Dose is not always what it seems: where very misleading values can result from volume CT dose index and dose length product.

Purpose: The volume CT dose index (CTDIvol) and the dose-length product, commonly reported for examinations performed on clinical CT scanners, should not be used as surrogates for patient dose. This is because significant under or overestimates of these actual values can occur when there is a mismatch between the actual body size of the patient and the 16 cm or 32 cm diameter CTDIvol phantoms. This mismatch can be exacerbated in pediatric body examinations because of the fact that some manufacturers use the large diameter phantom while other manufacturers use the small diameter phantom as the CTDIvol reference phantom.

Method: A clinical example is described for a pediatric patient with a 4-fold difference in CTDIvol between a presurgical CT examination and a postsurgical CT examination, even though the actual dose absorbed by the patient was about the same. Using methods published by the American Association of Physicists in Medicine, we calculated the size-specific dose estimate (SSDE), and compared the estimated measurement of dose using the SSDE with the CTDIvol.

Results: Using SSDE significantly reduced the discrepancy in radiation dose estimates of CTDIvol in the clinical study, and allowed dose estimate comparisons between scanners to be more meaningful.

Conclusions: Radiation dose estimates are more accurate when using the SSDE metric in lieu of the CTDIvol metric for reporting and comparing patient dose indices.