Dose indices: everybody wants a number.

This paper discusses the merits and weaknesses of the standard terms that have been developed to quantify CT dose: CT dose indices (CTDI), dose length product (DLP) and effective dose. The difference between the measured CTDIvol and the CTDIvol displayed on the CT scanner illustrates a clinical dilemma. Displayed CTDIvol represents the radiation dose delivered to a plastic phantom, which is significantly different from the dose delivered to the patient, depending on the size of the patient. Although effective dose is simple to calculate for an individual patient, it was never intended for this purpose. The need for a simple, appropriate method to estimate pediatric patient doses led to the development of the size-specific dose estimate (SSDE), the newest CT dose index. Here I compare SSDE and its merits to the use of effective dose to estimate patient dose. The discussion concludes with a few sample calculations and basic clinical applications of SSDE to better quantify pediatric patient dose from CT scans.