Taking Aim at Tumors, With Sharpshooter Precision

A team of accelerator physicists, radiologists and oncologists are calling for a cancer-killing weapon with sharpshooter precision to be built in the United States, which they say will blast hard-to-treat tumors in the brain, lungs, liver and pancreas while sparing healthy cells.

Approximately 40 percent of cancers are successfully treated by radiotherapy, but conventional X-rays also harm normal tissues on their way to and beyond the tumor site, causing severe side effects.

"For a long time, we've known we can cure a high percentage of patients with big tumors if we can give high dosages [of radiation], but healthy tissues can't tolerate it," said Kathryn Held, a radiation biologist at Harvard Medical School and Massachusetts General Hospital, at the 2014 AAAS Annual Meeting.

Ion particle therapy, using protons or carbon-ion beams instead of X-rays, can deliver high doses of destructive energy to only the tumor because the energy is deposited where the beam stops. Heavier ions such as carbon are especially appealing in this regard due to their unique characteristics.

"Imagine a thin person running through a crowded room," said Ken Peach, a professor at the Particle Therapy Cancer Research Institute at the University of Oxford, referring to protons. "He might collide with one or two people.

"Now imagine a very fat person running through the same room. He'll collide with much more people, much more frequently."

"As a carbon ion passes through a cancer cell, it puts a much bigger hole in the DNA, as opposed to a little pin prick [from a proton]," added Held. "It's harder for the cell to repair, and therefore delivers a better kill."

Carbon ions are also better at attacking hypoxic cells - cancer cells that have been deprived of oxygen and consequently become resistant to radiotherapy.

"The technology is available," said Hak Choy, a radiation oncologist at the University of Texas Southwestern Medical Center. "The biological benefits have been proven in animal models. Can it be used to treat cancer in humans?"

"So far preliminary data look very encouraging," said Choy, who added that the major challenge in answering that question conclusively is the small number of facilities capable of offering ion particle therapy, and the subsequent lack of data.

There are currently 36 facilities offering proton therapy and six offering carbon-ion therapy worldwide. Almost 94,000 patients have been treated with proton therapy and just over 10,000 have been treated with carbon-ion therapy as of 2013.

A handful of proton therapy facilities exist in the United States but none for carbon-ion therapy. Estimated to cost $300 million and powered by a synchrotron, a new carbon-ion facility here could treat approximately 1,000 patients per year and allow researchers to conduct randomized clinical trials that compare the efficacy of proton, carbon and conventional therapies, said Choy.

Building the center is expected to take the next five to 10 years. The National Cancer Institute (NCI) is providing planning grants to establish a research agenda and is considering co-funding trials in existing international centers to accelerate knowledge in the mean time, said James Deye, program director of NCI's Division of Cancer Treatment and Diagnosis.

"This is about delivering cancer care that's aimed at curing the disease, not just treating it," said Peach, who added that the reduction of side effects would also improve quality of life, especially for children and young adults with cancer.

"We know that [particle ion therapy] works, the question is can we make it work better. That's a challenge for the next decade."