Acoustic Radiation Force Impulse Imaging in the Differential Diagnosis of an Ectopic Intrathyroidal Thymus

Ectopic intrathyroidal thymus is caused by a hesitate in the migration of thymus during embryogenesis and is a rare cause of a nodular intrathyroidal lesion. Nodular thyroid lesions raise the suspicion of malignancy in the pediatric age group. Although rare, ectopic intrathyroidal thymus is usually misdiagnosed as a thyroid nodule and in addition to ultrasonography, further imaging techniques including MRI is performed. Fine needle aspiration biopsy or surgical excision, are also included in the management in thyroid nodules, because of the possibility of a malignancy. Acoustic Radiation Force Impulse-Imaging (ARFI) is a new method determining the tissue elasticity using short duration acoustic pulces and exciting the tissue in the selected region of interest. Acoustic Radiation Force Impulse-Imaging had been used in the differentiation of malign and benign thyroid nodules with or without in combination with gray-scale ultrasonography fi ndings. We here present a case of intrathyroidal ectopic thymus in a 4 year-old boy diagnosed with the use of Acoustic Radiation Force Impulse-Imaging in addition to gray scale and Doppler ultrasonography fi ndings. Case Report Acoustic Radiation Force Impulse Imaging in the Differential Diagnosis of an Ectopic Intrathyroidal Thymus Dilek Sağlam1*, Meltem Ceyhan Bilgici1, Cengiz Kara2, Muzaffer Elmali1, Asli Tanrıvermiş Sayit1 1Ondokuz Mayıs University, Department of Radiology, 55270, Atakum, Samsun, Turkey 2Ondokuz Mayıs University, Department of Pediatric Endocrinology, 55270, Atakum, Samsun, Turkey Dates: Received: 07 December, 2016; Accepted: 24 January, 2017; Published: 25 January, 2017 *Corresponding author: Dilek Sağlam, Ondokuz Mayıs University, Department of Radiology, 55270, Atakum, Samsun, Turkey, Tel: +905336355443; Fax: +903624576091; E-mail:


Introduction
Ectopic intrathyroidal thymus is caused by a hesitate in the migration of thymus from the angle of the mandible to the anterior mediastinum during embryogenesis [1]. If misdiagnosed as a thyroid nodule, unnecessary interventions and inappropriate treatment may cause unwanted results [2]. Acoustic radiation force impulse-imaging (ARFI) is a quantitative method in determining tissue elasticity and had been used in differentiation of benign and malign thyroid nodules [3]. We here report a case of intrathyroidal ectopic thymus and to our knowledge this is the fi rst case presenting the role of ARFI elastography fi ndings in the differential diagnosis.

Case Report
A 4 year-old boy was referred to our outpatient clinique for fi ne needle aspiration biopsy from a solitary thyroid nodule. The thyroid nodule was detected with ultrasonography in another institution. The patient was reevaluated in our clinique. The patient had no family history for goiter. Physical examination was normal and thyroid gland was non-palpable.
Technetium-99 m pertecnetate thyroid scintigraphy was performed and asymmetric uptake in the right and left lobes were detected.

Abstract
Ectopic intrathyroidal thymus is caused by a hesitate in the migration of thymus during embryogenesis and is a rare cause of a nodular intrathyroidal lesion. Nodular thyroid lesions raise the suspicion of malignancy in the pediatric age group. Although rare, ectopic intrathyroidal thymus is usually misdiagnosed as a thyroid nodule and in addition to ultrasonography, further imaging techniques including MRI is performed. Fine needle aspiration biopsy or surgical excision, are also included in the management in thyroid nodules, because of the possibility of a malignancy. Acoustic Radiation Force Impulse-Imaging (ARFI) is a new method determining the tissue elasticity using short duration acoustic pulces and exciting the tissue in the selected region of interest. Acoustic Radiation Force Impulse-Imaging had been used in the differentiation of malign and benign thyroid nodules with or without in combination with gray-scale ultrasonography fi ndings. We here present a case of intrathyroidal ectopic thymus in a 4 year-old boy diagnosed with the use of Acoustic Radiation Force Impulse-Imaging in addition to gray scale and Doppler ultrasonography fi ndings. was repeated for fi ve times and the mean value was recorded.

Citation:
The mean ARFI elastography velocities of the lesion and the mediastinal thymus were, 0, 63 m/s and 0, 60 m/s, respectively.
The mean ARFI elastography velocity of the normal thyroid parenchyma was 1, 25 m/s. The normality of data distribution was evaluated with Saphiro-Wilk test and Paired sample T test was used to compare the mean SWVs in normal distributed data. There was not a signifi cant difference between the mean SWV of the lesion and thymus (p=0.9). There was a signifi cant difference between the mean SWV of the lesion and thymus versus thyroid gland (p=0.001 and p=0.002, respectively). The lesion was diagnosed as ectopic intrathyroidal thymus and follow-up was recommended.

Discussion
In the embryogenesis, thymus originates from the third and the fourth branchial pouchs on both side of the neck and descends to its fi nal place, the anterior mediastinum. A hesitate in this migration process results in ectopic localization of the thymic tissue.
In children intrathyroidal lesions before puberty are rare and rise the suspicion of malignancy. Ectopic parathyroid gland, esophageal diverticula, lymphocytic thyroiditis, adenoma, nodular goiter, lymphoma and abscess are some of the lesions, which should be considered in the differential diagnosis of nodular thyroid lesions in children. Although rare, ectopic intrathyroidal thymus is one of the possible origins of a nodular lesion in the thyroid gland and several cases of intrathyroidal ectopic thymus in children had previously been reported [4,5].
Ultrasound is the fi rst imaging study performed in the case of nodular thyroid lesions. Classical fi ndings of welldefi ned margins, ovoid to fusiform shape on longitudinal axis, hypoechoic structure with multiple small echogenic foci are the typical ultrasonographic fi ndings suggesting thymus tissue. Also the low lying or mid-localization of the thymus tissue will give a clue [6]. In the study of Han et al., the classical gray scale ultrasonography fi nding; linear echogenic  septa in thymus was found to be the representative of the connective tissue septa and the vessels within the septa [7].
On Doppler ultrasonography thymus do not have prominent vascularity. ARFI elastography is a new method determining the tissue elasticity using short duration acoustic pulses and exciting the tissue in the selected region of interest. The tissue elasticity is measured in meters per second (m/s). This method provides quantitative information about tissue elasticity and is less operator dependent compared to strain elastography.
Most commonly used in liver fi brosis to replace liver biopsy, ARFI elastography had also been used for the differentiation of malign and benign thyroid nodules and diffuse thyroid lesions in the literature [8,9]. Previous studies suggest ARFI elastography as a useful tool in the differentiation of malign and benign thyroid nodules with or without in combination with gray-scale ultrasonography fi ndings [10][11][12].
Several cases of intrathyroidal ectopic thymus in children had previously been reported. Most commonly these lesions were misdiagnosed as a thyroid nodule on ultrasonography. In the previous cases, CT, MRI or sintigraphy had been used as conformational imaging studies [13]. Some of these patients underwent surgery for excision or biopsy was performed to exclude malignancy. To our knowledge this is the fi rst time in the literature that the diagnosis was supported with ARFI elastography fi ndings without the need for any other procedure.
Although studies were carried out for ARFI elastography measurements of thyroid nodules and diffuse thyroid diseases There are some limitations in the use of ARFI elastography in the differential diagnosis of intrathyroidal ectopic thymus.
As we mentioned above, there are studies evaluating the normal or abnormal thyroid gland in adults, but to our knowledge there is not a study evaluating these values in children and also ARFI elastography velocities of the normal thymus is not found, to make a reference point. Our decision in this case depends on our own clinical experience on elastography measurements of the thymus and thyroid glands. We think ARFI elastography helps in the differential diagnosis of ectopic intrathyroidal thymus and prevents unnecessary interventions.