Ultrasound features value in the diagnosis and prognosis of medullary thyroid carcinoma

Ultrasound (US) is the most important imaging in the preoperative diagnosis of medullary thyroid carcinoma (MTC). MTC are easy to be misdiagnosed due to lacking typical malignant US features. This study investigated US features, clinical characteristics, prognosis, and detection methods, aimed to explore the association between US features and biological behavior, and improve early diagnosis of MTC.

A total of 189 MTC patients were enrolled in the study. Based on US features, 29 MTC were categorized as "indeterminate" (i-MTC) and 160 MTC were categorized as "malignant" (m-MTC) according to Thyroid Imaging, Reporting and Data System published by America College of Radiology (ACR TI-RADS). We compared US features, clinical characteristics and prognosis between both groups. We analyzed cytological categories of fine needle aspiration (FNA) within each i-MTC and m-MTC group according to the 2017 Bethesda System for Reporting Thyroid Cytopathology (TBSRTC). We assessed the positive rate of FNA, frozen pathological examination, and preoperative serum calcitonin (Ctn) level in i-MTC and m-MTC groups.

Preoperative US features were significantly different in shape, margin, composition, echogenicity, and calcifications between i-MTC and m-MTC (p < 0.05). I-MTC showed a hypoechoic solid or solid-cystic nodule lacking malignant US features. While m-MTC was presented as a solid nodule with obviously malignant US features. There were significant differences in lymph node dissection, extent of tumor, lymph node metastasis, and TNM stage and prognosis between i-MTC and m-MTC (p < 0.05). Compared to m-MTC, i-MTC underwent central neck dissection more frequently rather than lateral neck dissection at the time of the initial operation; i-MTC had less extrathyroidal invasion and lymph node metastasis, earlier stage, higher rate of biochemical cure, and lower rate of structural persistence/recurrence (p < 0.05). The 2017 TBSRTC of i-MTC and m-MTC was significantly different (p < 0.05). Preoperative serum Ctn level had a higher diagnostic sensitivity for both i-MTC and m-MTC when comparing to FNA and frozen pathological examination (p < 0.05).

US features were associated with biological characteristics and prognosis of MTC. I-MTC lack malignant US features, preformed less aggressiveness, and better prognosis. TBSRTC according to FNA combined with serum Ctn were helpful for the detection of i-MTC.