Interobserver and intraobserver reproducibility in the histopathology of follicular thyroid carcinoma

We evaluated the interobserver and intraobserver reproducibility in the histopathology of follicular thyroid carcinoma (FTC). Forty-one anonymous FTC pathology slides were independently reviewed by 5 pathologists, and 31 of them were also evaluated twice by the same pathologist. A final consensus diagnosis (FCD) was made at the end of the study. Interobserver and intraobserver agreement was determined as the kappa statistic for qualitative data and intraclass correlation coefficient for quantitative data. The agreement between the 5 observers' initial diagnosis and the FCD was 0.69, 0.41, 0.35, 0.28 and 0.11, respectively, strongly suggesting a leadership phenomenon. The FCD classified 30 cases as malignant, including 24 cases diagnosed as FTC. There was unanimous agreement about 13 of the 24 FTCs. Diagnostic reproducibility was found to be acceptable for the nonminimally invasive FTC. Diagnostic discrepancies occurred in 57% of the seven cases classified as minimally invasive FTC by the FCD. FCD excluded malignancy in 11 cases including 6 atypical adenomas. Interobserver and intraobserver agreement for FTC diagnosis was 0.23 (standard error [SE], 0.04) and 0.68, respectively. Interobserver and intraobserver agreement for the presence of vascular invasion was 0.20 (SE, 0.04) and 0.51, respectively, contrasting with a moderate to substantial level of agreement when considering the number of vascular invasion. Interobserver and intraobserver agreement for nucleus optical clearing were slight and moderate, respectively. The importance of the study is the confirmation that diagnostic reproducibility of minimally invasive FTC is low and that this has clinical implications, and also implications for the design of studies into the treatment and outcome of FTC.