Benefits of diagnostic radiation outweigh the dangers

You may not realize it, but your life is radioactive. If you’ve eaten a banana lately, potassium-40 atoms in your body are shooting out thousands of particles every second. And if you’re anywhere near solid ground, you can assume that radon gas is pelting you with gamma rays.

With X-rays at the dental office, radon in the basement and cosmic rays beaming down from space, we’re literally surrounded by radioactivity and high-powered radiation. Invisible, exotic and — as nuclear weapons have so clearly demonstrated — potentially lethal, radiation seems perfectly suited to inspire fear, says Jerrold Bushberg, clinical professor of radiology and radiation oncology at the UC Davis School of Medicine. “Just about every monster on Saturday morning cartoons was created by mutations and radiation,” he says.

Recent reports of patients accidently receiving much larger-than-normal doses of radiation from cancer treatments or CT scans have only added to widespread concerns about radiation.

But, in fact, in low doses — the kind that most of us will be exposed to in our lives — the risks of radiation are meager compared with lots of other, more mundane hazards in life, says William Morgan, director of radiation biology and biophysics at the Pacific Northwest National Laboratory in Richland, Wash.

“There’s always some risk with radiation,” he says. “Everything in life has some risk. But when you get down to low doses of radiation, the risks are very, very, very small.”

Unlike the stable, unexciting atoms that make up most of the universe, radioactive atoms release either particles or rays of energy in fast, powerful bursts. In a similar way, X-ray machines shoot electron beams from a heated filament. Unlike the relatively weak microwave radiation that comes from cellphones or even the UV radiation in sunlight, this so-called ionizing radiation packs enough punch to break apart atoms, making it highly useful and potentially dangerous, at least in high doses.

Roughly 80% of the ionizing radiation most people get in their lives comes from radon gas, cosmic rays and other natural sources. (Yes, including bananas.) But barring a major nuclear attack or accident, almost all of us will receive our biggest one-time dose of radiation in a doctor’s office or hospital.

CT scans — tests that use a focused beam of X-rays to provide a sharp cross-section view of soft tissues — are a particularly powerful source of radiation. Roughly speaking, a single CT scan to the chest exposes a person to about 10 millisieverts (mSv) of radiation. For the sake of comparison, the typical person gets about that much radiation from natural sources every three years.

CT scans do have the power to cause cancer, at least in rare cases. It’s generally estimated that 1 out of 2,000 or 2,500 young adults who has a CT scan of the chest or abdomen will eventually develop cancer as a result of the scan, says Cynthia McCollough, a professor of radiologic physics at the Mayo Clinic in Rochester, Minn. Put another way, about 1,999 out of 2,000 patients will not have a problem. The risk is thought to be somewhat higher for children. Because of these dangers, McCollough urges doctors and patients, especially young patients, to avoid unnecessary CT scans.

Note that she emphasizes unnecessary scans: According to McCollough, worries about radiation are turning many people away from potentially lifesaving diagnoses and treatments. “People conflate X-rays with atomic bombs, and they’re scared,” she says.

Compared with CT scans, the radiation doses in other common scans are small. The dose from a spinal X-ray is about 1.5 mSv, a mammogram is about 0.7 mSv and a full- mouth dental X-ray will give you about .03 to 0.2 mSv.

At these low doses, the risks of radiation are especially controversial. John Leslie Redpath, director of radiation biology at UC Irvine, says that, in his view, “anything below 1 mSv is probably safe.” As for mammograms and dental X-rays, Redpath says, “radiation-induced cancers from such procedures would be extremely rare and could well be nonexistent.”

But in 2006, the National Academy of Sciences estimated in its “Biological Effects of Ionizing Radiation VII” report that giving annual mammograms to 100,000 women ages 40 to 80 could cause 20 to 25 cases of fatal breast cancers. This death toll is probably dwarfed by the number of lives saved by mammography. A study of more than 1 million Swedish women published online in the journal Cancer in September found that mammograms for women in their 40s cut the death rate from breast cancer by 29%.

A much-publicized study released in May suggests that dental X-rays may double the risk of thyroid cancer. Researchers compared dental X-ray histories of 313 thyroid cancer patients in Kuwait with a matched group of people who didn’t have cancer. The researchers didn’t have access to dental records, so they had to rely on their subjects’ memories. As the scientists noted, thyroid cancer is still rare despite the widespread use of dental X-rays. The National Cancer Institute estimates that fewer than 1% of Americans born today will develop thyroid cancer in their lifetime.

The American Cancer Society recommends that patients receive a CT scan or X-ray only “if there’s a good medical reason to do so.” CT scans, for instance, can often provide much clearer pictures of cancers and organs than any other type of scan, including radiation-free ultrasounds and MRIs. The ACS says that patients are likely to get more benefit than harm from recommended scans and X-rays.

The Food and Drug Administration urges consumers not to refuse any recommended scans or X-rays over fears of radiation. At the same time, the agency says, patients shouldn’t insist on scans that a doctor doesn’t think are necessary. According to FDA, dental patients who are concerned about radiation should find a dentist who uses high-speed X-ray films that use less radiation than slower films.

While such precautions make sense, it’s impossible to know the real toll from CT scans and X-rays, says John Boone, professor and vice chair of research radiology at UC Davis. “We don’t have a body count,” he says. If a person who had a scan comes down with cancer decades later, no one can tell if the scan had anything to do with it, he explains.

To estimate the risk of CT scans, scientists have sought help from a surprising group: survivors of the nuclear attacks on Nagasaki and Hiroshima, Japan. Clearly, there’s a big difference between a blast zone and a hospital radiology department. But tracking cancer rates in A-bomb survivors does provide a real-world glimpse at the long-term effects of radiation.

Studies found that, though the atomic bombs did raise cancer rates, they didn’t exactly unleash an epidemic of cancers among survivors in Japan. According to Brooks, the cancer risk in survivors is only a few percentage points higher than in similar populations who weren’t anywhere near the bomb.

It takes a significant leap to connect A-bombs to CT scans. Researchers generally assume that there’s a one-to-one relationship between the amount of radiation exposure and cancer risk. In other words, it’s assumed that if a patient getting a CT scan receives 10 times less radiation than a group of A-bomb survivors at the fringe of the blast zone, then the risk of cancer is 10 times smaller. This is the logic that led to the estimate that a CT scan will cause cancer once in every 2,000 to 2,500 cases.

But not all experts believe that atomic blasts really tell us much about the dangers of low-dose radiation. “The body responds very differently to low doses of radiation than high doses,” says Tony Brooks, professor emeritus of radiation biology and biophysics at Washington State University-Tri-Cities in Richland.

Redpath believes that comparing CT scans and X-rays to bombs greatly exaggerates the risk of the medical tests. Although it’s impossible to prove one way or another, he believes that doctors would have to scan far more than 2,500 patients to cause a single case of cancer.

Whatever the true risk of radiation, there’s no doubt that fear alone is putting people in harm’s way. McCollough recalls an 84-year-old man with an aneurism in his chest who recently refused a CT scan because he was afraid it could cause cancer. Since it often takes 20 or 30 years for a radiation-related cancer to show itself, McCollough feels that the man made an error of judgment. “It breaks my heart. I see patients crying [out of fear], and there’s nothing I can say to them.”

health@latimes.com