In very rare cases, using radiation to kill the primary tumor of a patient with metastatic cancer leads to the disappearance of tumors throughout the body.

Scientists can't explain this amazing collateral effect, but it seems to activate an antitumor immune response.

Mohan Doss, a medical physicist at Fox Chase Cancer Center, believes the distant tumors melt away because of incidental low-dose rays emanating from the high-dose therapy. And that bolsters a theory he has researched for years: radiation at or slightly above natural background levels can stimulate the body's disease-fighting defenses.

"When you have high-dose radiation, it suppresses the immune system," he said. "Low doses actually enhance the immune system."

That paradoxical concept, dubbed "radiation hormesis," was considered heretical in 1980 when biochemist Thomas D. Luckey wrote a book promoting it. Today, the theory remains controversial enough that Doss noted he was not speaking on behalf of his employer, but it has a growing number of adherents. Eight years ago, the International Dose-Response Society was founded to study the effects of low-dose exposures to radiation, as well as to chemicals and toxins.

In the public mind, ionizing radiation is mostly a frightening force, and with good reason.

Radiation produces invisible energetic particles that, at high intensities, bombard cells and damage DNA, leading to cancer, radiation poisoning, and, at worst, death. That destructive energy can be harnessed to destroy tumor cells, but even then, it can simultaneously do subtler damage to healthy cells, giving rise to another cancer years later.

At low levels, the effects of radiation are more speculative. For one thing, the risks are estimated by drawing on studies of atomic-bomb survivors and workers exposed to medical or nuclear radiation. For another, radiation exposure has a cumulative effect, and experts estimate we get the equivalent of 30 chest X-rays every year just from inescapable background levels in the United States. Finally, smoking and other lifestyle choices can compound the effects of radiation.

Nonetheless, radiation safety standards are based on the scientific consensus that any dose, no matter how small, is potentially harmful - and the harm increases proportionally with the exposure.

In 2006, that consensus was reaffirmed in a report sponsored by federal agencies including the Nuclear Regulatory Commission.

"There is no threshold below which low levels of ionizing radiation can be demonstrated to be harmless or beneficial," Harvard epidemiologist Richard R. Monson, an author of the report, said in a news release.

Many experts say this no-threshold model ignores our natural biological defenses.

"There is substantial scientific evidence that this [no-threshold] model is an oversimplification," says the Health Physics Society, composed of radiation safety professionals.

Radiation hormesis research goes farther, suggesting low exposures are not merely harmless, but beneficial. By turning on protective processes that would not otherwise be active, mild radiation might ward off cancer as well as other diseases of aging such as Alzheimer's.

Many of the studies were done in cell cultures or on animals. A 2009 study found many fewer lethal genetic mutations in the offspring of mildly irradiated male flies than nonirradiated ones.

In humans, much cited - and debated - data come from Taiwan, where apartment complexes were built in the early 1980s using steel that was later discovered to contain radioactive contamination. Several studies have found abnormalities in the DNA or blood cells of the inhabitants. But a 2004 study found cancer death rates there were 20 times lower than expected.

Doss recently analyzed updated data on atomic bomb survivors. When he plotted their cancer rates and radiation dosages on a graph, there was a blip that did not fit with the no-threshold model of radiation risk; survivors with low exposures had unexpectedly low cancer rates, Doss wrote in the International Dose-Response Society's journal.

David J. Brenner, a Columbia University radiation biophysics researcher, found fault with Doss' analysis.

"As the doses decrease, there are more and more statistical uncertainties in the data, so it becomes pretty easy to cherry-pick data and suggest it's consistent with some particular model of radiation response," Brenner said.

Brenner said recent large studies of cancer risks in children who underwent CT scans "show increased risks that are pretty consistent with the risks estimated from the A-bomb data."

Settling the debate won't be easy. Hormesis adherents have proposed testing the theory in pilot studies of cancer patients, or at least revising radiation safety regulations to increase the dosage deemed safe. So far, the mainstream has not been swayed.

Doss is not deterred. He is about to publish results of a study conducted with Temple University neurologists on rats with a Parkinson's-like disease. The scientists hypothesized that low-dose radiation would boost levels of antioxidants in the rats' brains and relieve disease symptoms.

"We have promising results," he said.

Contact Marie McCullough at 215-854-2720 or mmccullough@phillynews.com.