Answer to Question #12216 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Computed tomography (CT) scans from 2005–2007 seem to have had very high settings on their dose limits. I read that the dose from a head CT is 60 millisieverts (mSv). Is this correct? Isn't that the level where induced cancer is 100%? Are those people who have this type of scan then doomed?
No one is doomed to radiation-induced cancer from head CT scans performed a decade or more ago. While it is true that today's CT scanners expose patients to less radiation than earlier generation CT scanners, the effective dose from a head CT scan in the past would not have been anywhere near 60 mSv. The Conference of Radiation Control Program Directors (CRCPD) conducted a study of CT scanners in 2000 (NEXT 2000 CT Survey). The average effective dose for head CT scans in 2000 was 1.2 mSv.
And no one is doomed to radiation-induced cancer from an effective dose of 60 mSv. The Health Physics Society's position statement "Radiation Risk in Perspective" states, "Substantial and convincing scientific data show evidence of health effects following high-dose exposures (many multiples of natural background). However, below levels of about 100 millisieverts (mSv) above background from all sources combined, the observed radiation effects in people are not statistically different from zero." In other words, at effective doses under 100 mSv, the risk, if it exists, is too small to be seen.
The volume-weighted computed tomography dose index (CTDIVOL)for a head CT scan was around 60 milligray (mGy) in 2000. But the CTDIVOL is not a measure of risk; it is actually a dose measured in an acrylic phantom. Effective dose is a better risk metric.
Kent Lambert, CHP, FHPS