Rives’ classification is a common anatomical classification system used for intrathoracic goiters . Intrathoracic goiters are classified as substernal or mediastinal aberrant, regardless of whether continuity with the thyroid gland is present. Intrathoracic goiters account for 5% of all mediastinal tumors, and mediastinal aberrant goiters account for 2% of intrathoracic goiters (0.1% of all mediastinal tumors) [1, 2].
Aberrant mediastinal goiters are commonly reported in women ≥50 years. The tumor frequently arises from the anterior superior mediastinum, and blood tests often show euthyroidism [1, 3]. CT scans reveal a well-circumscribed tumor that may be accompanied with focal calcification deposition or cystic degeneration; moreover, enhanced CT shows prolonged enhancement [4, 5]. MRI is reliable for evaluating intrathoracic goiter extension but does not provide additional information in diagnostic yield to that from CT . Significant fluorodeoxyglucose accumulation has been reported on PET-CT . Preoperative differentiation between mediastinal aberrant goiters and thymomas, which is frequently made after excision, has been challenging due to the lack of specific radiological findings on both enhanced CT and MRI. Thyroid scintigraphy is a disease-specific examination for goiters, but limited accumulation has been reported for mediastinal aberrant goiters . The use of preoperative endobronchial ultrasound-guided transbronchial needle aspiration has been reported in cases of intrathoracic goiters located in the superior mediastinum (adjacent to the trachea) . However, a biopsy of intrathoracic goiters in the anterior mediastinum (similar to the goiters in our case) is difficult due to anatomical reasons.
In the thyroid gland, the iodine concentration in thyroid tissue is associated with a high radiodensity on non-enhanced CT [8, 9]. This finding may also apply to mediastinal aberrant goiters comprised of thyroid tissue. However, high radiodensity on non-enhanced CT (91.9 HU) was characteristic of the current case; thus, we suspected a mediastinal aberrant goiter. Mediastinal aberrant goiters have a blood supply arising from the thoracic vessels, such as the aorta or subclavian artery ; thus, preoperative diagnosis of mediastinal aberrant goiter allows for evaluation of the feeding vessel, which is necessary for safe surgery, especially in recent years with the increasing use of thoracoscopic and robotic surgery. Although we could not detect obvious blood supply from thoracic great vessels in the present case, an additional preoperative enhanced CT should be performed to preventing unforeseen bleeding.
The median CT value of four intrathoracic goiters on non-enhanced CT in our institution (including two cases of substernal goiter and a mediastinal aberrant goiter in the superior mediastinum) was 76.1 HU (interquartile range: 71.7–89.2). Although there are some reports on mediastinal aberrant goiters, only a few of these have presented the CT values for these goiters. A previous report determined the definite CT values of intrathoracic goiters to be 70–85 HU . This is consistent with our findings. On the other hand, the median CT value of a thymoma was reported to be 39.5 HU (interquartile range: 33.7–42.2) , which is lower than that of a mediastinal aberrant goiter. However, intrathoracic goiters with a low concentration of iodine per unit volume, for example, owing to cystic degeneration, may have lower CT values . Although further case accumulation is needed, preoperative non-enhanced CT values may be feasible for differentiating mediastinal aberrant goiters from thymomas.
Although mediastinal aberrant goiters are benign entities, 3–6% of these are known to undergo malignant transformation . Therefore, we believe that surgery should be performed even under preoperative suspicion of a mediastinal aberrant goiter.
We encountered a rare surgical case of a mediastinal aberrant goiter characterized by high radiodensity on non-enhanced CT. An anterior mediastinal tumor with a high radiodensity should be considered a mediastinal aberrant goiter.