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Medical radiation: Too much of a good thing?

It all started in 1895, when Wilhelm Roentgen discovered x-rays. Six years later, he was honored with the first Nobel Prize in Physics; by then, doctors were already using primitive x-rays to diagnose illnesses. The collaboration between physicians and physicists has continued ever since, resulting in amazing advances far beyond Roentgen’s wildest dreams.

Modern imaging techniques have saved countless lives. But like every medication and operation, these benefits come at a cost, both in dollars and sometimes in health. In the case of imaging, potential problems include both misdiagnosis and over-diagnosis, which often lead to excessive or unnecessary treatment. And additional side effects may result from the very radiation used to produce many types of diagnostic images.

Physicists tell us that ionizing radiation delivers energy in the form of photons that pack enough oomph to strip electrons away from the nuclei of atoms. We are continuously exposed to tiny amounts of ionizing radiation from cosmic rays and naturally occurring radioactive elements here on earth. We accept this so-called background radiation without blinking an eye, but we rightly fear exposure to devastating doses of ionizing radiation from nuclear accidents or atomic weapons. And research has focused attention on potential harm from the radiation used for diagnostic imaging.

The major worry is cancer. Ionizing radiation damages the DNA of every cell it strikes. Most cells can repair mild damage on their own, but if the insult is more severe, some cells may escape from the normal mechanisms that control cell division and growth. Repeated mild hits produce cumulative damage; for example, exposure to 1 millisievert (mSv) a year for 10 years produces the same DNA damage as a single dose of 10 mSv (see table below). Whether the damage occurs from one large dose of radiation or many small doses, the consequence can be unrestrained cell multiplication — cancer.

It’s not a common occurrence, and it shouldn’t scare us away from reaping the enormous benefits of diagnostic imaging. But it should remind patients and doctors alike to use these wonderful techniques with respect and care.

It took a while for doctors to recognize that x-rays can cause cancer. In the early 20th century, some scientists developed cancer as a result of their pioneering experiments with x-rays. Still, even in the mid-20th century, some shoe stores had fluoroscopic machines that allowed customers to use x-rays to see if their shoes fit, and some patients who got therapeutic head and neck radiation for problems as mild as acne and enlarged tonsils went on to develop thyroid cancer. Doctors have learned to minimize the side effects of ordinary x-rays, but in the 21st century, the large amount of radiation used for computed tomography, better known as CT scanning, is prompting a reevaluation of the risks and benefits of diagnostic imaging.

Let’s take a look at some imaging techniques that rely on ionizing radiation.

Overuse of imaging to stage prostate cancer

Prostate cancer will be diagnosed in about 218,000 American men this year. Because most cases are detected by prostate-specific antigen (PSA) screening, many will meet the criteria for low-risk disease (PSA less than 10 nanograms/milliliter, Gleason score 6 or lower, and clinical stage T1c or T2a). The American Urological Association, the American College of Radiology, and other expert groups say that such men do not need imaging studies because very, very few of these cancers have spread from the prostate to tissues like lymph nodes or bones. Even so, two 2011 studies reported that 36% to 48% of men with low-risk prostate cancer undergo one or more unnecessary bone scans, CT scans, or MRIs.

X-rays

Even in the era of CTs and magnetic resonance imaging scans (MRIs), ordinary x-rays are an important tool for diagnosing medical problems ranging from fractures to pneumonia. A stationary tube beams x-rays through the patient’s body. Tissues that are dense, such as bones, stop the x-rays from penetrating through the body, while less dense tissues, such as muscle and fat, allow them to pass through to a sheet of film behind the patient. When the film is developed, the dense tissues appear white, the less dense tissues black or various shades of gray.

New digital techniques that use less radiation have made x-rays clearer and safer than ever. Digital x-rays can be viewed and stored electronically, eliminating bulky film, and doctors can send the images to physicians anywhere in the world in an instant. The new scanning techniques (more on these below) share these advantages.

Digital technology, though, can’t overcome the intrinsic limitation of x-rays: they can’t produce images of tissues that are not dense. Healthy lungs, for example, appear uniformly black because they are filled with air, but if lungs fill up with fluid (heart failure) or pus (pneumonia), the abnormal area looks white on the film or screen. By using radiodense contrast materials, doctors can obtain images of tissues that otherwise allow x-rays to pass right through: examples include angiography for blood vessels, barium swallows and enemas for the gastrointestinal tract, and intravenous pyelography for the kidneys and urinary tract (but not the prostate). Although these techniques have improved diagnosis and therapy, they deliver more ionizing radiation than plain x-rays (see table).

Magnetic resonance imaging

Instead of relying on radiation, MRIs use the body’s natural magnetic properties to produce detailed images of any part of the body. Because hydrogen is so abundant in the body, especially in water and fat, it is used as the target. Like the planet earth, the hydrogen nucleus — just a single proton particle — rotates on its axis. Also like the earth, every hydrogen proton behaves like a bar magnet, with north and south poles. Under normal conditions, the magnetic poles are randomly aligned and don’t generate enough energy to produce images. But when the body is placed in a strong magnetic field, the proton “magnets” line up along an axis, like so many parallel compass needles. If the protons are then hit with a short burst of precisely tuned radio waves, they will momentarily turn around. Then, in the process of returning to their original orientation, they emit a brief radio signal of their own.

To obtain an MRI, a technician places the patient in a long tube that produces an intense magnetic field and generates pulses of radio waves. The tissues’ hydrogen protons resonate, emitting radio signals that are captured by detectors and processed by a computer into detailed images. By alternating the sequence of pulses, doctors can use MRIs to obtain images of tissues anywhere in the body. And by using a contrast agent (gadolinium), doctors can further enhance the MRI. Even so, some organs still manage to hide their secrets from MRIs. The prostate is an example, but new endorectal coil MRIs, in which the pulse generator is placed in the rectum right behind the prostate, may (or may not) change that.

MRIs sound scary, but since they don’t use radiation, they are safe for all of the body’s tissues. But some MRIs are performed with contrast agents, which can occasionally cause adverse reactions. And even without injections of contrast material, MRIs frighten some patients, who develop claustrophobia when they are alone in a room and confined to a narrow, noisy tube for up to 30 minutes. Sedatives can help, and new open scanners are less threatening (if a bit less accurate). Patients who have metal in their bodies — such as pacemakers, inner ear implants, clips on brain aneurysms, some artificial joints, and shrapnel — cannot be given MRIs (though they can have CT scans). MRIs are very expensive.

A leap forward: CT scanning

Computed tomography (CT scanning, sometimes called computer-assisted tomography or CAT scanning) burst on the scene in the early 1970s, altering the landscape forever. New techniques are making CTs better than ever, but they often deliver larger amounts of radiation as they produce images of superior quality.

CTs use x-rays to image the body, but their relationship to ordinary x-rays is something like the link between the space shuttle and the Spirit of St. Louis. In a first-generation CT scan (the axial CT), the patient lies on a table that moves through a doughnut-shaped tube containing the x-ray generator. The generator rotates around the patient, beaming x-rays through his body. X-rays that pass through are collected by detectors that channel the electrical signals into computers, where they are reconstituted into detailed images of the body.

In first-generation CTs, the table moves the patient for a short distance, and then stops while an image is obtained. The process is repeated until the scan is completed. But in second-generation scans, spiral (or helical) CTs, the process is a bit different: the patient holds his breath and the table moves him through the scanner without stopping, while the tube rotates around him continuously in a spiral fashion. The computer compensates for the effects of motion, constructing more detailed images than were formerly possible. Spiral CTs don’t expose the patient to any more radiation than ordinary CTs, and they are also much faster.

Spiral CTs can provide remarkably detailed images of the nervous system, chest, and abdomen. In the urinary tract, for example, they have replaced older methods of looking for kidney stones. Unfortunately, they are not good at picturing the prostate, though they can detect prostate cancers that have spread to lymph nodes in the abdomen.

Even as many hospitals are proudly deploying their shiny new spiral scanners, other centers are shunting them aside to install third-generation multislice CTs. First- and second-generation CTs use a single row of detectors to pick up the x-rays that pass through the patient; multislice scanners have many rows, along with improved computer software. As a result, they are remarkably fast, able to scan a patient’s entire chest, abdomen, and pelvis while he holds his breath for just a few seconds. By focusing on a slightly smaller area of the body, doctors can use the same few seconds to obtain much finer slices that provide remarkably detailed images of the body in 1-millimeter segments.

Spiral CT scan

illustration of spiral CT scan

The x-ray generator rotates around the patient in a spiral fashion.

Radiation, CTs, and cancer

The use of CTs has increased dramatically in the U.S. About 80 million scans are performed annually, and the number is increasing by about 10% a year. Although an enormous number of patients have benefited from their CTs, about one-third of these exams are medically unnecessary. In all, CTs now account for approximately two-thirds of medical radiation in the United States.

CTs contribute a substantial burden to the escalating cost of health care in America. They also account for a small but important number of cancer cases. The risk depends on how much radiation patients receive and on their age and gender. Since cancer often takes years to develop, children and young adults are most vulnerable. And because the female breast is at risk, women face a larger worry than men, but the risk for men is still appreciable. For example, a CT coronary angiogram using the standard technique is estimated to cause one case of cancer for every 1,007 exams in 40-year-old men; the risk for 60-year-olds is one in 1,241, and it falls to one in 3,261 for 80-year-olds. Although the individual risks are small, the huge number of CTs explains why they already account for almost 2% of all cancers in the U.S. Looked at another way, scientists expect that the CTs performed in 2007 alone will cause about 29,000 malignancies and 15,000 deaths over the coming years.

Ultrasound

Ultrasound is a painless, entirely safe, relatively fast and inexpensive way to picture the body. Compared to CTs, MRIs, and PET scans, ultrasound may seem like kid stuff. But in this case, at least, appearances are deceiving. For some organs, like the heart and the prostate, ultrasound provides valuable information that CTs and MRIs cannot. For other problems, like gallstones, ultrasounds are preferred to CTs because they can provide as much (or more) information with less expense and hassle. And interventional radiologists often use ultrasound to guide therapies for many disorders.

Instead of using radiation, ultrasounds (also known as sonograms or echoes) rely on high-frequency sound waves that are beamed into the body from a probe. The sound waves that echo back from the target organ are captured and processed by a computer, then projected onto a video screen and preserved in digital images.

In most cases, the ultrasound probe is positioned on the skin, but it can be placed in a body cavity to get a closer look. For example, a probe can be put on the chest wall to picture the heart, but for more detailed and accurate images, it can be passed down the esophagus (“food pipe”) and positioned just behind the heart.

Doctors began using ultrasound to examine the prostate several decades ago, but they didn’t get very far until they learned to place the probe in the rectum, right behind the gland. Even in the CT-MRI era, transrectal ultrasonography (TRUS) is the best way to picture the prostate. It’s still not good enough to detect small prostate cancers, but it does allow doctors to diagnose the disease with biopsies and to treat it with radioactive seeds.

Until something better comes along, doctors will go on echoing the praises of ultrasound.

PET scans and nuclear imaging

Medical imaging began with x-rays and took a great leap forward with CTs. In between came nuclear scans; newer imaging techniques have replaced many of these, but nuclear heart scans remain important and popular, and positron emission tomography (PET) scanning is among the newest of the new. Some 20 million nuclear scans are performed in the U.S. annually, contributing an important amount of medical radiation.

PET scanning has more in common with older nuclear imaging techniques than with CTs and MRIs. Nuclear medicine produces images by administering tiny doses of radioactive isotopes that are tagged to chemicals taken up by specific body tissues. The chemical emits radiation, which is picked up by a gamma camera that produces images of the tissue. For example, the bone scan is an old standby that remains the best way for doctors to detect prostate cancers that have spread to bones, and the ProstaScint is a newer and more controversial nuclear scan that is designed to detect prostate cancer in lymph nodes. Other important examples include nuclear imaging stress tests and SPECT scans for the heart.

PET scanning depends on the fact that malignant cells have faster metabolisms than normal cells; it’s why cancer grows so rapidly and why chemotherapy can destroy cancer cells without producing as much damage to normal cells. Patients who undergo PET scans receive injections of radioactive fluorodeoxyglucose (FDG), which is metabolized just like glucose (sugar). Malignant cells need more sugar to fuel their souped-up metabolism, so they take up more FDG, then emit radiation that is detected by the PET scan. Because normal tissues take up some FDG, they can sometimes light up on the scan, producing a false positive result. It’s particularly true of the white blood cells that enter tissues to fight infections; for now, that’s a problem for doctors hunting for cancer, but it may turn into an asset if occult infections become the target.

Although PET scans are new, doctors have already demonstrated that they can detect tiny lung cancers, both when they are confined to the lung (when surgery may be curative) and when they have spread to the lymph nodes (and cannot be cured surgically). PET scanning also can help in other malignancies, including head and neck cancers, gastrointestinal cancers, and lymphomas. And in the latest advance, PET and CT scans can be integrated, offering the potential advantages of each in a single session, though with the disadvantage of increased radiation exposure. To date, however, PET scanning has been a disappointment in detecting prostate cancer, in part because most prostate cancer cells grow — and metabolize glucose — so slowly.

Because PET scanning is new, it is not available at all hospitals, and it is very expensive. Above all, although it is a powerful and promising tool, doctors have still not learned when and how to use it to best advantage. For example, more research is needed to compare PET scans with multislice CTs in detecting early lung cancer — and to learn which patients might benefit from screening with either technique.

Radiation dose from medical imaging

Test or procedure

Typical radiation dose (mSv)*

Background radiation from natural environmental sources over one year

3

DXA scan for bone density

0.001

Plain x-rays

Dental

0.005

Chest

0.02

Hip

0.8

Mammogram

0.4

Lumbar spine (low back)

1.6

X-rays with dye

IVP (urinary tract)

3.0

Coronary angiogram

2–29

CTs

Sinuses

0.6

Head

2.0

Spine

6.0

Chest

7.0

Abdomen

8.0

Cardiac (64-slice)

7–23

Nuclear scans

Thallium stress test

41

PET scan

14

Bone scan

6.3

MRIs

0

Ultrasounds

0

*mSv = millisieverts, which reflect the biological impact of ionizing radiation. Radiation doses can vary substantially for the same exam, depending on equipment and technique.

Reducing risk

X-rays, CTs, and nuclear scans are critical components of modern medical care. They are here to stay, but there are simple ways to retain their enormous benefits while reducing the risks of medical radiation.

Part of the progress will depend on physicists; as they strive to develop even better imaging techniques, scientists should work on ways to reduce the radiation used to make their remarkable images.

Device manufacturers and radiologists can also help. In 2009, the FDA received reports of at least 50 patients who received up to eight times the expected dose of radiation during brain CTs. Even without such errors, there can be an astounding 13-fold difference in the amount of radiation patients undergoing a brain CT receive. Manufacturers should improve the efficiency and safety of imaging equipment, and radiologists should use the lowest radiation dose and obtain the smallest number of images that will provide the necessary diagnostic information. For example, techniques that are already available can reduce the radiation dose of CT cardiac angiographies by 53% without sacrificing quality. Similarly, nuclear stress tests that use thallium pack three times more radiation than scans using technetium. And while spiral CTs have become the gold standard for diagnosing kidney stones, they require a big dose of radiation. New techniques can reduce that jolt from the usual range of 6.5 to 17 mSv to just 0.5 to 4.5 mSv without any loss of diagnostic information.

Regulatory agencies can also play an important role in reducing radiation exposure. The FDA is already taking the lead in establishing standards and accreditation criteria for advanced-imaging facilities.

While all these measures are important, maximum protection will depend on steps taken by patients and doctors together. One way is to simply keep track of each patient’s cumulative radiation exposure. But the most important step is to use imaging techniques judiciously and wisely. Here are some steps that can help:

Where appropriate, use clinical criteria to see if x-rays are likely to improve care. For example, many patients would be spared unnecessary X-rays for ankle or knee pain if their doctors applied clinical guidelines known as the Ottawa Ankle and Knee Rules before ordering x-rays. Similarly, patients with back pain, sinusitis, or headaches are only likely to benefit from medical radiation if they have certain “red flag” warning symptoms, which are uncommon.

Avoid studies that are not likely to affect treatment and outcome. X-rays for suspected rib fractures are one example; the treatment for this type of pain is the same whether or not a fracture is demonstrated.

When appropriate, select diagnostic techniques that do not rely on ionizing radiation. Ultrasounds and MRIs are examples (see “Ultrasound”), and they sometimes provide even better information than CTs. In particular, when it comes to picturing the prostate, ultrasound is David to CT’s Goliath.

Guard against seductive tests of unproven merit, particularly when they are promoted as screening tests for an apparently healthy individual. Two examples are instructive: CTs that use either the electron beam (EBCT) or multidetector (MDCT) methods can detect tiny deposits of calcium in coronary arteries. Because they don’t require injections or contrast material, they are considered “noninvasive” tests and are offered to the public “just to see” if coronary artery disease is present. The temptation is understandable, but neither the American Heart Association nor the American College of Cardiology recommends routine screening, but the so-called Screening for Heart Attack Prevention and Education (SHAPE) plan argues for regular screening of asymptomatic men 45 to 75 years of age. The radiation dose from a single coronary artery calcium CT ranges from 0.8 to 10.5 mSv; if 100,000 men had scans every five years between the ages of 45 and 75, these scans would cause 14 to 200 cases of cancer depending on the radiation dose involved. That’s not a huge number, but if the SHAPE recommendations were followed by tens of millions of American men, the numbers would soar.

A growing number of facilities are advertising CTs to screen for cancer. Here, too, the appeal is obvious. In particular, a 2011 study reported that three consecutive annual spiral CTs could reduce a smoker’s chance of dying from lung cancer by about 20%. This important and hopeful study has been widely publicized, but its limitations have received less attention; although experts do not yet recommend widespread CT screening, even for smokers, many people have been tested, usually at their own expense. Despite concerns that CT screening for cancer may do more harm than good, fear-driven testing shows few signs of slowing.

Scanners for security

In our frightening new world, x-rays and CTs have become as important for airports as for hospitals. Hand luggage is put through x-ray scanners; because one pass exposes your items to only a tiny fraction of the energy used in a chest x-ray, these scanners are safe for photographic film — and for the security personnel who use them all day long. But checked baggage is passed through CT scanners that are far more powerful; they will damage your film, but not your medications.

The new full-body scanners have raised concerns about privacy and safety. The privacy issue is personal, but the safety question is a scientific matter — and scientists agree the scanners are safe. Two types are in use. Millimeter wave scanners use radio waves to generate images, and they don’t expose travelers to any ionizing radiation. Backscatter scanners do use low-intensity x-rays, but they bounce off the skin without penetrating the body. They deliver only a tiny amount of radiation, about the same amount as you get in three to nine minutes of daily living, or about 1/1,000 as much as an ordinary chest x-ray. A person would need to have 2,500 to 5,000 backscatter scans a year to reach the Nuclear Regulatory Commission’s annual safety limit.

Perspectives

CTs, nuclear scans, and x-rays have revolutionized patient care. Medical imaging is a good thing — in fact, a very, very good thing. But like all medical interventions, imaging can have unintended consequences and side effects. In particular, the cumulative risk of radiation is often overlooked in the face of understandable enthusiasm for “noninvasive” testing. That’s one reason the average lifetime dose of diagnostic radiation has increased sevenfold since 1980. We can’t put the genie back in the bottle, nor should we. But we should employ all medical technologies wisely, especially when they involve risks. New studies call attention to risks from ionizing radiation, and it’s up to all of us to use that information wisely.

Posted by: Dr.Health

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