The
sodium iodide symporter (NIS) has been characterized to mediate the active transport of
iodide not only in the thyroid gland but also in various non-thyroidal tissues, including lactating mammary gland and the majority of breast
cancers, thereby offering the possibility of diagnostic and therapeutic radioiodine application in
breast cancer. In this report, we present a 57-year-old patient with multifocal
papillary thyroid carcinoma, who showed focal radioiodine accumulation in a lesion in the right breast on a posttherapy (131)I scan following radioiodine
therapy. CT and MR-mammography showed a focal solid lesion in the right breast suggestive of a
fibroadenoma, which was confirmed by histological examination. Immunostaining of
paraffin-embedded
tumor tissue sections using a human NIS antibody demonstrated NIS-specific immunoreactivity confined to epithelial cells of mammary ducts. In conclusion, in a
thyroid cancer patient we identified a benign
fibroadenoma of the breast expressing high levels of functionally active
NIS protein as underlying cause of focal mammary radioiodine accumulation on a posttherapy (131)I scan. These data show for the first time that functional NIS expression is not restricted to lactating mammary gland and malignant breast tissue, but can also be detected in benign breast lesions, such as fibroadenomata of the breast.