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Category: Medical and Dental Patient Issues — Worker Issues

The following question was answered by an expert in the appropriate field:

Q

How can the absorbed dose be calculated using the parameters that are on the console of a computed tomography (CT) unit, e.g., computed tomography dose index volumetric (CTDIvol), pitch, precontrast series, contrast series, and delay series?

A

Generally, the CTDIvol number that appears on the CT scan is a weighted-average absorbed dose to a cylindrical phantom used to perform the calibration measurements. Using a 32-centimeter (cm)-diameter cylindrical phantom, measurements at a depth of 1 cm and at the center of the phantom are made. The CTDIvol estimates the mean tissue dose by adding one-third of the central measured dose to two-thirds of the peripheral measured dose.

The problem, however, is that generally the body cross sections of most people are closer to an oval than a circle and not everyone has a diameter of 32 cm. So to come up with an absorbed dose estimate for an individual, you would need to convert the CTDIvol dose from the cylindrical phantom to an appropriately sized anthropomorphic model. In addition, to generate an effective dose you would need to estimate the mean absorbed dose to each organ within the irradiated volume and then use the International Commission on Radiological Protection (ICRP) organ tissue-weighting factors to convert the absorbed doses into an effective dose estimate.

Consequently, the CTDIvol value listed on the scan is a good "ballpark" absorbed dose estimate for tissues within the x-ray field, but to convert it to an individualized absorbed or effective dose is a lot more complicated. Fortunately, there are many groups that have studied this problem in detail, and they provide various methods to convert the CTDIvol and/or dose-length product (DLP) information into an individualized dose estimate.

One method is available from the ImPACT website. ImPACT has developed an Excel spreadsheet system that uses Monte Carlo phantom data and machine characteristics to determine dose estimates for the various organs from the scan parameters and derives an effective dose estimate.

Also available are online CT dose calculators that use DLP or CTDI data; anthropomorphic phantoms for newborns, one-, five-, and 10-year-olds, and adults; CT study type; and diameter data to derive a size-corrected effective dose estimate. You can find one at radiation-dose.com.

The above cited sites are only examples and are not necessarily endorsed by the Health Physics Society.

Mike Bohan