Mediastinal Masses in Nuclear Medicine Studies: A Diagnostic Algorithm

DISCUSSION

The differential diagnosis of a mediastinal mass is widespread and most frequently includes the lymphoma, thymic mass, germ cell tumor, and ectopic thyroid tissue, with primary thymic neoplasms, thyroid masses, and lymphomas most commonly diagnosed in the adult population (1). The morphologic and radiologic features of each process help to differentiate and diagnose the mediastinal mass. When there is suspicion of a mediastinal mass, a posteroanterior and lateral chest radiograph is typically the first-line imaging. Although this basic modality provides limited tissue characterization, mass localization is achieved, which can aid in narrowing the differential diagnosis. CT is an important tool in evaluating a mediastinal mass and is typically the next step after chest radiography. CT imaging has the ability to further characterize the mass based on specific location; degree of soft-tissue vascularization; and attenuation of air, fat, water, and calcium. These specifications are often sufficient in achieving a diagnosis (1,2).

However, when the mass cannot be diagnosed radiographically, the next step in imaging is a physiologic assessment with radionuclide modalities. The importance of nuclear studies in determining the diagnosis of mediastinal masses is usually underestimated and is not suggested by many authors (3). In this case, we will emphasize the role of nuclear studies in determining the diagnosis of anterior mediastinal mass. ¹⁸F-FDG PET/CT is an efficacious tool with many applications, largely in the setting of malignancy. In our case, PET/CT was obtained because of suspicions that the mediastinal mass represented lymphoma. Both Hodgkin and non-Hodgkin lymphomas are ¹⁸F-FDG–avid, and in general, ¹⁸F-FDG PET is more sensitive (85%–95%) and specific (95%) than CT for detecting lymphoma (4). Because of this, it has the ability to differentiate between a lymphoma versus a more benign etiology.

Similarly, thyroid scintigraphy plays a role in evaluation of mediastinal mass when thyroid etiology is being considered. Thyroid scintigraphy is performed primarily with ^99mTc, ¹²³I, and rarely ¹³¹I. The use of ^99mTc over radioiodine is typically preferred for several reasons. One example is that ^99mTc has a faster acquisition time than ¹²³I because of its significantly higher allowable administered dose (185–370 MBq [5–10 mCi] vs. 7.2–11.11 MBq [200–300 μCi]). Subsequently, ^99mTc is considered more patient-friendly because of this shortened examination time and less time the patient is required to lie supine with an extended neck. Another reason that ^99mTc is preferred is that it is more convenient and more readily available to hospital radiopharmacies than ¹²³I and, as a result, is better able to accommodate last-minute scans (5).

As was the case with our patient, she underwent thyroid scintigraphy with ^99mTc initially, presumably because of the numerous above-mentioned benefits of ^99mTc over ¹²³I. However, when there are suspicions that a mediastinal mass represents functioning thyroid tissue, imaging with either ¹²³I or ¹³¹I is the method of choice, not ^99mTc. This is because of the increased background activity with ^99mTc caused by the activity of the salivary glands in the neck, the attenuation of γ-rays by the sternum and surrounding soft tissues, and also the substantial blood-pool activity from the heart and great vessels all interfering with the visualization of retrosternal or deep functioning thyroid tissue (4,6).