Malignancy Rate in Sonographically Suspicious Thyroid Nodules of Less than a Centimeter in Size Does Not Decrease with Decreasing Size

We evaluated the malignancy and nondiagnostic rates using fine needle aspiration cytology (FNAC) results in thyroid nodules smaller than 1 cm according to the subdivided size. We retrospectively reviewed the medical records of all subjects underwent FNAC from 2003 to 2009 in our hospital, and 2,756 patients of subcentimeter thyroid nodules with one or more suspicious sonographic features and 7,105 with nodule sized 1 cm or more were included. The malignancy rate was higher in those subcentimeter nodules with suspicious sonographic findings than the nodule sized 1cm or more (19.7% vs 7.8%, P < 0.001). We grouped the nodules based on size with mm interval and observed that the malignancy rate did not decrease but the nondiagnostic results increased its size decrement. When we divided the subjects arbitrarily into a 5 mm or smaller and a 6-9 mm sized group, nondiagnostic cytology findings were reported more frequently in the smaller group (24.3% vs 18.1%, P = 0.001), while the rate of "malignant" was similar (18.3% vs 15.5%, P = 0.123) and the rate of "suspicious for malignancy" was higher (6.8% vs 2.9%, P < 0.001). Therefore when we decide to perform FNAC or not in subcentimeter-sized nodules, we should consider sonographic findings and other clinical risk factors but not the nodular size itself.