There are many reports that diagnostic accuracy of fine needle aspiration (FNA) of thyroid is improved with ultrasound guidance, especially for impalpable nodules. Despite its general acceptance, routine use of ultrasound guided fine needle aspiration (USGFNA) has been the source of much controversy due to the lack of large-scale studies and lack of data on the natural course of welldifferentiated thyroid cancer of small size.

The aim of our study was to define the rate of malignancy in relatively large numbers of patients with incidentally detected impalpable thyroid nodules and to assess the extent of disease in patients with suspicious or malignant cytology on USGFNA of thyroid nodules by surgery. We retrospectively reviewed the medical records of the 267 patients who underwent USGFNA of incidental thyroid nodules from January 2000 through December 2001.

Three hundred and seventeen nodules from 267 patients were aspirated. The average size of nodules was 0.9±0.3 cm, a range of 0.2 cm to 1.5 cm. All 317 lesions were impalpable. Cytological diagnosis included 101 inadequate specimen (32%), 139 benign (44%), 29 indeterminate (9%), 4 suspicious of follicular or Hrthle cell neoplasm (1%), 42 papillary carcinoma (13%), and 2 others. The size of the nodule was not related to the probability of getting an adequate specimen for cytological diagnosis. Forty of 48 patients with suspicious or malignant cytology underwent surgery. All 35 patients with a cytological diagnosis of papillary carcinoma were confirmed to have papillary carcinoma on histological results. One of 3 patients with a cytological diagnosis of follicular neoplasm had a follicular carcinoma. In 36 patients with well-differentiated thyroid cancer, extrathyroidal extension was observed in 44% (16/36), regional lymph node metastasis was found in 50% (18/36), and multifocal tumors were found in 39% (14/36).

The rate of malignancy in incidentally detected impalpable thyroid nodules was 12% in retrospective analysis of our patients. Among those, 69% (25/36) of patients had either extrathyroidal extension or regional node involvement and 39% had multifocal tumors at surgery. This suggests that the small size itself could not guarantee a good prognosis in incidentally found thyroid cancers. USGFNA is a useful diagnostic method in those patients.