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. 2011 Dec;261(3):999. doi: 10.1148/radiol.11111055

CTDIvol, DLP, and Effective Dose Are Excellent Measures for Use in CT Quality Improvement

Rebecca Smith-Bindman *, Diana L Miglioretti †
PMCID: PMC6940005  PMID: 22096003

Editor

McCollough and colleagues, in their recent editorial in the May 2011 issue of Radiology (1), have dismissed volume computed tomography (CT) dose index (CTDIvol) and dose-length product (DLP) as useful measures of patient radiation dose, arguing that they do not measure the dose the patient absorbs. However, these indexes quantify the radiation dose to which a patient is exposed and thus dictate the dose absorbed by the patient. Although absorbed doses vary by patient size, they are primarily determined by the doses that come out of the machine and the region imaged. These types of readily available and controllable measures are critically needed to understand and improve the safety of imaging. Although these measures may vary by as much as twofold across patient size for the same type of examination to get images of similar quality, we found 10- to 100-fold differences in DLP for CT scans obtained for the same clinical indication among thousands of examinations we have reviewed, reflecting far more variation than could possibly occur owing to patient size. In fact, after accounting for patient weight and body mass index, a profound—and unacceptable—variation in these measures remained. Most of the variation in dose is due to variation in the adoption of multiphase protocols, larger scanning regions, or higher dose settings without awareness of the resulting dose burden these choices create. Thus, without even considering patient weight, we could greatly improve how we are conducting CT simply by assessing CTDIvol and DLP.

The authors also dismiss the use of effective dose (1), which can be calculated from DLP by using age- and sex-specific conversion factors, or more complicated methods that take into account patient size, arguing that it is too imprecise. Effective dose is a useful measure for identifying patients who receive unnecessarily high doses, tracking doses over time, and assessing facility performance and is easy to understand because it puts doses from scans of different body regions on an equitable scale. Furthermore, it is a useful measure for epidemiologic studies, where standard statistical methods exist for analyzing imprecise variables and can account for effect modification according to patient size, sex, and region imaged.

No measurement we use in medicine or research is perfect. The important question is whether a particular measurement is useful given its limitations. In this case, the answer is a resounding yes. CTDIvol, DLP, and effective dose are excellent measures of radiation dose from CT and could be used immediately to improve the safety of CT by identifying when doses may be higher than necessary and standardizing how we conduct CT examinations.

Footnotes

Disclosures of Potential Conflicts of Interest: R.S.B. No potential conflicts of interest to disclose. D.L.M. Financial activities related to the present article: institution received grants from the National Cancer Institute; institution has a grant or a grant is pending from the National Institutes of Health. Financial activities not related to the present article: none to disclose. Other relationships: none to disclose.

Reference

  • 1.McCollough CH, Leng S, Yu L, Cody DD, Boone JM, McNitt-Gray MF. CT dose index and patient dose: they are not the same thing. Radiology 2011;259(2):311–316. [DOI] [PMC free article] [PubMed] [Google Scholar]
Radiology. 2011 Dec;261(3):999–1000. doi: 10.1148/radiol.11111055a

Response

Cynthia H McCollough *, Shuai Leng *, Lifeng Yu *, Dianna D Cody †, John M Boone ‡, Michael F McNitt-Gray §

We respectfully disagree with the assertion by Drs Smith-Bindman and Miglioretti that we have dismissed CTDIvol and DLP as useful tools; they are incredibly valuable measures of scanner output and the first-line tools for dose management in CT (1). They are not, however, measures of patient dose. The quantities used to describe amounts of radiation are all precisely defined in international standards (2–4), as are quantities used to reflect risk (3,5). In CT, these are all related to scanner output and, hence, correlated with CTDIvol. However, each is fundamentally different. The use of precisely defined quantities in a manner that is inconsistent with their meaning, and without accounting for factors affecting their values, can lead to misleading conclusions.

For example, in a recent safety audit, we observed a factor of 8 difference in CTDIvol (5–40 mGy) between patients having the same examination, a single-phase abdomen and pelvis CT examination, for similar clinical indications. On the surface this would appear to be an inappropriate difference in “dose.” However, these patients were significantly different in size (5 feet 8 inches tall male patient weighing 110 lb vs 4 feet 10 inches tall female patient weighing 177 lb), and examination of the CT radiographs and cross-sectional images demonstrated that the attenuation through the pelvis was much greater in the second patient, justifying at least a factor of 8 increase in scanner output.

Image Gently (6,7) and the U.S. Food and Drug Administration (8) correctly stress that patient size must always be considered when prescribing scanner output. Between a neonate and obese patient, at least a factor of 50 in scanner output (CTDIvol) is required to obtain comparable image quality. In addition, differences in patient height between babies and adults lead to even greater differences in DLP. Finally, differences in patient history and examination indication can often explain why one patient received two scan phases and another only one scan phase.

Although we agree that reviews of protocols and patient cases may reveal systematic overuse of multiphase protocols, larger scanning regions, or higher-dose settings, we disagree that this can be done by simply reviewing CTDI and DLP values, which do not account for patient size and clinical factors. It is essential to record and account for patient and examination variables to determine if observed variations in dose indexes are or are not appropriate. CTDIvol and DLP are essential parts of that process, but of themselves are insufficient metrics.

With respect to effective dose, however, it has no place in quality management processes. The International Commission on Radiation Protection, which developed the concept of effective dose and defines the manner in which it is calculated (3,5,9), states unequivocally that it is not to be calculated for any individual patient, nor is it to be used for individual risk estimation or for epidemiologic purposes. Effective dose was developed for occupational radiation safety, and its values are derived from factors that average risk over all ages and both sexes. Its application in the field of medicine is tenuous at best (10). We all want to use CT effectively and safely for the benefit of patient care. But oversimplifying complex issues and metrics does not serve the best interest of the patient. Only by understanding precisely what these quantities mean and how they are to be used can we ensure that patient safety decisions are made based on accurate data.

Disclosures of Potential Conflicts of Interest: C.H.M. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: institution has a grant or grants are pending from Siemens Healthcare. Other relationships: none to disclose. S.L. No potential conflicts of interest to disclose. L.Y. No potential conflicts of interest to disclose. D.D.C. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: received money for expert testimony; received payment for lectures including service on speakers bureaus from MTMI. Other relationships: none to disclose. J.M.B. No potential conflicts of interest to disclose. M.F.M.G. Financial activities related to the present article: none to disclose. Financial activities not related to the present article: none to disclose. Other relationships: the UCLA Department of Radiology has a master research agreement with Siemens Medical Solutions.

References

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