Partially cystic thyroid nodules on ultrasound: Probability of malignancy and sonographic differentiation

Background: Thyroid nodules are very common and a large portion are mixed echoic, with both solid and cystic areas. There are few studies regarding the reliability of ultrasonographic criteria for evaluating solid nodules as they apply to mixed echoic nodules. The object of this study was to evaluate the frequency of malignancy in mixed echoic thyroid nodules and ascertain the ultrasound findings that help distinguish benign from malignant nodules. Methods: Among 1056 thyroid nodules undergoing ultrasound with fine-needle aspiration (FNA) biopsy, 392 nodules (37.1%) were mixed echoic. From this group of 392, the nodules that were read as benign or malignant on histopathology examination after surgery and the nodules that were not resected but were considered to be benign or malignant on cytology were analyzed for their ultrasonographic features. The nodules were divided into the following three groups. Group 1 (n = 93) included nodules in which the solid portion was <50%; group 2 (n = 216) comprised nodules in which the solid portion was ≥50%; and group 3 (n = 26) included mixed echoic (spongy) nodules. We also analyzed features of the solid portion of the nodule, namely, its position (eccentric or not), shape, margin, and echogenicity, and whether there were micro/macrocalcifications. Results: In the FNA sample, 52 nodules were inadequate for cytological diagnosis, but the remaining 340 (86.7%) were adequate. Eighteen of the nodules were malignant and 317 were benign, yielding a malignancy rate of 5.4%. By group, the malignancy rate was 2.2% (2/93) in group 1, 7.4% (16/216) in group 2, and 0% (0/26) in group 3. There were more malignancies in group 2 than the other groups (p = 0.040). Among sonographic findings, eccentric placement (p = 0.007) and the presence of microcalcifications (p < 0.001) were significantly correlated with malignancy. Conclusions: About 5% of partially cystic nodules in our series were malignant. When more than 50% of the nodule is solid and the solid portion of the nodule is eccentric, the risk of malignancy is greater. As has been noted for completely solid nodules, microcalcifications are associated with an increased risk of malignancy.