Every one of us is exposed to radiation simply by virtue of living. The Environmental Protection Agency says that, like a steady drizzle of rain, we're constantly showered by radiation from space, and the closer we get to space, the more we're exposed. That's why exposure increases when we fly and why those who live at higher elevations are more exposed. And for those of us with cancer, we voluntarily submit to radiation from some medical imaging devices such as CTs and PETs. How can we evaluate the risk we are taking for secondary cancers as a result of this additional exposure?

To help answer that question, I turned to Dr. Ella Kazerooni, professor of radiology and the director of the Division of Cardiothoracic Radiology at the University of Michigan.

Dr. Kazerooni begins by explaining that "most of what we know about the risk of cancer for diagnostic radiation comes from an extrapolation of what happened with the atomic bomb in Japan," referring to a study that "everybody cites as an explanation about radiation and radiation risk."

The study indicated that the part of that population which received radiation doses from the bomb similar to that in CTs had a higher incidence of cancer than those who had no exposure, leading the researchers to conclude that the risk of CT imaging was the same.

But that methodology is controversial. "There's a huge debate," says Dr. Kazerooni, "amongst the scientific community as to whether that's valid," and she explains why. "It's not as if we took 10,000 people who got a CT when they were 2 years old and 10,000 who didn't and waited 50 years to see who got cancer and who didn't to see if there's a difference. It would take randomizing people who did and didn't get a CT and they would have to live exactly the same way for the rest of their lives and then be compared 50 years later to see if there's a difference."

Given various lifestyles, it would be almost impossible to conduct a study that definitively points to any single reason for getting cancer.

And so I ask Dr. Kazerooni to explain the factors that influence risk. Age is an obvious factor since younger people have a longer time to live, during which their cumulative risk would increase.

The second factor, says Dr. Kazerooni, is the intensity of the radiation exposure, and I ask why it varies. "It depends on the question you're trying to ask or answer," she says, "so depending on what doctors are trying to see, more or less X-rays are needed and the amount of radiation varies."

She elaborates, "Different tissues in the body are more and less susceptible to being hit by X-rays. If you think about the lungs, they are kind of a big bag of air, and X-rays go right through them, so we don't need many X-rays to take a picture of a lung. But if you're trying to do a CT scan of the brain, which is inside that really thick skull bone, you need more X-rays to get through the skull just to be able to see the brain."

Another recent study caused alarm when it showed that the amount of radiation varied as much as 13-fold among different institutions. At Michigan, Dr. Kazerooni says, "We follow the principal of ALARA — which is 'as low as reasonably achievable' when we set up our protocols, but the question being asked will impact the type of protocol we use."

All this means that it's impossible to quantify the risk for any one individual. There are simply too many variables. And for us survivors, that can be frustrating. After undergoing horrible treatments to get rid of cancer — some of which are known to cause secondary cancers down the road — how can we possibly understand the risk we are taking when we undergo these tests? The answer is that we'll never know exactly, but understanding some basics about radiation exposure can help us weigh their risks and benefits.

The biological risk of radiation exposure is measured in millisieverts (mSv), but who, outside of the scientific community, understands that? To help put it in perspective, we are naturally exposed to between two to five mSv per year, depending on where we live. Depending on altitude, latitude and cosmic activity, 100 hours of flying equals about 1mSv of exposure. And the average CT of the abdomen and pelvis is approximately 15 mSv.

But where's the tipping point for risk? No one knows exactly, and, again, there are too many unknown variables to say with certainty. What we do know is that the National Cancer Institute says that "people exposed to radiation at their jobs are monitored and limited to an effective dose of 100 millisieverts (mSv) every 5 years (an average of 20 mSv per year, with a maximum of 50 mSv in any single year)."

However, patients who undergo medical imaging procedures are not monitored or limited, and since the amount of radiation varies from patient to patient, from test to test and from machine to machine, there's no way of knowing exactly how much we're getting. We can, though, use a chart which approximates exposure for various imaging procedures. It was developed for patients by the Radiological Society of North America and the American College of Radiology.

And what about the radiation exposure in PET scans? Reporting agencies simply say that it is a "small" amount, but what is "small?"  Dr. Kazerooni explains that the radiation exposure from a CT scan is limited to the area of the body that is being scanned whereas the whole body is exposed during a PET scan because the radiation comes from a "tracer" which is injected and travels throughout the entire body. This makes it impossible to compare exposure from a CT to that of a PET, and reporting of radiation exposure is different for PETs, but Dr. Kazerooni says that the exposure, "In a pure PET scan, when people do throw out a number, is about 5 to 7 millisieverts." That, at least, gives us an idea.

So what, I ask Dr. Kazerooni, should patients know about the cumulative risk of CT scans? "It's a very complex question," she answers. "I think the most important thing is to understand the downstream risk from the CT exposure versus the benefit that can be gained from having the test."

It also helps, she says, to understand that "if you take all the cancers that occur in the United States — of every body part — an individual's risk of cancer is 42 percent over his or her lifetime." She adds, "If your baseline risk of cancer is 42 percent, and we're doing a CT scan to find out if somebody has cancer — and they do — it's diagnosed earlier than if we didn't have this technique at all, and hopefully earlier diagnosis would improve survival."

But what if we undergo several scans? Dr. Kazerooni answers, "If you are a cancer patient and you're undergoing multiple CTs, the risk of your having a problem from the cancer you already have is worse than the risk of getting another cancer 20 to 40 years later because you had the CT scans."

Can we diminish our risk? "I think it's reasonable," Dr. Kazerooni remarks, "for anybody whose physician has indicated they should undergo medical imaging to ask their physician how important is it that they undergo this test."

She adds, "Some people just go get a test when their doctor says they're supposed to, but I think patients and physicians could do this in a more participatory manner." For example, she suggests that patients ask the following questions: "What are the benefits of the test and are there any downsides to it? What are you looking for? How is this going to help? Is it really going to answer the questions that we have about what might be going on with me? Is it really necessary?"

Dr. Kazerooni also mentions that — depending on the type of cancer someone had — if they've been cancer-free for five years and have been getting scanned for five years, it's reasonable to ask physicians, "Given that I've had no evidence of cancer for the last five years, do you think I still need to continue to get an annual CT?"  She adds, "It may be that the type of cancer somebody had is known to come back, but it may be the type that's unlikely ever to come back so maybe they don't need the annual CT. And those are personal discussions that people should have with their physicians." 

We can also ask, when we have scans, if the ALARA principle is followed and for the effective dose in millisieverts that we'll be getting. (Watch the surprised look on the technician's face when you ask!)  We can then keep a record of exposure and discuss cumulative doses with our doctors. This information can be particularly helpful years after treatment when we're talking with our doctors about followup scans. And it's all part of taking an active role in our care, as Dr. Kazerooni suggests.

The bottom line? There is no definitive answer about the risk for any given individual, but Dr. Kazerooni offers this advice: "People should not be afraid of the radiation exposure all by itself, given what the information found can do to help save someone's life or help reduce the burden of an illness that they may be facing."

Put another way, experts agree that the risk associated with these tests, when used wisely and considered individually, is small compared to the benefit of diagnostic information gained.

Next Friday, March 4: Scanxiety

Previous installments of Candid Cancer are archived here. Betsy de Parry is the author of The Roller Coaster Chronicles, a book about her experience with cancer and the shorter, serialized version she wrote for annarbor.com. Find her on Facebook or email her.