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The Murky World of Effective Dose for Cardiovascular CT

We thank Drs. Castellano and Nicol for their interest in our paper (1) and their many insightful comments, which focus specifically on the radiation dose for patients undergoing cardiovascular computed tomography (CT). The central themes of their letter are the challenges involved in estimating the effective dose from the dose-length product (DLP), the present lack of and need for diagnostic reference levels (DRLs) for cardiovascular CT, and the great variability in radiation doses from coronary CT angiography (CTA). We share all of these concerns.

The thoracic conversion factors of 0.017 and 0.014 mSv · mGy⁻¹ · cm⁻¹, which are routinely used in published reports, are indeed inadequate for estimating an effective dose from CTA, resulting in substantial underestimation. As we have previously pointed out (2), these values were derived from Monte Carlo simulations of ancient single-slice scanners incapable of performing CTA and using an older definition of effective dose. Figure 1 in the letter by Drs. Castellano and Nicol is a wonderful illustration of why the effective dose from coronary CTA should be higher than that from a chest CT with the same DLP. In any event, just because radiation dose is difficult to estimate does not mean that it should not be openly discussed with patients, especially because it is a public concern. We are currently working on a more accurate approach for estimation of effective dose from DLP for coronary CTA for virtually all contemporary scanners.

One of the goals of our paper was to stimulate awareness of the need for DRLs in all cardiovascular procedures involving ionizing radiation, of which coronary CTA is an important example. This is an area in which the cardiology community has fallen behind other fields (e.g., pediatric radiology). DRLs for individual examinations are often framed in terms of the 75th percentile of the distribution of patient doses in a particular population. However, with such high between-center variability, as pointed out by Drs. Castellano and Nicol, careful thought is needed for the most constructive approach to developing such reference levels. If a population-based 75th percentile threshold were used uniformly, most patients would be flagged as exceeding the DRL in some laboratories; consequently, the concept of DRL could be rendered of little use for patient radiation protection.

Another important implication of the great variability in radiation doses is that, in real-world practice, doses may not be nearly as low as those reported by experts using the latest equipment in selected patient populations. Thus, to paraphrase Mark Twain, rumors of the demise of the “radiation problem” in cardiac imaging have been greatly exaggerated. The UK laboratories’ median coronary CTA DLP range of 200 to 1000 mGycm suggests that, using a cardiac-specific conversion factor, the median coronary CTA effective dose at these sites ranged from ∼5 to 30 mSv. The existence of such dose levels in contemporary practice underscores both the need for continued efforts to reduce doses to patients and the need for patient-centered imaging, shared decision making, and better communication with our patients.

• American College of Cardiology Foundation