Answer to Question #12032 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

At my hospital in Spain, we are creating a new low-dose protocol for computed tomography (CT) for patients with illnesses such us multiple myeloma. To keep the protocol easy to perform, studies are obtained in only one acquisition that includes head and neck, chest, abdomen, pelvis, and part of the limbs and legs. We want to estimate effective dose for the whole study, but we found that we only know the global dose-length product (DLP). Conversion factors are different for different anatomic areas, and we don't know DLP for each area. Do you know any way to estimate DLP by anatomic regions?

A

Unfortunately, there is no method to estimate individual, anatomy-specific DLPs from the whole-body, single-scan DLP. In this situation, I would recommend the following two approaches:

  1. The first approach is to use anthropomorphic phantoms, such as those from Computerized Imaging Reference Systems (CIRS), Inc., in Norfolk, Virginia, and the Alderson Radiation Therapy (ART) phantom and its earlier version, the Alderson RANDO phantom. Scan each anatomic region (i.e., head and neck, chest, abdomen, and trunk) of a phantom using your low-dose scan protocol to obtain the DLP. To determine effective dose (ED), use the conversion factor for each region, as found in American Association of Physicists in Medicine (AAPM) Report No. 96. Sum each ED from different regions to obtain the total ED. If you don't have anthropomorphic phantoms in your imaging department, consult your radiation therapy department; they usually have an anthropomorphic phantom.
  2. The second approach is to use commercial software, such as CT Expo, to simulate your protocol. The price of the software is very reasonable. If you are using an automatic tube current modulation routine in your CT scanner, you may need to find the scan parameter for each anatomical region and obtain the ED for each anatomical region, as CT Expo may not perform tube current modulation (check the latest version.)

Terry Yoshizumi, PhD, FAAPM
Professor of Radiology/Radiation Oncology/Medical Physics (Health Physics)

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