OBJECTIVETo characterize the morphological features of thyroid papillary microcarcinoma (PMC) and assess the significance of expression of CK19, HBME-1, Galectin-3, CD56 and p63 in differential diagnosis of PMC from benign thyroid lesions.

METHODSClinicopathologic features of 78 cases PMC were reviewed. Immunohistochemical analysis of CK19, HBME-1, Galectin-3, CD56, and p63 in 78 cases of PMC and 48 cases of benign thyroid lesions (18 cases of papillary hyperplasia, 17 cases of nodular goiter and 13 cases of lymphocytic thyroiditis) was conducted. The patients were followed up for from 6 to 269 months after surgical operation.

RESULTS69 cases nuclear atypia and overlapping nuclei (88.5%), 67 cases nuclear grooves (85.9%), 50 cases nuclear pseudoinclusions (64.1%) and 60 cases papillary architecture (76.9%) were detected in 78 cases of PMC. Moderate to strong co-expression of CK19, HBME-1 and galectin-3 was observed in 98.0% (50/51) in the PMC group but in none of the benign disease group. The expression of CD56 and p63 was negative in both groups. In the postoperative follow-up period of 6-269 months, 7 cases (9.0%) developed intrathyroid recurrence, 3 cases (3.8%) developed lymph node metastasis, no distant metastasis or death was observed. In 12 cases (15.4%) the PMC lesion smaller than 3 mm in diameter was not found by frozen section diagnosis.

CONCLUSIONSOverlapping nuclei, nuclear atypia, polar disorder, ground glass nuclei, nuclear grooves and nuclear pseudoinclusions are most important for the diagnosis of PMC with or without papillary architecture. The appearance of definite interstitial invasion, interstitial sclerosis and true complex papillary architecture are more helpful to make right diagnosis. Intraoperative frozen section is of limited value for a reliable diagnosis of PMC in diameter < or = 3 mm. Moderate to strong co-expression of CK19, HBME-1 and Galectin-3 is a very useful indicator for differential diagnosis of PMC from benign thyroid lesions.