KI-67 Antigen Expression Predicts Survival and Correlates with Histologic Subtype in the WHO Classification of Thymic Epithelial Tumors

We performed an immunohistochemical study with monoclonal antibodies to Ki-67 antigen and p53 protein on 45 cases of thymic epithelial tumors classified according to the recent World Health Organization (WHO) classification system to evaluate whether there is correlation between the expression of these markers and prognosis, histologic subtype, and myasthenia gravis (MG). We also correlated histologic subtype with sex, age, MG, and survival. Ki-67 and p53 labeling indices (LIs) were expressed as a percentage of positive nuclear immunostaining by counting 1,000 epithelial tumor cells. Statistically significant differences were found between Ki-67 LI and survival (p = 0.007), whereas the prognostic implication of p53 could not be demonstrated, although there appeared a trend that patients with tumors of higher LIs had worse survival. Significant correlations were also found between Ki-67 (p < 0.0005) and p53 (p < 0.0005) LIs and histologic subtypes. No correlation was found between these parameters and MG. Histologic subtypes of the WHO classification also correlated with survival (p = 0.01), whereas no correlation was found with sex, age, and MG. In conclusion, our results indicate that the proliferative activity, assessed by Ki-67 LI, and the histologic pattern, according to WHO classification system, seems to represent reliable parameters in the prognosis of thymic epithelial tumors.