False positive findings on post thyroidectomy whole body I-131 scans

Learning Objectives To be familiar with false positive findings of post thyroidectomy whole body I-131 scans.

Whole body I-131 scintigraphy is sensitive to detect thyroid tissue remnants, residual tumor or metastatic foci in post thyroidectomy setting. However, I-131 can also be retained by ectopic thyroid tissue, glandular or serous epithelium, and sites of Inflammation, which might decrease the specificity of I-131 scan. Cases from our practice demonstrate several nonspecific physiologic or pathologic accumulation of radioiodine. Some types of uptake are common: Foci of ectopic functioning thyroid tissue along the route of migration may be stimulated by high TSH levels after thyroidectomy, and produce focal activity in the neck/mediastinum. In one case thyroglossal duct tissue mimics a lymph node. Contamination with saliva and nasal secretions can also mimic metastatic disease. Cases include hair contaminated with radioactive saliva, and a handkerchief in the pocket. Other sites of tracer accumulation are less common: Activity at midline of neck after I-131 ablation was due to pooled secretions around a tracheostomy tube. Atypically located gastric mucosa can still accumulate radioiodine. A patient with Barrett’s esophagus demonstrates banana shape activity in the thorax. A patient with hiatal hernia shows activity above the diaphragm and mimics lung metastasis. Although the serous epithelia of cyst linings are not able to actively transport I-131, they may be permeable to passive diffusion, which leads to the accumulation of radioiodine in cysts and pleural or peritoneal effusions. A case with simple cyst in left thigh shows focal activity mimicking femora metastasis. Increased blood flow and capillary permeability at sites of inflammation can lead to deposition of I-131 in the extracellular space soon after administration. Two cases of alpha-1-antitrypsin deficiency demonstrate radioactive activity in tracheobronchial tree and lung parenchyma. In summary, being aware of causes of false positives can improve the accuracy of interpretation of whole body I-131 scans.