I131 WBS and TG levels are useful tools for the detection of functioning metastases in patients with differentiated thyroid carcinoma [1–2]. TG level determination has both high sensitivity (88%) and specificity (99%) in the recognition of metastases [2,3]. However, I131 WBS has high specificity (80–100%) but less sensitivity, 45–57% in papillary cancer and 60% for follicular cancer [1,3–6,8].
False positive I131 body scans have been reported from contaminated body secretions such as saliva retained in the esophagus, galactorrhea, pathologic transudates, and inflammation as seen in skin burns [1,12,16]. Neoplasms of nonthyroidal origin, such as lymphoepithelial cyst, gastric adenocarcinoma , Warthin tumor , primary lung adenocarcinoma , or undifferentiated bronchogenic carcinoma , have also been reported to cause false positive WBS . Table 1 shows causes of false positive WBS based on a review of MEDLINE 1966–2002.
Several reports have shown that the thymus whether hyperplastic (one case) or nonhyperplastic (four cases), can be the cause of an increased mediastinal uptake on WBS [1,24–29]. It has been observed among patients of relatively young age (19–44 years). They also have CT scans showing enlarged thymus without other abnormalities, and low to absent serum TG after thyroid hormone withdrawal . Among the most common causes of false positive uptake in the chest are disorders involving the esophagus such as achalasia, Zenkers diverticulum, hiatal hernia, and Barrett esophagus. However, our patient’s clinical presentation was not compatible with these conditions. Radiologic imaging including a chest CT scan failed to reveal any other abnormality except a hyperplastic thymus.
Because there have been more cases of thymic uptake in patients with normal thymus glands than thymic hyperplasia after posttherapy I131 scans, uptake in this location must always be assessed as to the potential of it not being thyroid tissue. Verifying this, we failed to find thyroid tissue in our case of thymic uptake of I131 either histologically or by thyroglobulin staining.
In summary, we present the second case of thymic hyperplasia causing a falsely positive posttherapy I131 scan. Typical thymic histology and absent thyroglobulin staining of the thymus confirmed the lack of thyroid cells.
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