Answer to Question #15440 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Pediatric Issues

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

Q

When my daughter was 4 ½ years old, she had a computed tomography (CT) of her temporal bone. I agreed but received little information from her doctor or radiologist on radiation exposure. I called recently to get the dose, and they said she received 55.75 mGy and the report shows DLP 420 with a note that says, "pediatric reductions used." On the phone, a radiologist from the facility said, "Ooh, I usually turn it down" when reading the CTDI mGy which has caused me great distress the past year. My question is, should I be concerned about the amount of radiation for this single exposure for my daughter and approximately how many mSv is this?

A

I want to begin by reassuring you that you do not need to worry about the amount of radiation your daughter received with this CT scan. It isn't enough dose to cause harm.

The dose metrics used for CT scans can be confusing. The machine indicates the CT Dose Index to a volume (CTDIvol) in mGy and the Dose Length Product (DLP) in mGy-cm. Neither of those is the 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.

We can use the DLP value to estimate the effective dose in mSv because it includes the dose and a measure of the amount of the body that was exposed. Effective dose also depends on the sensitivity of the part of the body that was imaged and is used to compare the cancer risk of different exams. None of these dose metrics, CTDIvol, DLP or effective dose, are the dose to any individual patient and they should not be used to calculate cancer risk for an individual. They are based on machine outputs and population averages for risks.

The CTDIvol value of 55.75 mGy is higher than many protocols used today, but it is not completely out of line. The CTDIvol is lower than is typically used in adult protocols.   

For a temporal bone CT scan in a 4 ½ year old, a DLP of 420 mGy?cm results in an effective dose of approximately 1.7 mSv. For populations that have received effective doses below 100 mSv, there is no statistical difference in risk compared to populations who have received no radiation exposures other than natural background. The Health Physics Society Fact Sheet on Radiation Exposure from Medical Exams and Procedures has additional information you may find helpful.

With an estimated effective dose of 1.7 mSv, you do not need to worry about an increased risk of cancer. Please keep in mind that there was a very real benefit to your daughter from the CT scan. It provided important information to her doctor so she could receive appropriate care.

Deirdre H. Elder, MS, CHP, CMLSO

Answer posted on 12 September 2024. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.