate will
fall off as the system
as
a cobalt 60 decays.
And so that's the key difference, whereas with
other delivery systems,
such as a linac based solution, you have
a linear accelerator that's producing the fixed dose
rate,
over time to be able to do the
actual delivery. In the planning process, which is
part of, how you would sort of modulate
the dose from these go out sources,
you have combinations of different collimator settings as
well as different sectors. And so there's eight
sectors that are geometrically spaced around,
the patient head, and each one has 24
sources. And by turning on and off these
sectors, you can modulate the dose and get
different dose distributions,
and that's all done as part of the,
treatment planning system and the, dedicated treatment planning
software for Gamma
Knife.
Okay. And Gamma Knife is
mostly used for,
treating brain cancers. So what why is it
so particularly suited for that cancer type?
Well, I I I think in in general,
the system, from his inception was designed, to
replace the surgeon's knife and the term gamma
knife and to be able to
target,
lesions, that are deep inside the brain are
very difficult, challenging to get to.
And with that,
the design of the system was thought, with
the understanding that, it would have very high
accuracy,
very good precision,
and would be dedicated to cranial radiations. And
so
diseases that we treat are obviously those that
are particularly with brain tumors, so metastatic brain
tumors. So here at MCI, we treat about
eighty five percent of our volume as brain
tumors. However, there's other types of diseases that
it's been shown to be very efficacious for.
So meningiomas,
vestibular schwannomas,
some pituitary adenomas,
and some gliomas, which are very common. On
top of that, there's other indications,
that are, not necessarily malignant tumors,
but, you know, AVM, so any vascular malformations
and or any functional disorders like trigeminal neuralgia
or essential tremor, those that are being sort
of explored,
using resurgery approaches.
And Gamma Knife has been a device that's
been used to, treat, many of these for
for a number of years and has been
a proven platform to have,
good success in treating these diseases.
Oh, that's interesting. And, it's one of the
reasons it's particularly suited for the brain that
you can do this sort of complete
immobilization
to make sure that the the patient is
held completely
stationary during the treatment.
Correct. And I I think one of the
big aspects of that is the accuracy.
And so many of these lesions that you're
targeting are very small,
and especially when you get into the functional
disorders where your target volumes are two to
three millimeters.
And so the resolution is, the resolution and
accuracy is very important
in doing so. And, you can accomplish that
by having frame based immobilization,
or frameless now with,
with the
addition of the
CBCT
capabilities and the mass based solution that we
have there.
But, historically,
the frame based solution has shown exceptional accuracy,
submillimeter accuracy. And I think one of the
key attributes and one of the reasons why
we sort of looked,
to Gamma Knife as our our workhorse with
radiosurgery delivery is the ability to have, you
know, sustained and stable accuracy.
The system inherently is very heavy.
There are, minimal moving parts. The couch and
the sectors are principally the ones that move
the most. And and the idea there is
because of this, the accuracy is sustained over
amount of time. So,
we aren't so concerned with potential parts wearing
over time.
Then we've seen that as we've sort of
published our own results on the stability and
the accuracy
of systems, and we are able to sustain,
submillimetric
accuracy,
over a number of years. And this has
been our case for us since we've been
treating with it for over eight years.
Okay. And
so why is there a a need for
a dedicated system such as this for the
brain,
when linear accelerators can do intracranial and extracranial
stereotactic radiosurgery?
No. Very, very fair point and very good
question, especially now in in modern day era
where, we see a number of,
universal delivery systems, such as CRM Linacs,
that have the capability of delivering radiosurgery. We
also see other,
dedicated radiosurgery platforms that are available in the
market.
I think part of it comes with, the
abilities,
or the capabilities of the delivery system and
also the volume.
You know, generally, busier practices such as Miami
Cancer Institute see a high volume, so we
typically treat over three hundred fifty patients in
cases per year. And so for us, having
a dedicated platform to sustain
and to be able to support this volume
is very key.
At the same time, I think one of
the big attributes with the GavenLife platform is
the dedicated and and really efficient workflows.
And so, for us, given that we treat
principally,
brain metastases
as as our common disease on the platform,
one of the things that we stay true
is to ensure that we have very accurate
imaging before we treat, and so all our
MRIs are within forty eight hours of the
treatment of the Gamma Knife platform.
And so in order to do that, we
have to be able to be very efficient,
and Gamma Knife does allow us to,
gives us that that ability to have a
platform where we can quickly take an MR,
create a plan,
and have the plan ready for execution within
forty eight hours. And when you do that
for a large number of patients, it actually
is really a staple to, what we're able
to do.
Couple that with our sort of ability to
ensure that we deliver high quality treatment plans.
And so given its limited mobility, we also
firmly that the quality assurance of the treatment
system itself is very stable,
and very accurate.
So these are some of the the factors
that we sort of, incorporated as we made
the decision to use,
the gamma knife esprits, our our our radiosurgery,
sort of platform of choice.
Yeah. I mean, that was that was my
next question, that you're using the the latest
generation of the Lexel Gamma Knife, which is
the Electro Esprit. How long have you been
using that system, and what what are you
finding its key benefits to
be? Sure. So when we first launched the
center in 2017,
we did a launch with the
Gamma Knife Icon platform.
And at the time, we, launched and and
and really found a nice balance between frame
and frameless cases. So it was sort of
the first time that we'd seen that,
frameless treatments could be performed,
on the Gamma Knife unit.
Since then,
obviously, there's been the
introduction to the Esprit.
And so, once the opportunity presented itself to
upgrade,
we ended up looking at an upgrade for
that system. And what we found so far
is, exactly, what we've seen in the past
is very accurate and consistent,
quality insurance and quality performance of the platform.
There are some changes to the system to
get a refresh. The treatment couch
is updated.
We also have seen that there are some
cover changes with the inclusion of a new
monitor that's now mounted on the treatment unit,
which facilitates
information for the therapists that are signing up
the patients.
I think one of the biggest things have
been the improvements in the
Vantage frame, so changing from the g frame
to the Vantage
frame for stereotactic localization,
as well as the introduction to improvements at
the treatment planning system, most notably,
with lightning.
And,
from a workflow ease, the integrated console has
been something that
our therapists have, have commented on, being that
it's much easier with, you know, single monitor
and the keypad is much easier to use
and more intuitive to use,
compared to its predecessor. So,
you know, for these reasons,
it seems like, you know, our our team
has been very happy with the,
with the upgrade, and and it really has
helped us improve in a number of aspects
in terms of, our overall treatments.
And what what's the Vantage frame that you
mentioned?
So, you know, over the years, there's, in
terms of the frame based treatments,
there's been, sort of the the g frame,
which has been,
the tried and true,
stereotactic frame used for positioning. And,
Elekta has now moved towards using the Vantage
frame, which is a more patient friendly focus,
Vantage frame. And, and so it's actually or
stereotactic frame. And so, it's one that allows
for, you know, openness in front for the
patient. It's a lighter frame,
and it's sort of the new platform moving
forward for Electa to be able to use
for stereotactic localization
when when when when using a frame based
approach.
It's also made of a lighter material,
and it's less,
you know, less prone to artifacts
when doing CT scans. And so overall, the
the the frame is, has been, useful in
terms of,
you know, patient satisfaction and also for, a
number of, you know, treatment planning considerations,
in in the sense of reducing artifacts.
Okay. So, how does the treatment planning for
gamma knife, how does that differ from other
radiotherapy devices?
Well, given that it's a sort of a
a a unique, delivery,
system,
the
gamma knife has its own, Lexile, gamma plan
system. So, the idea there, the LGP as
it's referred
to, is a dedicated trim and planning system
that allows you to do,
essentially,
forward planning,
I e, where you're able to sort of
place ISO centers at shots.
And the principle in which, you know, the
gamma
knife system works is that
once you have a lesion identified, you place
shots or
and you and you and you spheres back
these targets or the contours to be able
to place a number of shots on top
of one another and
the convolution or the sum of all these
shots creates a dose distribution in which you're
targeting
that particular lesion.
And so this historically has been done
forward plan. And so part of the art
of of treatment planning
was to be able to know how to
place these these isosenders and shots.
And over time, there's sort of been improvements
in the way of doing so, mostly in
having inverse planning approaches. And so there's been
a couple of generations of,
inverse planning, most notably,
the,
fast inverse planning or or or lightning as
the trademark is has been a solution that
has been,
has done has done very well.
We've, we were one of the first adopters
of the lightning treatment planning system, and the
idea here is that we're able to get
very high quality plans very, very fast.
And so the inverse planning approach there gave
good quality,
plans,
for many of our cases.
We have published on this or our our
our group has done extensive work with Lightning,
and we've shown that we can, in essence,
improve the quality of our clinical plans with
Lightning.
We could do it substantially faster. I think
over, the cases that we evaluated, plans were
completed in less than five minutes, especially for
multi targets.
At the same time, we also showed that,
there's sort of a
emphasis on quality of these plans. And so
these plans,
required, you know, minor tweaking after the fact.
And and,
as solutions came from Lightning,
they were very, you know, high quality.
At the same time, we also noted that,
it was somewhat
not dependent on the planner and so we
did a study where we evaluated,
you know, a novice, an intermediate, and an
expert planner,
And if they were able to plan manually
compared to lightning,
the lightning plans were
essentially
eliminated the need of experience. And so we
were able to generate very fast high quality
plans
with Lightning. So that's a testament to a
lot of the work that's been done with
Electa to be able to show that, the
improvements in in in in quality are there.
And we feel confident that that helps standardize
us irrespective
of the number or the different types of
physicists that are planning that we can obtain
very high quality plans for our patients.
Okay. And then so you're mentioning these shots.
I presume these are the shots of radiation
from the cobalt sources.
So
how does, how is this radiation
steered onto the target to get it really
conformal?
So the,
the way that it works is the,
sources, the 92 sources are focused to an
isocenter,
and they're collimated. And the way that the,
sources
are you know, the
shots are placed is,
you can have a combination of all these
eight sectors being open with the same collimator,
and so that would produce a shot of,
for instance, eight millimeters,
if you have that. So that would be
one shot. You can, in essence,
block some of these sectors so you can
have, you know, one or two or a
couple of these sectors be off, and the
remainders could be turned on, to eight millimeters,
and then you would have a different shape
of a dose distribution.
And similarly, you can have combinations of four,
eight, and sixteen
in any given pattern
to be able to
create different dose distributions.
There are some benefits, of course, to using
larger shots, so you tend to reduce some
of the treatment time. So there are improvements
in efficiency as you use larger shots. And
so I think one of the key things
that we found with lightning was
the combinations
of these shots in sectors,
change, when you have an inverse planning solution
versus a forward plan or manual plan. As
you can imagine, computers do a great job
and do it much faster, in terms of
determining solutions. And so what we've noticed is
that, with Lightning, we're able to get, you
know, you know, as a equivalent or better
quality plans in a much faster time than
someone that would do it manually because we
end up seeing the use of, you know,
you know, unconventional
sort of distributions where we tend to see
bigger shots more commonly used than smaller shots,
which allows us to reduce the overall treatment
time. So,
with that said, you know, the manipulation of
being able to generate the optimal combination,
obviously improves with lightning as opposed to an
individual doing a manual plan. And and you
see this more case when the lesions get
bigger and bigger. For very small lesions,
you don't, you know, tend to see a
lot of modulation
in terms of selection of these different shots,
but as they become more complicated, you do.
And then that's really where you see the
testament of, of using the system like the,
like lightning for that.
Okay. So, basically, you've got this array of
cobalt sources around the skull, and then you
use the collimators
to shape the beams coming from these different
sources,
based on the treatment planning to to sort
of really target as conformally as possible.
Correct. And and and I think the the
two, you know, key parameters typically
are, you know, conformity at the prescription dose
and and the fall off. And so,
you wanna ensure that you're covering the target.
So target coverage is key and principle, to
make sure that you're not missing part of
the tumor.
Typically, you have very little margin of error
there, so you have to ensure that the
accuracy and the delivery and your and and
the coverage of the lesion is is paramount.
Then, you look at how conformal is your
dose,
sorry, is your is your shape of your
dose cloud to your
target, and that's at the prescription level.
And then,
typically, half of the prescription level, you make
sure that that's also very tight, and that
really just is an indicator of the measure
of the fall off of the radiation because
you wanna make sure that you also don't
spread a lot of lower doses
without really controlling it. And so
that typically is measured by sort of a
gradient index and then you have a conformity
index that that quantifies both of these parameters.
And so
that's something that we actively monitor, in our
in in in all our treatment plans, and
we make sure that, we have very tight
prescription
and dose fall offs for all our treatment
plans.
Right. And and the tight dose fall off
is to stop, the radiation getting to the
normal tissue in the brain?
Correct. And and and I think in part
of the reason is is typically,
when you have these lesions that are kind
of in in the brain,
the only other,
normal tissue is the brain that's surrounding
it. When it's normally
adjacent to the brain stem, optic eyes and
optic nerve,
you do have constraints there and you and
and you do plan to ensure that you
meet those radiation dose constraints.
But when you don't have something,
adjacent to it, in reality, you just have
to make sure that you have the fastest
fall off. You know, typically, the concern, of
course, with normal brain is is brain necrosis,
and we know there's correlations between increased volumes
of, you know, various dose levels, leads to
higher rates of necrosis.
And so you really wanna ensure that you
can, minimize the amount of normal brain tissue
that's being rated.
Okay. And then in terms of ease of
use, how does the Gamma Knife compare to
other technologies?
You know, I I I think, you know,
one of the key aspects in terms of
of of any type of, dedicated platform is,
they tend to approach it with an integrated
solution. And so,
what you end up seeing is that you
have a dedicated,
you know, software package that follows a workflow
of how you wanna do the deliveries.
You have an integrated treatment planning system that
supports the way in which you deliver,
and then you have, you know, QA devices
and or this delivery system,
that's able to facilitate its overall delivery. So
you don't have much of a mix and
match, and so the system is sort of
built for a purpose. And I think that's
one of the key things in terms of
the overall workflow
of of the system is that it allows
for it to be,
you know, very efficient.
And I think for us, that's key, you
know, given the the the idea that we
talked about is ensuring that we can, you
know, treat patients,
rather quickly,
once we know that there's a diagnosis.
On top of that, I I would say,
you know, in terms of the ease of
use on the physics side of things, there's,
although there's a QA program,
associated with it, it is very simple to
use. I mean, I think,
given its longevity and years of history, the
overall performance is well characterized for a Gamma
Knife unit.
And, Electa was
does provide
a very efficient QA tools to be able
to quickly test key parameters
such as, you know, cone beam CTS eccentricity,
radiation focus point,
in congruence with imaging isocenter.
And in reality, as you mentioned, we we
don't expect to see fluctuations in dose rate
because it is a, you know, a radioactive
source that decays with time.
At the same time, as I mentioned before,
given sort of the the minimal amount of
moving parts,
you don't expect there to be significant wear
with a lot of these these components. And
so,
for us, as we've shown, it's been very
stable. Although we have a very robust quality
assurance program with it, it is very efficient
in terms of the metrics that we measure,
to be able to make sure that the
the system is consistent over time.
And why is it so important that, the
radiosurgery
is both easy to perform and efficient?
No. I I I think, you know, part
of it is time. You know, every everyone's
time is, is important.
And so,
in an ever evolving platform within radiotherapy where,
I would say on the physics side physicist
side of things, procedures are becoming more complicated
and more involved.
Obviously, anytime that you can make things more
efficient, and maintain quality,
I think no one would ever be be
against that. And I think from our end,
that's why it's a very similar case for
for radiosurgery. So,
we wanna make sure that, we can facilitate
the overall efficiency, And I think we see
that at least with the Gamma Knife platform
using lightning,
and so we're able to be very efficient
in acquiring MRs for our patients, in in
time.
We can generate plans very quickly. We can
ensure that the machine,
is QA'd and ready to go with treatment,
and then we could feel confident with the
quality of the treatment that we're delivering,
you know, over time. So, I think for
us, those are some of the factors that
are that are very important.
And at the same time, it does save,
you know, what I would say,
it it ensures that, you know, people are
more focused on the key aspects of the
treatment delivery versus
smaller processes that just take more time.
So so what are the key success factors
when you're integrating a gamma knife into a
radiotherapy
department?
You know, I I would say for for
MCI's experience, I mean, I I think we
have sort of a a a unique approach
to it. I I think one, it comes
with,
in general,
you know, understanding that the machine is a
component of the radiosurgery program and it's not
the radiosurgery program.
And so for us, you know, we took
a very programmatic approach where we ensured a
very multi multidisciplinary
approach with racial oncologists, medical physicists, neurosurgeons,
neuro oncologists to make sure that we developed
a program that, you know, had key pillars
in terms of overall quality, make sure that
the that the
that the machine was being utilized to its
full potential and it was appropriate in terms
of the treatment diseases that we what we
gave,
make sure that, we, you know, ensure that
specific quality metrics for all treatment plans were
insured, you know,
conformity, dose fall off,
and we ensure that we deliver appropriate doses,
in terms of, the different disease types that
we were, you know, going to treat.
At the same time, you know, this is
not sort of a one and done. You
know, we do have,
peer meetings and peer review.
And so, on a weekly basis, we present
all the cases that are, you know, going
to go up to gamma knife for for
gamma knife treatment to make sure that they're
appropriate. We also discuss location, immobilization
type, not just within physicists or radiation oncologist,
but also as a multidisciplinary
care team, to make sure that, you know,
we we're looking at it as a programmatic
approach and not just because we have a
device that can treat radiosurgery. We wanna treat
everything with radiosurgery. So I I think for
us, those have been very important success factors,
in terms of how we've, you know, what
we've done.
It's also important for us to understand how
well we're doing so ensuring that we have
good follow-up,
and and understanding our local control of the
diseases that we treat. And so for us,
we've had a fair number of publications
looking at how accurately we're we're targeting, looking
at how well our our patients are doing.
And so I I think all of those
are just, you know, a handful of of
of key,
you know, factors that, if you really wanna
ensure a very
successful program that you take into account.
And just like with anything, you wanna make
sure you plan for it,
and, you know, you have goals in mind
in terms of what you would wanna accomplish,
and then you just follow-up on that. So,
I I think I would attribute all those
factors to to the success we've had so
far.
Excellent. And then the final question,
both now and looking into the future,
are there any disease types that could be
treated using the gamma knife that perhaps weren't
treatable before?
And I I will say, I mean, as
as as I mentioned earlier in in the,
in the podcast, I I I do think
there's a large number of indications in which
radiosurgery has has has has proven and shown
itself. I think for us, one of the
key things that, we've been able to do
is, you know, there's been sort of a
movement sort of for preoperative surgery. So, obviously,
for cases that have larger, you know, lesions
and are gonna go to resection to be
able to remove them or to be able
to, you know, provide an irradiation or do
a preoperative surgery before the removal, and there's
a lot of advantages. And I think one
of the key aspects of, you know, the
Gamma Knife platform is the ability to do
that. Right? Because it is very efficient,
has a very streamlined workflow,
and you can, you know, turn around and
read a surgery plan in a day.
And, you know, there are other platforms that
you could do it, but when you do
four or five patients, the efficiency is very
important. And so being able to do many
patients very quickly,
has been something that historically has been beneficial
to us. So I would say that if
we look at the diseases that we, likely
wouldn't be treating with other platforms, it would
be these preoperative surgery cases. And, you know,
there are, you know, there is growing data
to to ensure that, you know, that there's
benefits associated with preoperative surgery.
Obviously, one of them being that volumes are
smaller and smaller. You don't you're not worried
about
potential seeding
after the fact
and not knowing what the disease is. And
so for us, this is an area that
we have
to have done here and have a growing
need for it and, have been successful in
terms of being able to accomplish it in
a very efficient time time period.
Excellent. Right. Well, thank you very much. Thanks
for your time speaking to us today.
Oh, my pleasure. And, thank you very much
for for doing this podcast, and and and
great to be able to share this information
to to all the users out there.
That was Alonso Gutierrez
in conversation with Physics World's Tammy Freeman.
I'm afraid that's all the time we have
for this week's podcast.
Thanks to Alonso and Tammy for a fascinating
discussion,
And thanks to our producer,
Fred Isles.
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