Giving cancer the needle

A 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 fast treatment-planning method were developed by UW-Madisonengineering physics professor Douglass Henderson and medical physics associate professor Bruce Thomadsen.

Together, this suite of inventions could mean on the spot treatment reoptimisation, which is said to be the holy grail of prostate cancer seed placement.

To eradicate diseased tissue, physicians implant up to 100 radioactive seeds in the prostate. Like a tiny grain of rice, each seed is cylindrically shaped and emits radiation in all directions-increasing its likelihood of damaging healthy tissue, too.

Borrowing a concept from nuclear materials handling, Henderson and Thomadsen designed directional seeds-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 millimetre 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 company to develop experimental prototypes. To keep the seeds from rotating once they’re implanted, the group also hopes to modify their 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 a challenge. 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. 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 3D grid, use optimisation software to calculate several sets of possible seed locations, and determine which configuration will work best. But current optimisation methods are iterative methods in that 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.

Ultimately, these advances could represent a major leap in precise prostate cancer treatment, 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 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.”