Inter-Observer Variation in the Pathologic Identification of Minimal Extrathyroidal Extension in Papillary Thyroid Carcinoma

Background: Extrathyroidal extension (ETE) is a significant prognostic factor in papillary thyroid carcinoma (PTC). Minimal extrathyroidal extension (mETE) is characterized by involvement of the sternothyroid muscle or perithyroid soft tissue, and is generally identified by light microscope examination. Patients with mETE, identified pathologically, are automatically upstaged to pT3. However, the prognostic implications of mETE have been a source of controversy in the literature. Moreover, there is also controversy surrounding the identification of mETE on pathological specimens. The objective of this study was to determine the level of agreement among expert pathologists in the identification of mETE in PTC cases. Methods: Eleven expert pathologists from the United States, Italy, and Canada were asked to perform a review of 69 scanned slides of representative permanent sections of PTC specimens. Each slide was evaluated for the presence of mETE. The pathologists were also asked to list the criteria they use to identify mETE. Results: The overall strength of agreement for identifying mETE was slight (κ = 0.14). Inter-pathologist agreement was best for perithyroidal skeletal muscle involvement (κ = 0.46, moderate agreement) and worst for invasion around thick-walled vascular structures (κ = 0.02, slight agreement). In addition, there was disagreement over the constellation of histologic features that are diagnostic for mETE, which affected overall agreement for diagnosing mETE. Conclusions: Overall agreement for the identification of mETE is poor. Disagreement is a result of both variation in individual pathologists' interpretations of specimens and disagreement on the histologic criteria for mETE. Thus, the utility of mETE in staging and treatment of PTC is brought into question. The lack of concordance may explain the apparent lack of agreement regarding the prognostic significance of this pathologic feature.