Primary intrathoracic goiter: Incidental diagnosis on 99m-Tc pertechnetate radioisotope thyroid scan.

Sir,

In true primary intrathoracic or aberrant goiter, presence of accessory thyroid tissue in the mediastinum is congenital. It has no direct connection to the cervical thyroid gland. This group represents less than 1% of surgically removed goiters.[1] They usually present with clinical symptoms due to pressure on adjacent structures as the goiter enlarges within the rigid thoracic inlet. Here, we report a case of large retrosternal primary intrathoracic goiter found serendipitously on Technetium-99m (99m-Tc) pertechnetate radioisotope thyroid scan, which was missed on USG neck, who presented with symptom of neck heaviness.

A 44-year-old female presented with complaint of neck heaviness since 1 week. Further interrogation revealed that neck swelling was present for 10–15 years. She was asymptomatic without any history of dysphagia, hoarseness, or stridor. On examination her vital signs were stable. Neck examination examination revealed a moderate-sized thyroid swelling which was nontender, nonpulsatile with nodular surface and palpable lower pole of thyroid. No bruit was audible over the thyroid swelling. No cervical lymph node was palpable. Laboratory investigations confirmed euthyroid status. USG of neck was performed which showed multinodular goiter without any definitive evidence of retrosternal extension.

Radionuclide thyroid scan was obtained 20 min after i.v. administration of 5 mCi of 99m-Tc on a dual head gamma camera equipped with low-energy general purpose parallel hole collimator. Image showed moderately enlarged thyroid gland with overall reduced and patchy uptake and multiple areas of further reduced tracer uptake within [Figure 1]. In addition, a large area of low-grade tracer uptake was seen in the mediastinum, without any obvious path of continuous tracer uptake between mediastinal mass and thyroid gland [Figure 1]. A diagnosis of multinodular goiter with a functioning primary intrathoracic goiter was made. Computed tomography (CT) scan of the neck and chest confirmed the radioisotope thyroid scan findings [Figures 2 and 3]. Mediastinal biopsy confirmed it to be benign colloid goiter. Sternotomy was performed to excise the retrosternal goiter. She had an uneventful postoperative recovery.

Figure 1

Technetium-99m pertechnetate thyroid scan: (a) flow, (b) blood pool, and (c) delayed images, showing moderately enlarged thyroid gland (thin arrow) with overall reduced, pachy uptake and multiple areas of further reduced tracer uptake within. Large area of low-grade tracer uptake is seen in the mediastinum (arrow head) without any obvious path of continuous tracer uptake between mediastinal mass and thyroid gland

Figure 2

(a-i) Transaxial sections of computer tomography neck and thorax showing MNG (thin arrow) and large without any continuity between the mass and cervical thyroid gland

Figure 3

(a) Coronal and (b) sagittal sections of computer tomography neck and thorax showing MNG (thin arrow) and large mediastinal mass without any continuity between mass and cervical thyroid gland (thick arrow)

Retrosternal goiter is usually referred to as enlarged thyroid gland with greater than 50% of its mass below the thoracic inlet. It is classified into two groups. One is the truly primary intrathoracic goiter. The second group represents the acquired retrosternal goiter, which arises in the cervical thyroid gland, and while growing, it descends along a fascial plane through the thoracic inlet into the mediastinum.[1] Retrosternal goiter has a clinical importance because its compressive symptoms may cause diagnostic problems and the selection of surgical approach is sometimes difficult. Asymptomatic large retrosternal goiters are very uncommon. It is important that surgeon develops a consistently reliable technique of examination by which he can properly assess the whole gland, its extent, and functioning status. The diagnostic procedures used for evaluation of intrathoracic goiter include thyroid function tests, ultrasonography, chest radiography,[2] CT,[3] angiography, mediastinoscopic biopsy, and radioisotope scans.[4] However, their order of preference and indications differ from case to case.

The routine chest X-ray is a valuable initial study for compartmental localization of a mediastinal mass and also demonstrates tracheal displacement or compression and areas of calcification. CT scan reveals continuity of the mediastinal mass with the cervical gland, borders, focal calcifications, high-contrast attenuation values of the goiter, and post-contrast enhancement. Radionuclide scintigraphy is capable of detecting most intrathoracic goiters,[5] provides an excellent estimate of the functional status of a mediastinal goiter, its nature and extent. It is considered a best preoperative investigation as it can be performed quickly, reliably, and with very low radiation exposure. Though the 131I scan is preferred over other radioisotopes for demonstration of intrathoracic goiter due to its low background activity, we currently prefer 99m-Tc to 131I/123I for the following reasons: 99m-Tc is much more readily available, imaging can begin in 20–30 min after injection, and it is less expensive. Radioiodide thyroid imaging, however, is preferred to 99m-Tc as the choice of investigation of patients with retrosternal goiter with unsatisfactory 99m-Tc images due to poor radionuclide concentration.

Prognosis of retrosternal goiter is very good if diagnosed and treated at a proper time. The specific indications for resection include compression of adjacent structures, prevention of future complications, obtaining a diagnosis, and suspicion of malignancy.

A useful investigation is one in which the result will alter the management or add confidence to clinical diagnosis. As these cases are rare, it has been suggested that in all patients with either a diffuse or multinodular goiter, radionuclide scanning should be done as first line of investigation, followed by chest roentgenogram or USG to explore the possibility of an intrathoracic goiter. Here, we emphasize the role of radioisotope thyroid scan over USG neck to diagnose retrosternal goiter.