Future of Proton Beam Therapy Shines Among Tough Cancers

Research from Japan documenting remarkable survival rates among patients with inoperable lung cancer may only hint at the potential of proton beam therapy.

“Protocols are being developed not just for lung cancer but [also] for cancers of the esophagus and pancreas, liver tumors and gynecological cancers,” said Dr. Tim R. Williams, chair of the board for the American Society for Radiation Oncology (ASTRO). “It will really be an exciting thing over the next five or 10 years as these protocols come online and patients get treated.”

Data reported this month in ASTRO’s official journal, the International Journal of Radiation Oncology-Biology-Physics, documents high survival rates for 55 patients suffering from stage I inoperable non-small-cell lung cancer (NSCLC). They were treated at the Proton Medical Research Center in Tennoudai, Japan, from November 2001 to July 2008 with different doses. Peripherally and centrally located tumors were irradiated. The two-year progression-free survival rates among these patients were 88.7% and 97%, respectively.

These rates contrast starkly with survival rates for NSCLC patients treated with conventional radiotherapy. Reported survival rates for these patients at five years range from 6% to 31.4%. The difference may be due, at least in part, to the time differences in the available data, two years for proton beam therapy patients and five for conventional therapy. But even for those treated with stereotactic radiosurgery, a highly precise form of radiation therapy, the survival rate is just is 54.7% after two years.

Stereotactic radiosurgery uses focused radiation beams to target a well-defined tumor. It relies on detailed imaging, computerized 3D treatment planning, and precise positioning to deliver doses of radiation with extreme accuracy. Proton beam therapy is similarly precise with the additional advantage that protons deposit most, if not all, their energy in the patient rather than exiting through the body as does the radiation delivered conventionally or by stereotactic radiosurgery.

Williams, who also serves as medical director of radiation oncology at Boca Raton Community Hospital, explains that high-energy protons penetrate to a certain depth in the body, which is calculable and controllable, and then they stop.

“For this reason, there’s always been interest in using them for localized tumors,” Williams said.

Advances in computers have made proton targeting more precise. So has technology that controls the proton beam, turning it on and off to match the rhythm of a patient’s respiration.

“[Using it on] small lung tumors is a perfect application because you can exactly target the tumor and control for respiratory motion, eliminating the tumors without the need for surgery,” he said. “So the results (obtained in Japan) are very exciting. They are a natural reflection of the inherent potential advantages of the particles themselves.”

Proton beam therapy typically focuses on localized tumors because protons are best applied when the tumors are highly defined. “If you have regional spread, then local control obviously is not as relevant,” Williams said.

Reports of success with proton beam therapy have seemed almost anecdotal because of the relatively few sites that offer this type of therapy. Much of the work has focused on cancers of the prostate and brain. But lately, with the rising number of proton beam centers in the U.S. and around the world, other applications are being tried.

“This increasing number of proton beam therapy facilities allows us to explore new levels of dose intensity and capitalize on increased local control of the particle,” Williams said.

More research is needed to compare the relative effectiveness of proton beam and stereotactic radiosurgery, say the Japanese investigators, just in regard to patients with stage I inoperable NSCLC. They are urging a randomized clinical trial to clarify the survival benefit that might be achieved from using proton beam therapy.