Answer to Question #14391 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

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

Q

I just had a neck computed tomography (CT) and a chest CT. I was told the CTDIvol was 18.52 mGy and the total exam DLP was 745.89 mGy-cm. I cried when I saw the numbers. Please help me. I am so worried. Will this increase my risk for cancer-induced disease?

A

I want to begin by reassuring you that you don't need to be worried about the radiation from your neck and chest CT scan. 

The dose metrics for CT scans can be very confusing. For CT scans, the machine indicates the CT Dose Index to a volume (CTDIvol) in milligray (mGy) and the Dose Length Product (DLP) in mGy-centimeters (mGy-cm). Neither of those are patient dose—they are measures of the radiation that was output by the CT scanner. The CTDIvol is a dose value that was measured at a specific location in a phantom (an acrylic cylinder used for CT quality assurance testing) and the DLP is the CTDIvol multiplied by the length of the scan. The CTDIvol is very useful for comparing the radiation output during different types of scans and between different CT scanners. 

The DLP in mGy-cm is more useful for calculating effective dose. However, the effective dose is not the actual dose to any specific patient; it is a useful measure to compare risks from radiation exposures to only part of the body based on averages across the population. The effective dose depends on the radiation sensitivity of the organs and tissues that were exposed to the radiation as well as the dose absorbed in the tissue. 

For the neck CT, I estimate an effective dose of 2.2 millisievert (mSv). For the chest CT, I estimate 5.3 mSv. These are within the normal range for the CT scans. The risk of health effects from doses below 100 mSv are either too low to measure or do not exist.

Deirdre H. Elder, MS, CHP, CMLSO

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