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UC Davis Health System

UC Davis Health System

Fighting tumors — and saving sight — with charged protons

Photo of patient Michael Bone during an eye examination
Patient Michael Bone during a follow-up examination with Susanna Park, associate professor of ophthalmology.

On Sunday, April 2, 2006, Michael J. Bone, an athletic Sacramento-area veterinarian, noticed flashes of light in his right eye as he went about his weekend activities. Reading the newspaper Monday morning, he spotted a crescent-shaped shadow in the central vision of the same eye. On Tuesday evening, the shadow was still there. Bone's wife called the doctor the following morning.

By Thursday afternoon, following a battery of tests, Bone had a diagnosis. The avid skier, fly fisherman and father of three had choroidal melanoma, a cancer of the pigmented layer of the eye that lies behind the retina. The tumor was about one-third of an inch in diameter and just under a quarter-inch deep.

For decades, surgical removal of the eye, known as enucleation, was the only treatment for ocular melanoma. Enucleation remains a standard treatment for the disease today, but Bone also had an alternative choice: UC Davis is one of six centers nationwide that can treat the lesions with proton-beam therapy.

For Bone, the choice was clear. “Number one, proton-beam therapy has a higher success rate,” he said. “Number two, I keep my eye. Three, because the radiation is so controlled, I don't have to worry about collateral damage to healthy eye tissue, and there's very little risk of vision loss.”

Protons vs. gamma rays

Conventional radiation therapy kills cancer cells using gamma radiation. However, it has limitations as a treatment for tumors near delicate structures like the optic nerve or retina: Gamma rays deliver energy to all the tissues they travel through, from the point they enter the body until they leave it.

Proton beams, in contrast, drop almost all of their energy on their target. Dosing is so exact that tissue just one-tenth of an inch from the target receives almost no radiation. Because damage to healthy tissue is minimized, doctors can treat cancers with higher — and therefore more effective — radiation doses.

Since 1994, the cyclotron housed in the Crocker Nuclear Laboratory on the UC Davis campus has generated protons to treat about 800 eye tumors in patients from as far away as New Zealand. Tumors have diminished or disappeared in more than 95 percent of cases, with better long-term survival rates than those seen in patients who have their eyes removed. In addition, most proton patients retain useful vision in the treated eye.

Collaboration with UCSF

The first step in Bone's treatment was a 75-minute operation at UC Davis Medical Center, in which Susanna Park, an associate professor of ophthalmology and vision sciences, stitched four tantalum rings to the back of his sclera, the tough white outer coating of the eye, along the perimeter of the tumor. The rings, which don't require removal and remain in place, show up in bright relief on X-ray, serving as clear guideposts for the proton-beam procedure.

Step two, a few weeks later, was a trip to UCSF, where experienced technicians fitted Bone with the custom components he would need during the treatment: a bite block and partial face mask, to help immobilize his head, and a propeller with graduated blades to modulate the proton beam to the precise depth of his tumor. Additional advance work took place at the Davis cyclotron lab, where machinists fashioned a hole the shape of Bone's tumor in a brass fitting known as a collimator; the collimator molds the beam to hit just the tumor and a small safety margin of tissue bordering it.

UCSF has offered proton-beam therapy for ocular melanomas at the Crocker Nuclear Lab since 1994. When radiation oncologists and physicists at UC Davis Cancer Center take over the jobs now being done in San Francisco, proton-beam patients will get all of their care in Sacramento and Davis.

"We're trying to make proton-beam therapy more accessible to patients in the Central Valley and inland Northern California," said Park, who trained in proton-beam therapy as a resident and retina fellow at Harvard University/Massachusetts Eye & Ear Infirmary.

At the same time, researchers at UC Davis Cancer Center and Lawrence Livermore National Laboratory are at work on a new-generation proton-beam machine that promises to make the therapy more available worldwide. The machine, powerful enough to treat cancer anywhere in the body, will be tested at UC Davis Cancer Center.

Instructed to stare

Bone's first proton-beam treatment took place on Tuesday, June 6. That afternoon, he drove himself to the Crocker Nuclear Laboratory from his Fair Oaks veterinary practice. He took a seat in the straight-backed metal chair in the lab's 8- by 12-foot treatment room, tilted his head back for an anesthetic eye drop, then leaned into the rigid face mask and bit down on the bite block. A belt securing his forehead to the back of the chair was cinched tight; clips were attached to his eyelids to prevent blinking. Bone was instructed to stare fixedly at a small red light on the proton therapy apparatus. A camera, focused on his eye, stared back. Any movement of Bone's head would be relayed immediately to a monitor on the other side of the door.

After assuring themselves that Bone was in proper position, the medical team — a radiation oncologist, physicist and two technicians — cleared the room and took up positions at a control center around the corner.

Photo of patient Michael Bone back to his beloved fly fishing
Just days after his final proton-beam treatment, Michael Bone was back to fly fishing.

Medical physicist Inder Daftari activated the beam. Exactly two minutes later Daftari turned it off.

Talking about the procedure immediately afterward, Bone said it was easier than he thought it would be.

“I just thank God I have these great doctors, great machines, great people to take care of me,” he said.

Over the next three days, he returned to the lab for three more two-minute treatments. Five weeks later, Bone was back at the UC Davis Health System Eye Center for a follow-up appointment with Parks. He had no vision loss. The tumor's growth had been arrested. And he had better than a 90-percent chance for a full cure.

His family and friends are as relieved as he is. Bone's best friend and fishing buddy on first hearing about the melanoma, had quipped: “Well … just so I don't have to tie on your flies for you.”

A few days after his final proton-beam treatment, Bone was waist-deep in the American River, casting flies. He tied them on just fine. He didn't even need to squint.