A 53-year-old man has a chest x-ray taken for acute cough and clinical concern for early pneumonia (Fig. 1). The chest x-ray reveals soft tissue fullness in the superior mediastinal region, displacing the trachea to the right. There is no evidence of pneumonia. A CT scan of the thorax is performed with contrast to further evaluate the incidental mass lesion, and a select image is shown above (Fig. 2).
What’s the most likely cause of this superior mediastinal mass?
c) intrathoracic thyroid goitre
d) pericardial cyst
e) thoracic aortic aneurysm
f) mediastinal teratoma
c) intrathoracic thyroid goitre
Intrathoracic thyroid goitre is the term used when at least 50% of the thyroid tissue resides in a subclavicular location within the thoracic confines, typically in the retrosternal space in the anterior mediastinum superiorly. An “incomplete intrathoracic goitre” is where there’s a connection to normally located thyroid tissue in the lower anterior neck region. A “complete intrathoracic goitre” describes ectopic thyroid tissue, with no connection to the coexisting normal thyroid gland. The latter is significantly less common than the former. These mediastinal mass lesions are usually discovered incidentally on imaging performed for other reasons, although they can produce symptoms and signs, depending on their size and location, such as dysphagia from esophageal compression, dyspnea from tracheal narrowing, and occasionally superior vena cava syndrome from extrinsic compression of that vascular structure. Medical treatment with exogenous thyroid hormone can be attempted to suppress the size of the goitre, and can reduce the enlarged gland up to 30-35%, although the results are often only temporary. Treatment with sodium iodide-131 may be effective but can sometimes cause acute respiratory reactions in the elderly, leading to distress. Most symptomatic cases will go to surgery and can be successfully treated with a cervical incision.
This case also raises a controversial topic of investigation of incidental lesions discovered on imaging performed for other reasons. Many researchers will state that lesions seen in the thyroid gland on thorax CT should at the very least be investigated by thyroid ultrasound, with consideration for biopsy depending on sonographic characteristics, as up to 11-13% will be malignant. Such lesions in patients 35 years and younger have an even higher rate of malignancy. Other medical authors suggest that going after “incidentalomas” can lead to a costly cascade of further testing and potential patient harm.
Anterior mediastinal masses are not uncommon and the main differential can be summarized as the 5 “T’s”: thyroid, thymoma, teratoma, T-cell lymphoma (a form of non-Hodgkin’s lymphoma) and thoracic aortic aneurysm. In reality, the CT study would be reviewed in full, revealing a connection between the presenting thoracic lesion and the lower neck thyroid tissue, sealing the diagnosis. However, in the single axial image from the CT scan, it could be said that this lesion might still represent thymoma, teratoma or lymphoma, although those lesions more commonly arise in the pre-vascular space of the anterior mediastinum, more inferior in the thoracic cavity. Intrathoracic lymphoma can have a variety of appearances and can enhance prominently like the presenting lesion, although they are more commonly more hypodense on CT. Thymomas are also typically more hypodense and often contain a prominent central cystic component. Mediastinal germ cell tumours, or teratomas, are typically markedly inhomogeneous, frequently hypodense and can contain intra-lesional fat, calcification and even bone. Pericardial cysts can develop within the mediastinal confines but would be more common more inferior, closer to the cardiac structures. The present lesion is solid and enhances and as such a pericardial cyst should not be considered. A thoracic aortic aneurysm would reveal intense homogeneous enhancement and the additional axial and coronal images would reveal no connection to the underlying aortic arch.
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