Sonographic and Elastographic Features of Extra- and Intrathyroidal Ectopic Thymus Mimicking Malignancy: Differential Diagnosis in Children

Abstract

Thyroid nodules with ultrasound (US) cancer risk features and extra-thyroid lesions suggesting malignant lymph nodes, require prompt diagnosis, especially in children. The US pattern of intrathyroidal ectopic thymus (IET) can strongly suggest papillary thyroid carcinoma (PTC). The extra-thyroid ectopic thymic tissue (EET) can mimic pathological lymph nodes in US. The aim of the study has been to demonstrate US features and diagnostic methods, allowing finally to confirm the presence of IET and EET in children. The US and elastographic features of 16 ectopic thymic tissue (ET) lesions were analyzed so as to describe the typical characteristics of ET and to define the best method to differentiate ET and malignant lesions. Among 16 analyzed lesions, 11 lesions were IET, and 5 were EET adjacent to the thyroid connective tissue capsule. Most of IET were located in the middle part of the right lobe and were fusiform or oval in shape. All the lesions were solid, hypoechoic, and heterogeneous with bright internal echoes. Among IET, 73% of lesions had well- or very well-defined margins. In strain elastography of IET lesions, the strain ratio was similar in all lesions, and its value ranged from 0.95 to 1.09. Despite the low prevalence of IET and cervical EET, clinicians and radiologists should be aware of US characteristics of such lesions. The confirmation of their benign character is absolutely required. Elastography is a useful tool to initially differentiate PTC and IET. However, due to high risk of malignancy in thyroid lesions in children, similarity of US features of PTC and IET, and due to the possibility of malignancy in ET, only cytological evaluation provides definitive diagnosis.

Long-Term Follow-Up Ultrasonographic Findings of Intrathyroidal Thymus in Children

“…Some studies have confirmed that the presence of an intrathyroidal thymus through cytology or surgery due to the fear that children with suspicious thyroid lesions are misdiagnosed ( 4 5 12 13 15 16 21 22 ). However, a few studies on the ultrasound characteristics and the prevalence of intrathyroidal thymus were based on US images only, without cytological evaluation, and suggested that US could be used as a common diagnostic tool to identify a benign intrathyroidal thymus ( 11 13 17 23 ).…”

“…The thymus has certain characteristic ultrasonographic features, which include a homogeneous background echogenicity similar to that of the liver or spleen, with scattered hyperechoic foci resembling a starry sky ( 4 5 6 7 8 9 10 ). Ectopic thymic tissue can be located anywhere along the descending path of the thymopharyngeal ducts ( 5 11 12 ). When ectopic thymic tissue abuts on the inferior or posterior surface of the thyroid or is enclosed by the thyroid parenchyma, the thymus is described as intrathyroidal ( 4 5 6 7 8 9 10 ).…”

“…The incidence of ectopic intrathyroidal thymus is reported to be between 1% and 5% on screening ultrasonography (US). However, because US has become more frequently used in children, the characteristic features of intrathyroidal thymic tissue used to distinguish it from thyroid nodules have been reported more often, and this intrathyroidal thymus is now more frequently found as a common variant ( 8 11 12 13 14 15 ). Until 2015, 59 cases of ectopic thymic tissue mimicking thyroid nodules or neoplasia detected on US had been published and more cases have been reported since ( 12 13 15 16 17 18 ).…”

“…The US appearance of an intrathyroidal thymus can overlap with that of a thyroid nodule. Certain, characteristics leading to malignancy suspicion and misdiagnosis have resulted in children undergoing thyroid surgery for suspected malignancy that proved to be benign thymic tissue on surgical pathologic analysis ( 7 8 12 13 14 19 20 ). A few papers have reported on follow-up US of the intrathyroidal thymus in small numbers of patients ( 1 6 11 13 15 18 ).…”

Ectopic thymus as a cause of Horner's syndrome

“…The thymus is the primary lymphoid organ in infancy and early childhood and facilitates the differentiation and maturation of T lymphocytes [1]. The thymus arises from the third pharyngeal pouch, eventually forming thymic primordial that descends into the anterosuperior mediastinum.…”