Radiation Doses and Risks

 

<<  Introduction

 

The World Health Organization officially classifies x-rays as a known carcinogen. The potential risks of ionizing radiation in high doses causing cancer is well known based on studies of Japanese WWII atomic bomb survivors. The potential risks of medical radiation directly causing cancer is much less established and have been extrapolated from the Japanese survivor studies.

 

A great limitation of this approach is that the atomic bomb survivors were exposed to not only x-rays, but also gamma rays and particulate radiation. To date there has been no study showing a direct relationship of medical x-ray radiation and cancer. Of concern, however, is that Japanese survivors subsequently developing cancer had been exposed to doses as low as 50 mSv (millisieverts).

 

Patients receiving a single CT of the abdomen and pelvis commonly receive doses between 10 and 25 mSv, and some trauma patients receive doses in excess of 50 mSv.

 

Unfortunately, there is no proven way to measure the exact dose that a given patient receives. The x-ray output from the scanner is easy to measure but the actual absorbed patient dose, which is determined by patient size, amount of body fat, and many other patient parameters, is difficult to determine. Currently we can only estimate the "effective absorbed dose" in mSv by multiplying the radiation output, known as the DLP,* by a coefficient, which is based on an idealized 140 lb. patient. As a result, the reported doses are mildly exaggerated in average patients and greatly exaggerated in obese patients. An obese patient receiving a body CT may have a DLP double that of an average weight patient but the actual absorbed dose may only be increased 20 percent to 30 percent. The reported mSv however, would indicate 100 percent increase in "absorbed" dose. Nevertheless, at this point in time, this is the only accepted way to determine patient exposure.

Radiation is part of our natural environment. We are exposed to radiation from many naturally occurring sources, including from minerals, food, and outer space.

 

Exposure is measured in sieverts (Sv) and millisieverts (mSv) as a way to evaluate the biological effects of ionizing radiation. The average dose per person from all sources is about 6.20 mSv per year. (American Nuclear Society)

 

Average radiation exposures

Yearly background in U.S.   3 mSv / year
Airline pilots   2 mSv / year
Evacuees of 1986 Chernobyl disaster   33 mSv / year
Living within 10 miles of Three Mile Island   0.8 mSv / year
Maximum allowed radiation for radiation workers   50 mSv / year
Chest x-ray   0.1 mSv / year
Evacuees of 1986 Chernobyl disaster   33 mSv / year

 

CT scan average doses

Chest

  6-10 mSv / year

Abdomen / Pelvis

  14 mSv / year

Head

  2 mSv / year

 


In the future, we anticipate that more-accurate measurements for patient absorbed dose will become available, and that cumulative doses will become easier to track. Currently there is limited commercial availability of products that can collate patient doses.

 

 

Cancer Risk

Actual cancer risk from medical radiation is only an estimate.  No long term studies have been conducted; it is estimated that an accurate study would require following 1 million patients for 30 years.

 

Since many patients receive frequent CT scans, the doses can add up quickly. The currently accepted theory that cancer risk increases directly with increased exposure. Risk is also thought to be inversely proportional to age, with children at the highest risk. A common risk assessment suggests that 10 mSv of exposure increases lifetime cancer risk by 0.05 percent in a 50 year old and 0.2 percent in a 5 year old.

 

A 2010 report in the New England Journal of Medicine suggested that up to two percent of malignancies might be the direct result of medical radiation exposure. A recent article in the Canadian Medical Association Journal suggested that the cancer risk is much greater than previously reported.

 

The baseline cancer risk for an American male is 44.9 percent;  female 37.5 percent. Thus, the cancer risk for a male after receiving a PE CT is 44.947 percent. Additional cancer risk from ionizing radiation is estimated to be 0.047 percent per 10 mSv exposure; 1 percent per 250 mSv.

 

See www.xrayrisk.com for information for patients and healthcare providers about the risks from radiation exposure from medical imaging.

 

 

Next: Dose Reduction Strategies >>

 

 

* http://www.medscape.com/viewarticle/572551_2
Two related measures of CT radiation dose are typically used: the CT dose index (CTDI) and the dose length product (DLP). The CTDI represents the radiation dose of a single CT slice.