Re: Effectiveness of Computer-Aided Detection in Community Mammography Practice
Authors: R. M. Nishikawa, M. L. Giger, Y. Jiang, C. E. Metz
Journal: JNCI Journal of the National Cancer Institute (2011)

Measuring the clinical benefits of computer-aided detection (CAD) in mammographic screening is challenging, in part because metrics used to quantify the effectiveness of mammographic screening do not always apply in a straightforward way to comparisons of screening with and without CAD. This was illustrated in a recent article in the Journal by Fenton et al. (1) on the effect of introducing CAD to mammographic screening in the community setting.

One of the principal conclusions of Fenton et al. (1) was that CAD increases detection of ductal carcinoma in situ (DCIS). Fenton et al. imply that this is an undesired result because DCIS may lead to overdiagnosis. However, their data showed that the increased rate of detection of DCIS in women whose mammograms included CAD was accompanied by a decrease in the rate of detection of invasive breast cancers. This result is consistent with the goal of CAD, the use of which should lead to the detection of DCIS that might ultimately progress to invasive cancer. Thus, the increased detection of DCIS by CAD should not be considered overdiagnosis, but rather earlier detection, which is an important goal of screening mammography.

More importantly, perhaps, the detection of DCIS by CAD (in a CAD vs no CAD comparison) is not the same as mammographic detection of DCIS (in a screening vs no screening comparison). All cancers detected by CAD are mammographically detectable; hence, if the radiologists does not detect the DCIS (with or without using CAD), it will eventually be detected at a future screening exam. Therefore, CAD will not lead to potentially more overdiagnoses, as the authors imply, but rather to earlier treatment of women with CAD-detected DCIS who would be treated even if CAD were not used. The downside is that women with indolent DCIS will be treated one or two years earlier. The upside is that some of the additional treated women will have DCIS that would have progressed to invasive cancer if it were not for CAD.

Fenton et al. (1) used the accepted standard method to calculate sensitivity of screening mammography. When CAD was used, both the numerator and the denominator of the sensitivity calculation appropriately included cancers that were detected only with the aid of CAD. However, that method introduces bias when CAD is compared with no CAD because without CAD, the estimated number of cancers in the screened population (the denominator in any calculation of sensitivity) included only cancers detected by screening and interval cancers, those cancers that were detected in the interval between routine screening. Although some other cancers are likely to have been missed, they were not counted in the denominator of the sensitivity calculation without CAD because there was no way of determining their number.

Whether CAD is an effective clinical tool is an important question that remains unanswered. Careful attention must be given to the methodology used in the evaluation and interpretation of results. The article by Fenton et al. (1) presents a snapshot of the effect of CAD at the introduction of the technology. Improvements in the performance of CAD systems, radiologists who have gained more experience in using CAD, and the introduction of full-field digital mammography have greatly changed the landscape.