Answer to Question #12685 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
My four-year-old son just had a head computerized tomography (CT) scan at a pediatric hospital. The dose length product (DLP) was 345.4 mGy cm and the computer tomography dose index (CTDI) was 20.1 mGy. While the scanning technician used the lowest dose possible, I am concerned about the cancer-causing effects of the exam. It is my understanding that mSv is the figure that correlates to increased probability of having CT-induced cancer. I would like your help to understand what mSv is, if you can compute it based on the information I provided, and the incremental probability of my son being diagnosed with cancer over the course of his life due to the CT. Many thanks in advance.
You asked for an explanation of mSv. The mSv is a unit of effective dose, one of many types of radiation doses that we use. Effective dose is calculated based on doses to individual organs and the risk factors for each organ. Explaining by example, the dose to the brain is multiplied by the brain's risk factor and the dose to the bone marrow is multiplied by the bone marrow's risk factor, etc. Then, for all the exposed organs and tissues of the body, these are added together to get the effective dose. So, the effective dose is a measure of overall cancer risk.
Yes, the effective dose in mSv can be estimated from the DLP. Using the values you supplied and the methods described in American Association of Physicists in Medicine Report No. 96, The Measurement, Reporting, and Management of Radiation Dose in CT, the effective dose is around 1.4 mSv.
Also from the AAPM website, "At the present time, epidemiological evidence supporting increased cancer incidence or mortality from radiation doses below 100 mSv is inconclusive. As diagnostic imaging doses are typically much lower than 100 mSv, when such exposures are medically appropriate, the anticipated benefits to the patient are highly likely to outweigh any small potential risks." The diagnostic imaging procedures referred to by the AAPM include CT scans.
The Health Physics Society (HPS) position statement Radiation Risk in Perspective states, "Substantial and convincing scientific data show evidence of health effects following high-dose exposures. However, below levels of about 100 mSv above background from all sources combined, the observed radiation effects in people are not statistically different from zero." In other words, the risks, if they exist, are too small to be seen.
Kent Lambert, CHP, FHPS