The importance, and important limitations, of ultrasound imaging for evaluating thyroid nodules.

The care of a patient with a thyroid nodule has become increasingly complex. Although early detection and interventionare sought formosthumanmalignant tumors, thesemandateshaveproved less certain for thyroidcancerbecausemany affectedpatients (evenwhen untreated) do not die of their disease. Thus, physicians increasingly seek noninvasive means by which to identify patients who can avoid unnecessary diagnostic or therapeutic intervention. During the last 2 decades, sonographic imaging of the thyroid has become the most valued tool. The investigation published in this week’s JAMA Internal Medicine by Smith-Bindman and colleagues1 lends further support. The authors performed a large-scale analysis of all thyroid ultrasongraphic examinations performed at a single institution during a 5-year period. Radiologic findings were linkedwith the California Cancer Registry, allowingmaskedassessmentof sonographic features that conveybenignormalignantdisease.This investigationconfirmsthe importance of analyzing parenchymal tissue (solid or cystic), measuring nodule size, and identifying sonographic features, suchasmicrocalcifications, irregularmargins,andincreasedvascular flow. These results are important because the large-scale andmasked analysis by Smith-Bindman et al validates results from less stringently performed, retrospective analyses. However, thedataabout suspiciousnodule characteristicswerederived from a total of only 96malignant tumors. Accurate interpretation and effective translation of these findings intoclinicalpractice is critical.Currently, expertguidelines generally recommend diagnostic fine-needle aspiration (FNA)only for thyroidnodules larger than1.0 to 1.5 cminmaximaldiameter.2 This is becausedata confirm the indolent, nonharmfulnatureofmost thyroidmicrocarcinomas smaller than 1 cm in diameter.3 Furthermore, FNA cytologic diagnoses are nonspecific, often conveying indeterminate results in which cytologic abnormalities raise concern for malignancy, althoughmost areultimately found tobebenign.4 In suchcases, the risk of cancermust also be balanced against the risk of unnecessary surgical intervention. In the current study, the retrospective, case-control design mandates readers carefully compare cohort characteristics anddiagnosticprocesses toexpected standards. In doing so, some features of this investigation are notable. For example, it has been well established that the prevalence of thyroid cancer in a population of consecutively evaluated nodules approximates 8% to 15%.5 The current study, however, analyzed a cohort in which cancer prevalencewasonly1.6%,whereas29%to48%ofnoduleswere smaller than 1 cm. In addition, the number of nodules in the thyroid increased with age in patients with cancer but not amongcontrols.These facts suggest anonrepresentative study population and an increased potential for sampling bias. It alsoappearsunlikely thatall euthyroidpatientswith thyroidnoduleswereevaluatedwithultrasonography-guidedFNA. Because this would be necessary to fully understand the tissuecharacteristics (benignormalignant)ofeachnodule, theauthor’s assumption that all patients not listed in the cancer registryhavebenigndisease isunfounded.Thus,notwithstanding thehigh-qualitynatureof theauthor’s sonographic reviewand dataanalysis, the study’s epidemiologic interpretationsarenot readily transferrable to clinical practice. Specifically, testmetrics, such as positive or negative predictive value and calculations of number needed to diagnose, are influenced by population characteristics and the prevalence of disease. Does this imply that the author’s conceptual framework should not be applied to thyroid nodule evaluation? The answer ismost certainlyno.Sonographic featuresare, andshould remain, the principal tool to guide thyroid cancer risk assessment.2,6 Solid nodules larger than 1 cm, especiallywith microcalcifications, should be the primary focus of diagnostic evaluation. Conversely, purely cystic nodules canbe safely monitored.Unfortunately, the authors alsodonot address the spongiform nodule, which also has a high benignity rate.7 Although they combine cystic honeycomb nodules with purely cystic nodules, it is unclear that such nodules are the same as spongiformnodules. All sonographic findings should be considered so that nodules can be prioritized for possible tissue evaluation. Similarly, to allow clinicians to use such sonographic characteristics in clinical decision making, reporting by radiologists must also include asmany descriptors as possible. All too often, ultrasonographic reports provide nodule Related article page 1788 Research Original Investigation Thyroid Cancer Risk