Answer to Question #15518 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

How do I interpret Size Specific Dose Estimates (SSDE)? I had an abdominal/pelvic computed tomography (CT) scan and was given a SSDE. I have no idea what to make of the combined 40.9 mGy SSDE for the two scans. Does 1 mGy = 1 mSv in this case? The hospital did give me some additional information based on AAPM report 96 and AAPM report 204, which I don’t really understand. If you could provide an explanation, it would be greatly appreciated.

A

Size Specific Dose Estimate (SDE) is an attempt to account for the fact the Computed Tomography Dose Index (CTDI) is only representative of the dose if a person's body was equivalent to the 32 cm phantom that is used for measuring CT radiation dose. Since most individuals are not the same size as the phantom and body composition is not exactly the same as the acrylic that is used in the phantom, an attempt was made to "correct" for the size of the individual and have a CTDI value more closely match the individual's dimensions. The American Association of Physicist in Medicine (AAPM) Report No. 204 provides a table of correction factors to convert CTDI to SSDE which is what was done to convert the DLP from your exam to SSDE.  

A Sievert or millisievert (mSv) is a unit of dose equivalency that accounts for not only the energy absorbed but also the radiosensitivity of the organs and tissues exposed. A Gray or milligray (mGy) is a measure of energy absorbed per unit mass. Though both mGy and mSv are described by the same units (joules per kilogram) they are not the same in the context of CTDI and equivalent dose. For example, if an individual had two CT examinations with the exact same technical factors, e.g., the kilovolt potential (kV), the milliampere-seconds (mAs) and scan length (cm) were set exactly the same, but the areas scanned were different, e.g., one scan of the chest and one of the upper thighs, these two exams would have the same exact CTDI and DLP, however the effective dose would be different due to the fact that different areas of the body were scanned.   

Another problem arises for two different individuals scanned using the same techniques and having the exact same area of their body scanned. The methodology used to convert DLP to effective dose used in AAPM report 96 does not account for individuals with different sizes. Let's assume one individual has an effective diameter that is twice that of the other, but they are both scanned using the exact same technical parameters. In this case the methodology used in AAPM Report 96 to convert DLP to effective dose would result in identical effective doses for these two individuals, even though one person is twice the size of the other. However, the actual effective dose to the individual who is larger is lower than the smaller individual, since radiation dose is a measure of energy absorbed per unit mass and an individual with twice the mass would only have half the absorbed dose.   

A CTDI of 40.9 mGy does not result in an effective dose of 40.9 mSv as CTDI doesn't account for the area of the body scanned. 

AAPM Report 96 does not specifically account for patient size when calculating effective dose from a CT exam. AAPM Report 204 is a way to estimate CTDI for different sized patients (SSDE) but does not provide a methodology to use that information in order to calculate and effective dose estimate.   

A paper published by the Oxford University Press in the Journal of Radiation Protection Dosimetry in Jan 2017 "Body Size-Specific Effective Dose Conversion Coefficients for CT Scans' provides a method to account for patient size. 

I hope this provides some clarity regarding your CT dose. In summary CTDI reported dose is not the same as effective dose and the typical method for calculating effective dose does not take into account patient size specific considerations. The paper cited provides one method to use patient size information to adjust the k factor used to calculate effective dose. Using this method the effective dose from your examination would be estimated to be 24 mSv. 

John Gough, MS, DABHP, DABMP

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