Despite guideline recommendations, repeating fine-needle aspiration biopsy (FNAB) for thyroid nodules with more than 50 percent growth is unlikely to result in a diagnosis of malignancy, according to a study published online Jan. 16 in Radiology.

“A positive FNAB result for malignancy is significantly more likely in the presence of suspicious US (ultrasonographic) features,” wrote Soo-Yeon Kim, MD, of Yonsei University College of Medicine, Seoul, Korea, and colleagues.

Nodules diagnosed as benign at initial FNAB typically don’t need a routine immediate follow-up diagnostic exam. However, due to variability in the performance of FNAB and false-negative rates as high as 10 percent, Kim and colleagues explained that the general recommendation is for all benign nodules to be followed with serial US exams. In 2009, the American Thyroid Association released guidelines indicating that a 50 percent volume increase of nodules at follow-up US performed six to 18 months after initial FNAB should serve as the threshold for performing another FNAB.

To investigate the 50 percent threshold as an indicator of malignancy, Kim and colleagues conducted a study of 854 FNAB-confirmed benign thyroid nodules. Initial mean diameter of the nodules was 19.92 mm and initial mean volume was 3.19 cm³. Suspicious US features were tracked.

Results showed that nearly 80 percent of the nodules did not grow more than 50 percent in volume over a mean four years of follow-up. Factors associated with growth were a follow-up of more than four years, younger age, cystic component of less than 25 percent and nodule size of 1 cm or larger.

Of the 172 thyroid nodules that increased in volume at least 50 percent over the follow-up time, just one was malignant, for a malignancy rate of 0.6 percent. The overall malignancy rate for all the nodules was higher at 1.2 percent.

Eight of the 10 total malignancies were detected on the basis of suspicious US features rather than growth. The authors suggested that these suspicious features—including marked hypoechogenicity, irregular or microlobulated margin, microcalcification and taller-than-wide shape—could serve as a more cost-effective indicator of malignancy than focusing on the nodules that grew more than 50 percent.

“In the actual clinical situation, it is not always easy to follow-up thyroid nodules with regular intervals between US examinations, but the concept of the use of suspicious US features for diagnosing malignant thyroid nodules has been well established,” wrote Kim and colleagues. “Therefore, when we consider repeat FNAB to detect false-negative results of initial FNAB, it may be more simple and reasonable to assess the nodule for suspicious US features rather than to calculate volume growth with consideration of heterogeneous interval times between US examinations.”