Iodine concentration by the thymus in thyroid carcinoma.

A 14-yr-old boy underwent a total thyroidectomy with bilateral neck dissection for a papillary carcinoma with lymph node metastases. Total-body scanning with 3.7 GBq 131I revealed radioiodine accumulation in the anterior mediastinum. CT and MRI demonstrated a mediastinal mass which corresponded to the area of increased radioactivity. Five months later, another therapeutic dose of 131I was followed by a sternotomy and removal of the thymus because a hand-held radiodetecting surgical probe demonstrated that the thymus was the mediastinal structure which concentrated iodine. Thymus histology was negative for thyroid cancer metastases (as further confirmed by the negative immunostaining) and showed cystic Hassall's bodies. Secondary ion mass spectrometry microscopy demonstrated that iodine was located only in the Hassall's bodies, bound to proteins. This finding suggests that an acquired "thyroid follicle-like" structure, as that observed in cystic Hassall's bodies, could be responsible for the epithelial cell iodine uptake. In conclusion, we have provided evidence for the iodine-trapping property of the cystic Hassall's bodies of the thymus, which may be a possible cause of misleading mediastinal radioiodine uptake.