Background: With the use of May-Grunwald-Giemsa staining, cytologic features of the follicular variant of papillary thyroid cancers (FVPTCs) on thyroid fine-needle aspiration (FNA) often resemble those of hyperplastic and adenomatous nodules. Detection of reduced staining after thyroid peroxidase (TPO) immunohistochemistry with monoclonal antibody MoAb47 has been shown to be a helpful diagnostic marker. The purpose of this study was to assess the value of TPO immunohistochemistry for the diagnosis of FVPTC.
Methods: Of 3505 patients with adequate FNA samples, 1576 patients underwent surgical procedures. Histologic examination of the surgical specimen demonstrated papillary thyroid cancer in 227 cases, including 42 FVPTCs (18%). The diagnostic accuracy of standard thyroid FNA and TPO immunohistochemistry, which we use routinely, was compared in these 42 FVPTCs.
Results: Standard FNA allowed accurate diagnosis of malignancy in 32 of the 42 FVPTCs and in 170 of the 185 typical papillary thyroid cancers. TPO-immunohistochemistry staining was accurate in all 42 FVPTCs and in 182 of 185 typical PTCs.
Conclusions: FVPTC is a frequent source of false-negative results on standard thyroid FNA. This study shows that TPO immunostaining accurately demonstrates malignancy in these tumors.