Interobserver and Intraobserver Variation Among Experts in the Diagnosis of Thyroid Follicular Lesions With Borderline Nuclear Features of Papillary Carcinoma

Distinguishing follicular variant of papillary carcinoma (FVPC) from follicular adenoma and follicular carcinoma can be difficult if nuclear features of papillary carcinoma are not well developed or only focally present. We assessed interobserver and intraobserver agreement among 6 thyroid experts by using 15 cases in which original pathologists suspected FVPC. There was unanimous expert agreement in diagnosing FVPC in only 2 cases (13%) and majority agreement in 6 cases (40%). Unanimous agreement on benign and malignant diagnoses was seen in 4 cases (27%) and majority agreement on malignancy in 8 cases (53%). Intraobserver agreement ranged from 17% to 100%. Histologic features considered most helpful in diagnosing FVPC were nuclear clearing, nuclear grooves, nuclear overlapping and crowding, nuclear membrane irregularity, and nuclear enlargement. This considerable interobserver and intraobserver variability in the diagnosis of FVPC seems to result from lack of agreement on the minimal criteria needed to diagnose FVPC, even among experts.