Radiological Diagnosis:

Intrathoracic Goitre with Colloid Cyst

The chest radiograph shows a mass in the superior mediastinum, on the right. CT shows a spherical, cystic mass with peripheral calcification, seen posteriorly, extending in the superior and the posterior mediastinum on the right. The mass was continuous with the right lobe of the thyroid.

Mediastinal goitres are multinodular goitres, which represent intrathoracic extension of cervical thyroid tissue [1]. Rarely, an intrathoracic goitre arising from ectopic thyroid tissue can occur [2],[3]. These are commonly found in the superior mediastinum; however, if they are large enough, they extend into the anterior mediastinum. They deviate, compress and/or circumferentially narrow the trachea. In the posterior mediastinum, they extend posterior to or between the trachea and esophagus [4]. A case report described a posterior mediastinal goitre with high CT density in continuity with an enlarged right lobe of thyroid [5]. Glazer et al [6] demonstrated the following characteristic features of mediastinal goitres:

1. Anatomic continuity with the cervical thyroid.
2. Focal calcification.
3. Relatively higher density than surrounding soft tissues e.g, muscles.
4. Enhancement CT intravenous contrast administration.
5. Prolonged enhancement.

Similar findings were observed in our case. Iodine-131 scanning is capable of detecting mediastinal thyroid tissue in almost all cases [7]. On MRI, mediastinal goitre is easily seen [8]. However, because of the ability of CT to detect the high density of iodine and calcification, CT is the preferred imaging modality for evaluating mediastinal goitres.