Intrathyroidal thymic carcinoma with delayed cervical lymph node metastasis 5 years postoperatively: a case report with focus on ultrasonic features

Intrathyroidal thymic carcinoma (ITTC) is a rare thyroid malignancy with nonspecific imaging features. The ultrasonographic characteristics of its metastatic lymph nodes, particularly in cases of delayed metastasis, remain poorly understood. To report a case of ITTC with cervical lymph node metastasis occurring 5 years after surgery, with a focus on the ultrasonographic features of both the primary and metastatic lesions. A 32-year-old woman presented with hoarseness and choking. Ultrasound revealed a solid, hypoechoic mass in the lower pole of the left thyroid lobe. The lesion exhibited ill-defined margins, an irregular shape, heterogeneous echogenicity with short linear hyperechoic internal strands and predominantly peripheral vascularity on colour Doppler flow imaging (CDFI). Total thyroidectomy with lymph node dissection was performed, and histopathology confirmed ITTC. During the 5-year follow-up period, surveillance ultrasound detected an abnormal lymph node in the left level IV of the neck. This node showed well-defined borders, loss of corticomedullary differentiation, an absent hilum and no significant vascularity on CDFI. Fine-needle aspiration cytology and subsequent surgical pathology confirmed metastasis from ITTC. The characteristic ultrasonographic features of primary ITTC lesions include a solid hypoechoic mass in the mid-to-lower pole of the thyroid, with ill-defined margins, an irregular shape, internal short linear hyperechoic strands and typically peripheral vascularity, often without calcification or cystic change. Metastatic lymph nodes may demonstrate well-defined borders, loss of corticomedullary differentiation, an absent hilum and scant vascularity. Recognition of these ultrasonographic patterns is crucial for the early identification, accurate diagnosis and long-term management of ITTC.