Intrathyroidal ectopic thymus tissue: a diagnostic challenge

Objectives: The prevalence of thyroid nodules in pediatric population is 0.2–2%, which is lower than adults. However, the probability of the nodule to be malignant is higher than adults (20–73%). Differential diagnosis of thyroid lesions in children includes intrathyroidal ectopic thymus tissue (ITT). ITT can present as a thyroid nodule, and be confused with malignancy with its hyperechoic pattern; this might cause unnecessary fine-needle aspiration biopsies and/or surgical interventions. In the current study, we mainly aim to define both US and color Doppler ultrasonography (CDUS) characteristics of ITT. We also aim to describe the most sensitive and most specific diagnostic parameters of ITT. Methods: We have evaluated US examination reports of 56 children for whom differential diagnosis included ITT between February 2015 and August 2018. We have recorded sonographic characteristics of the lesions, CDUS data, and thyroid hormone levels. Results: Study population consists of 56 patients (22 ITT, 34 other diagnoses). Median age of the population is 10 years. Age, sex, laboratory results, and follow-up change in lesion diameters do not show any significant difference between ITT and other diagnosis groups. Typical US appearance, fusiform lesion shape, and isovascular CDUS characteristics are higher in ITT group. The median value of the lesion’s highest diameter is smaller in ITT group. The most valuable criteria to predict ITT presence were the fusiform shape and the longest diameter of the lesion. Conclusions: Fusiform shape and a maximum diameter of ≤ 9 mm are the most selective criteria to predict ITT diagnosis. © 2019, Italian Society of Medical Radiology.