Ultrasonographic findings of medullary thyroid cancer: differences according to tumor size and correlation with fine needle aspiration results

Background: Complete initial tumor resection is a key prognostic factor in patients with medullary thyroid carcinomas (MTCs), hence precise preoperative diagnosis is very important. Thyroid ultrasonography (US) is a first-line modality and can lead to the appropriate next diagnostic procedure. Purpose: To evaluate the US characteristics of MTCs, to evaluate whether or not there is a difference in US findings according to tumor size, and to correlate the US findings with fine needle aspiration (FNA) results. Material and Methods: Thirty patients with 36 MTCs who had a preoperative US and a MTC diagnosis based on surgery were included. The US findings of each nodule were retrospectively analyzed in terms of size, internal content, shape, margin, echogenicity of solid portions, the presence of calcifications, the type of calcifications, and the presence of a halo by two radiologists. Each lesion was classified as suspiciously malignant, indeterminate, or probably benign according to known US criteria. The FNA results for the 28 MTCs were reviewed from the cytology reports. Results: The lesion size varied from 5.7-90.0 mm (mean 22.4 +/- 14.4 mm). The predominate US findings included solid internal content (91.7%), round-to-oval shape (63.9%), smooth margins (52.8%), hypoechogenicity (72.2%), and micro- or macro-calcifications (61.1%). Twenty-six nodules (72.2%) were classified as suspiciously malignant, and 10 nodules (27.8%) were classified as indeterminate. Of the 27 MTCs classified as suspiciously malignant, 21 MTCs (77.8%) had micro- or macro-calcifications, which was the most common malignant feature. MicroMTCs (<= 1 cm) had spiculated margins more frequently than macroMTCs (66.7% vs. 13.3%; P = 0.014) and macroMTCs (> 1 cm) had smooth margins more frequently than microMTCs (63.4% vs. 0%; P = 0.006). Twenty-one MTCs with positive FNA results showed smooth margins and a larger size more frequently than seven MTCs with negative FNA results (66.7% vs. 14.3%; P = 0.029; mean diameter, 24.28 +/- 13.45 mm vs. 11.74 +/- 4.73 mm, P = 0.004, respectively). Conclusion: Based on US, the majority of MTCs can be classified as suspiciously malignant due to the presence of micro- or macro-calcifications. Small MTC size (<= 10 mm) and a smooth margin may be factors predicting false-negative FNA results.