In a room at Northwestern Medicine Chicago Proton Center, Robert Johnson keeps a small collection of plastic heads. At first glance, they look like they’ve been lopped off the top of department store mannequins. But they’re more lifelike than that—made of materials that mimic bone, flesh, and brain. “One of them even has a gold filling,” he says.

For the last six years, Johnson, a physicist at the University of California, Santa Cruz, has been working on a machine that shoots protons through the human skull. His goal: to use protons instead of conventional X-rays to take 3-D images inside cancer patients. But first, he has to perfect the technology on his model skulls.

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His prototype can map the dummy’s head in about six minutes. It can find the gold filling inside the dummy’s mouth. And most importantly, it can recognize a tumor. While his machine isn’t yet good enough to make a diagnosis—X-ray images still have better resolution—that’s not the point. Johnson thinks that a proton-based image, even a blurry one, can guide a cancer treatment known as proton therapy better than a conventional X-ray.

Proton therapy fights cancer by bombarding tumors with, well, protons. But before doctors send in the protons, they have to design a treatment plan based on a 3-D image of the tumor. Right now, these images are CT scans, which see inside a patient with X-rays. From that scan, doctors calculate how much energy the protons need to hit the tumor—a complicated sequence of conversions and estimates to translate an image into a treatment.

That’s where Johnson’s prototype comes in. If you have a proton-based image, you can skip those conversions and design a more precise, more effective treatment plan, Johnson says.

Advocates of proton therapy say that it’s the most advanced form of radiation therapy today. In many ways, it’s safer and more effective than chemotherapy and conventional X-ray-based radiation therapy. Protons don’t really damage healthy tissue, because doctors can target them to release most of their energy at a specific depth inside the patient. “You don’t get any damage beyond the tumor itself,” says Bill Hansen, the director of proton therapy marketing at Varian, a company that makes cancer therapy machines for hospitals. X-rays, on the other hand, damage tissue wherever they go, sometimes causing serious side effects. Breast cancer treatment with X-rays increases the risk of a heart attack, for example, because of the left breast’s proximity to the heart.

Critics of proton therapy, however, say it’s highway robbery. Proton therapy machines are behemoths that require a circular particle accelerator the size of a room and expensive superconducting magnets. All together, they can cost $20 million or more—about 10 times the cost of a conventional X-ray radiation machine. While Medicare does cover proton therapy, some patients have trouble getting insurance companies to cover it because of its cost.

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That’s why researchers like Johnson are tweaking the technology in the hopes of making the therapy more mainstream. Johnson’s prototype was a long time coming—his collaborator, oncologist Reinhard Schulte of Loma Linda University, began working on this prototype all the way in 1998. Back then, the US only had one hospital-based proton therapy machine, installed at Loma Linda in 1990.

Proton therapy has since gotten more affordable, Hansen says. In recent years, companies have cut costs five times by shrinking the machines. Loma Linda’s first proton machine, still in use, accelerates protons around a circular track with a diameter the length of a tennis court. More recent models are almost 10 times smaller. And because protons are more precise, a patient may not have to schedule as many appointments on a proton treatment plan compared to a conventional radiation one.

As prices dropped, demand for the therapy rose a bit. In the US, only two medical centers offered proton therapy in 2003. Now more than 25 do. Because radiation damage of healthy tissue in growing children is especially harmful, doctors often recommend proton therapy to kids with cancer. “It’s now the gold standard for treating children,” Hansen says. But most cancer patients aren’t kids, and the technology still hasn’t really taken off.

Johnson and Schulte’s prototype doesn’t fix the cost barrier—it actually makes proton therapy more expensive. But their hope is that an even more precise proton therapy machine, aided by their proton imaging, will make it more attractive to hospitals. Proton therapy is capable of extremely powerful performance, but no one knows how to make it shine yet. “It’s kind of like driving an airline jet on the ground instead of flying it,” Hansen says. “To a certain extent, it’s a misuse of the technology.” Hard to think of a better use than on mannequin heads, though.