Advances may enable on-the-spot prostate cancer
treatment
trio
of innovations may enable physicians to plan prostate cancer patients’
treatment in real time and to implant cancer-killing radiation “seeds”
more accurately and efficiently. Directionally emitting radioactive
sources, a device for placing needles and seeds, and a super-fast treatment-planning
method were developed by Professor Douglass
Henderson and Associate Professor Bruce
Thomadsen. Together, this suite of inventions could mean on-the-spot
treatment reoptimization—the holy grail of prostate cancer seed
placement.
To eradicate diseased tissue, physicians implant
up to 100 radioactive seeds (like those pictured above) in the prostate.
Like a tiny grain of rice, each seed is cylindrically shaped and emits
radiation in all directions—increasing its likelihood of zapping
healthy tissue, too.
So, borrowing a concept from nuclear materials
handling, Henderson and Thomadsen designed directional seeds, or sources
with vertical shielding along one side. “I think nobody’s
done it before because they look at these sources, which are only eight-tenths
of a millimeter in outer diameter, and they say there isn’t enough
space to put shielding,” says Thomadsen. “We found you can
compress things and you can do it.”
As a result, they can implant seeds, particularly
at the boundaries between healthy and diseased tissue, that steer radiation
where it’s needed most.
With graduate student Liong Lin, the two developed
prototypes and conducted successful radiation simulations. Now they
are working with a leading brachytherapy products manufacturer to develop
experimental prototypes. To keep the seeds from rotating once they’re
implanted, the group also hopes to modify its design to incorporate
a wedge-shaped anchor along one vertical side. “It only has to
hold the source about three days, and after that time, tissues start
sticking to it,” says Thomadsen.
Implanting the seeds accurately is no small
feat. With a hole-studded grid mounted over the patient as a guide,
physicians use a hollow needle to insert the seeds manually. They rely
on real-time ultrasound images of the prostate to ensure proper seed
location and depth.
But both the confines of the grid and the ultrasound
itself limit the process, meaning that the radioactive seeds may not
make it to the correct locations, says Thomadsen. “Because the
needles are constrained to only be in those half-centimeter-by-half-centimeter
holes and only parallel to the ultrasound probe, when you do the treatment
plan, you can see very obviously that you don’t get a real optimal
plan—you’d want to put seeds where you can’t because
there are no holes,” he says. “And sometimes when you’re
doing the implant, you can’t get the needles where you want to
go because the pelvic bones are in the way,” he says.
So he and his graduate students abandoned the
grid and built a robot that could deliver seeds more precisely than
a physician could by hand. “There’s an additional impetus
that came along when we started working on the directional sources,”
says Thomadsen. “In order to get sources in the patient in the
right orientation, it would be very hard for a physician to get the
angles precisely.”
Graduate student Michael Meltsner built a prototype
robot and has perfected it by programming it to implant seeds into oranges.
“It’s a really basic prototype, and he’s at the point
where we have to test to make sure that, in the simple form we have,
it’s going to perform exactly how we want,” says Thomadsen.
By next year, when the system is complete, it
will provide countless angles for inserting seeds and will enable physicians
to properly orient seeds that contain shielding.
To plan the seed placement for maximum effectiveness,
physicians currently map an ultrasound view of the prostate on a 3-D
grid, use optimization software to calculate several sets of possible
seed locations, and determine which configuration will work best. But
current optimization methods are iterative methods—that is, they
calculate a solution, make a change, calculate a new solution, make
a change, and so on.
Inspired by a reactor physics technique called
adjoint, or “backward” transport, Henderson, Thomadsen and
their graduate students developed a method that could reduce the time
of this treatment-planning step from as long as 40 minutes to just a
few seconds. “The adjoint function plays a big role in the selection
of the seed position,” says Henderson.
Considering how both tissue and tumor in the
region of interest will react to radiation delivered by one seed, a
“greedy” algorithm optimization software chooses the best
location for the first seed. Based on that choice, the software evaluates
the best location for the second seed, then the third, and so on. In
addition, the technique can combine more than one type of radioactive
isotope and will specify seed angle and position based on whether it
contains shielding.
Together, the suite of advances could represent
the holy grail of seed implants: live-time reoptimization, says Thomadsen.
“A plan would tell you where to put the seeds,” he says.
“And each time you put in a seed, it would recalculate where to
put the next seed based on where you actually put the first one.”
For patients, he says, that level of interactivity
means less hassle and more peace of mind. “The patient wouldn’t
have to come in days early for a pre-scan,” he says. “They
could just come in for the procedure. Everything could happen right
then, in live time.”
The group received funding for its projects
from the Department of Energy Nuclear Engineering Education Research
program, the UW-Madison Graduate School, Best Medical International,
and the Wisconsin Alumni Research Foundation (WARF). Thomadsen, Henderson
and colleagues are patenting the innovations through WARF.
