Intrathyroidal ectopic thymic tissue mimicking thyroid cancer in children

To increase awareness of the important differential diagnosis between thyroid cancer and intrathyroidal ectopic thymic tissue when performing neck ultrasound (US) in pediatric patients.

We report 2 cases of intrathyrodal thymic tissue that were initially referred as possible papillary thyroid cancer (PTC). US and cytologic features of these lesions are described, and the distinguishing US characteristics of intrathyroidal thymic tissue versus PTC are discussed.

Two pediatric patients who underwent thyroid US were diagnosed with thyroid nodules suggestive of PTC. The suspected lesions in both patients exhibited an echo texture identical to the normal thymus gland. Fine-needle aspirate (FNA) biopsy of the nodule in the first case revealed numerous lymphocytes with positive immunocytochemical staining for CD3, confirming a T cell phenotype. The second patient underwent follow-up US imaging 8 months later that showed stability of the intrathyroidal nodule.

Intrathyroidal ectopic thymic tissue can be mistaken for PTC as both conditions have similar US features. Increased awareness of this unique mimicry may help to avoid unnecessary invasive investigations and surgery in these young patients.