Most thyroid goitres are in the neck, yet between 3 and 17% of goitres extend into the thorax.
Most thyroid masses in the mediastinum represent downward extensions of either a multinodular colloid goitre or, occasionally, an adenoma or carcinoma. Intrathoracic thyroid masses usually have a well-defined spherical or lobular outline (Fig. 1). Rounded or irregular, well-defined areas of calcification may be seen in benign areas, whereas amorphous cloud-like calcification is occasionally seen within carcinomas.
FIGURE 1 ■ Value of multiplanar reformations. A 45-year-old woman with dyspnoea: (A) frontal radiograph and (B) oesophogram demonstrate displacement of the trachea and oesophagus to the right, by a large mediastinal mass in the thoracic inlet. (C) Transaxial contrast medium-enhanced CT shows a large goitre arising from the left lobe of the thyroid. (D) Coronal reformat depicts the craniocaudal extent of the mass and its relationship with the adjacent structures.
Almost all intrathoracic thyroid masses displace the trachea and may cause tracheal narrowing. The direction of displacement depends on the location of the mass. Thyroid masses are most commonly anterior and lateral to the trachea. Posteriorly, masses often separate the trachea and the oesophagus, and such separation by a localised mass rising into the neck is virtually diagnostic of a thyroid mass.
CT imaging features of mediastinal thyroid goitres are:
1. Continuity of the mass with the cervical thyroid gland;
2. Foci of heterogeneous attenuation (cystic areas and calcifications);
3. High attenuation on unenhanced CT (higher than muscle), reflecting high iodine content of thyroid tissue; and
4. Intense and prolonged enhancement.
The most important of these features is to demonstrate continuity of the mass with the cervical thyroid. It is usually possible to diagnose a thyroid origin by noting a well-defined mass in the paratracheal or retrotracheal region, almost invariably being continuous with the thyroid gland in the neck. It is not possible to distinguish between a benign and malignant mass on CT unless the tumour has clearly spread beyond the thyroid gland. It should, however, be noted that multiple masses are a feature of benign multinodular goitre, though carcinoma can develop in multinodular goitre. MRI of intrathoracic goitre, like CT, can identify cystic and solid components, and in addition can demonstrate haemorrhage. In most practices, evaluation of the questionable thyroid usually relies on ultrasound with biopsy.
Radionuclide imaging with I or I demonstrates the presence of thyroid tissue within the mediastinum in almost all intrathoracic goitres. Although radionuclide imaging is a sensitive and specific method of determining the thyroid nature of an intrathoracic mass, CT is more useful as the initial investigation because it provides more information should the mass prove to be something other than a thyroid lesion and is almost as specific as nuclear medicine in diagnosing a thyroid origin. CT optimally demonstrates the shape, size and position of the mass(Fig. 2).
FIGURE 2 ■ Comparison of various techniques in assessment of goitre. A 90-year-old woman presents with swelling of her face and shortness of breath. (A) Transaxial and (B) sagittal reformatted images on contrast medium-enhanced CT imaging demonstrate a large intrathoracic goitre. Ultrasound evaluation of the neck using colour Doppler shows an enlarged left lobe of the thyroid with a mildly heterogeneous echogenicity. There are numerous tortuous venous collateral vessels surrounding the goitre, rendering safe fine-needle aspiration under ultrasound guidance impossible (C). 123I radionuclide imaging of the face and neck demonstrates iodine uptake of the goitre (D).
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