Clinical History
A patient presented to the emergency room with symptoms of dysphagia and dyspnea.
Imaging Findings
A 55-year-old female presented to the emergency room with symptoms of dysphagia and dyspnea. The PA chest X-ray demonstrated the presence of a large anterior mediastinal mass. The superior extent of
the mass extended above the thoracic inlet and was associated with a discrete rightward deviation of the trachea. The mass extended inferiorly to the level of the base of the heart. Axial CT images
of the lower neck showed a heterogeneous mass that was contiguous with the left lobe of the thyroid gland. Axial CT images at the level of the aortic arch demonstrated the large substernal component
of the mass which displaced the ascending aorta and superior vena cava posteriorly.
Discussion
It is known that thyroid abnormalities account for the majority of thoracic inlet in adults. They may extend inferiorly into the anterior, middle, and posterior compartments of the mediastinum. When
located in the anterior mediastinum, thyroid masses are almost always located posteriorly to the great vessels, usually in a paratracheal location. The majority of mediastinal masses of thyroid
origin represent a thyroid goiter, and they almost always extend inferiorly from the thyroid gland. A truly ectopic thyroid goiter known to be rare. Other thyroid abnormalities, such as thyroid
adenomas, and malignant thyroid neoplasms, infrequently extend into the mediastinum. Because a thyroid goiter accounts for the majority of mediastinal masses of thyroid origin, the demographics of
thyroid mediastinal masses are similar to those of a thyroid goiter with a tendency to occur predominantly in middle-aged female patients. The following are the CT imaging features of a mediastinal
thyroid goiter: a) continuity of the mass with the cervical thyroid gland; b) foci of high attenuation on noncontrast examination, reflecting a high iodine content of the thyroid tissue; c) foci of
heterogeneous attenuation ( low attenuation cystic areas and high attenuation foci of calcification); d) intense and prolonged enhancement following administration of an intravenous contrast; e) as
on plain radiographs, deviation or compression of the trachea is frequently identified on CT. Felson described the hilum overlay sign and the hilum convergence sign in order to distinguish between an
enlarged heart and enlarged pulmonary artery from a mediastinal mass. Felson stated that the proximal segment of the visible left pulmonary artery lies lateral to the cardiac shadow or just within
its outer edge in over 98% of individuals and lies slightly more than 1cm within the silhouette in the remainder. A similar situation occurs on the right side. Even with pericardial effusion or
cardiac enlargement, this relationship holds, with only an occasional exception. The configuration of an anterior mediastinal mass may closely resemble that of an enlarged heart or a pericardial sac,
but the mass can hardly lie directly medial to the pulmonary artery, since the heart and the pericardium pre-empt this location. Such a mass will overlap the main pulmonary arteries which then become
clearly noticeable within the margins of the mass. On the other hand, if the pulmonary artery derives from the lateral edge of such an anterior mediastinal shadow, an enlarged heart or pericardial
sac is indicated. A corollary of the hilum overlay sign is sometimes useful in distinguishing between a large pulmonary artery and a juxtahilar mediastinal tumour. If the pulmonary artery branches
converge toward the mass rather than toward the heart, then it is an enlarged pulmonary artery that is being dealt with. The reverse indicates a mediastinal mass and pertains the hilum convergence
sign.
Differential Diagnosis List
