Background: CD44 isoforms serve as a marker for cancer stem cells. CD44 variant 9 (CD44v9) contributes to the defense against reactive oxygen species, resulting in resistance to chemoradiotherapy. However, the significance of CD44v9 in patients with lung adenocarcinoma is unknown. Methods: We used immunohistochemical analysis to retrospectively analyze CD44v9 expression in 268 surgically resected lung adenocarcinomas and investigated the association between CD44v9 expression and patients’ clinicopathological features. Results: The expression of CD44v9 in 193 of 268 (72.0%) patients was significantly associated with early-stage cancer, low-grade tumors, absence of vessel and pleural invasion, and a mutated epidermal growth factor receptor (EGFR) gene. Multivariate logistic analysis revealed that CD44v9 expression was significantly associated with early-stage disease [odds ratio (OR) 0.29, 95% confidence interval (CI) 0.14–0.59; p < 0.001] and mutant EGFR (OR 2.53, 95% CI 1.06–6.04; p = 0.036). The percentage of CD44v9-positive tumors was higher in the earlier stages of disease; however, there was no significant difference in the survival of patients in each stage of disease who had positive or negative CD44v9 expression. Conclusion: CD44v9 was highly expressed in EGFR-mutant tumors, particularly in early-stage lung adenocarcinoma, suggesting that CD44v9 expression may play an important role in EGFR-mutant tumors.
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