A solid thyroid benign nodule that showed a significant decrease in size and ultrasonographic findings mimicking papillary carcinoma during 16-year follow-up.

Recent advances in ultrasonography and fine needle aspiration biopsy (FNAB) have facilitated accurate diagnosis of thyroid carcinomas that require treatment. However, we often encounter nodules evaluated as malignant on ultrasonography but diagnosed as benign on cytology, for which the optimal treatment strategy remains uncertain. A 28-year-old female had solitary and solid thyroid nodule measuring 6 cm in maximal diameter in February 1994. The lesion was cytologically diagnosed as benign. From September 1998, the nodule spontaneously decreased in size but ultrasonographic findings suspicious of malignancy such as peripheral and intra-tumoral calcification, low internal echo and irregular border gradually appeared. In July 2010, the volume of her nodule showed 97% decrease but was evaluated as papillary carcinoma on ultrasonography. FNAB was performed again and the nodule was diagnosed as benign. When we encounter a nodule showing ultrasonographic findings suggestive of malignancy with negative cytology, we should consider the possibility of a benign nodule degenerating over time.