Diagnostic criteria of well differentiated thyroid tumor of uncertain malignant potential; a histomorphological and immunohistochemical appraisal.

Background: Well differentiated thyroid tumor of uncertain malignant potential (WDT-UMP) represents a true "gray zone" of "follicular patterned" thyroid lesions, that needs to be characterized in order to outright the diagnosis of carcinoma and avoid unnecessary aggressive treatment.

Aim: To emphasize on the histomorphological criteria for more accurate diagnosis of WDT-UMP. Also to compare the immunohistochemical expression of CK19 of WDT-UMP versus adenoma and papillary thyroid carcinoma (PTC).

Materials And Methods: The study included 60 thyroid specimens; 18 WDT-UMPs, 24 PTC (18 classic variant and 6 follicular variants) and 18 benign thyroid lesions (8 adenoma, 6 Hashimoto's thyroiditis and 4 hyperplastic nodules). H&E stained sections were assessed according to the published major and minor criteria of malignancy in the thyroid. CK 19 immunostaining was examined and evaluated according to the proportion and intensity scores.

Results: We could detect the absence of nuclear inclusions, presence of characteristic nuclear groove, nuclear clearing, ovoid nuclei, nuclear crowdness, nuclear enlargement and pleomorphism as important reliable features for diagnosis of WDT-UMP with p value (<0.0001 for each). WDT-UMP showed moderate to strong CK 19 immunostaining with proportion scores 3 and 4; an intermediate expression profile; higher than adenoma and less than papillary carcinoma (p<0.0001).

Conclusion: The constellations of both major and minor criteria of malignancy are important clues for WDT-UMP diagnosis which could be ascertained by CK 19 immunostaining.