Imaging technique reveals primary mediastinal goiter

Issue: December 2015

ByStephanie L. Lee, MD, PhD, ECNU

A 65-year-old man was referred for a mediastinal mass in the right paratracheal area that was found on a routine chest radiograph (Figure 1). His primary care physician ordered a CT scan (Figure 2) that showed a 5.5-cm oval mass with smooth borders that displaced the upper lobe of the patient’s lung in the anterior mediastinum. The mass showed marked peripheral enhancement with the possibility of central necrosis.

The radiology differential diagnosis included a primary tumor, such as a paraganglioma, or metastatic cancers, including melanoma, thyroid cancer or a neuroendocrine tumor.

Endocrine referral, imaging

The patient was referred to the endocrine clinic for further evaluation. Review of his medical history was significant for Graves’ hyperthyroidism diagnosed 15 years earlier. A nuclear thyroid scan had been performed that showed an elevated 24-hour thyroid uptake of 79%. The report described an iodine-avid mass inferior to the right lobe, but the images were not available for review; images are destroyed at our hospital after 10 years.

The patient was treated with radioactive iodine and became hypothyroid. His thyroid-stimulating hormone level was 1.49 uIU/mL while assigned levothyroxine 100 µg daily.

Figure 1. Chest radiograph: Anteroposterior view shows an ovoid mass (yellow arrow) along the right paratracheal margin of the anterior mediastinum. The lung fields are otherwise clear of focal consolidation or effusion. The heart is normal in size.

A thyroid ultrasound performed in the endocrine clinic showed a small gland with changes of thyroiditis without nodules. The patient’s levothyroxine therapy was suspended for 3 weeks, and a nuclear thyroid scan was performed with planar and single-photon emission CT (SPECT) images.

The anteroposterior planar imaging (Figure 3A) with backed-off images to include the anterior mediastinum showed a small thyroid in the cervical neck and a separate iodine-avid mass in the anterior mediastinum. The 24-hour iodine uptake of the thyroid gland (excluding the mediastinal mass) was slightly low at 10.9%. The CT imaging was performed without contrast with lower energy (“low dose”) than the diagnostic CT scan and showed the mass in the anterior mediastinum (Figure 3B). The fusion of the planar nuclear scan and reconstructed CT scan confirmed that the tissue in the anterior mediastinum was iodine-avid thyroid tissue.

Figure 2. CT scan of the neck and chest with contrast. (A) Axial image shows an avidly enhancing lesion (blue arrow) within the right anterior upper mediastinum measuring 5.0 x 4.5 x 5.3 cm. This lesion demonstrates central nonenhancement, concerning for necrosis. (B) Reconstructed coronal image shows the enhancing 5-cm mass has well-defined margins. The mass compresses the internal jugular vein (yellow arrow) and displaces it anterolaterally. The mass also abuts the innominate artery (green arrow), which is normal in caliber and does not appear to be displaced. The thyroid gland (red arrows) is very small and separate from the anterior mediastinal mass.

Mediastinal thyroid tissue

Ectopic thyroid tissue may occur anywhere along the course of the embryologic migration of the thyroid gland along the thyroglossal duct, but can be found more inferiorly from the thoracic inlet to the diaphragm. Ninety percent of ectopic thyroid tissues are found in the base of the tongue. The rest may occur in the larynx, trachea, esophagus, mediastinum and heart.

Primary mediastinal goiter with no connection to the normal thyroid gland is very rare and accounts for less than 1% of substernal goiters. Patients with ectopic mediastinal goiter are usually asymptomatic, and these structures are found incidentally on a chest radiograph. Occasionally, mediastinal goiter causes symptoms, including pain, retrosternal mass, dyspnea and cough. Histological sampling is important for accurate diagnosis. Surgery for primary mediastinal goiters should always be considered if malignancy is suspected or if there are vascular or tracheal compressive symptoms from the mass.

Figure 3. Four-hour I-123 thyroid and SPECT CT scans. (A) Anteroposterior planar image of the neck and upper chest showing iodine uptake into a small heterogeneous thyroid gland (red arrows) that is separate from a right mediastinal mass (yellow arrow). (B) Reconstructed coronal low-dose CT scan without contrast showing the right anterior mediastinal mass (yellow arrow). (C) Fusion of the I-123 and CT scans confirming the mediastinal mass is iodine avid consistent with a primary mediastinal goiter without connection to the cervical thyroid gland.

Resection is usually through median sternotomy or thoracotomy to ensure all thoracic vessels supplying the mass are ligated, but emerging new approaches using thoracoscopy are being used. This patient had a CT-guided fine-needle aspiration biopsy that confirmed the anterior mediastinal mass was unremarkable, benign thyroid tissue. Although the CT scan showed heterogeneous enhancement with concern for central necrosis, the radioactive iodine uptake on SPECT scan showed fairly uniform uptake.

The patient wanted to avoid surgery, and based on the longstanding presence of mediastinal uptake for more than 15 years, it was agreed to repeat his CT scan in 6 months for growth or signs of obstruction.

This interesting case illustrates the use of radioactive iodine thyroid scan with backed-off planar images of the anterior mediastinum and SPECT/CT to diagnose a benign ectopic primary mediastinal colloid goiter.

References:
De Felice M, Di Lauro R. Endocr Rev. 2004;25:722-746.
Pilavaki M, et al. Cases J. 2009;doi:10.4076/1757-1626-2-8554.
Thuilier F, Venot J. Ann Endocinol. 2012;doi:10.1016/j.ando.2011.12.003.

For more information:
Stephanie L. Lee, MD, PhD, ECNU, is an Endocrine Today Editorial Board member. She is Associate Professor of Medicine and Associate Chief, in the Section of Endocrinology, Diabetes and Nutrition at Boston Medical Center. Lee can be reached at Boston Medical Center, 88 E. Newton St., Endocrinology Evans 201, Boston, MA 02118; email: stephanie.lee@bmc.org. Lee reports no relevant financial disclosures.