Using CIC and HoCIC as diagnostic biomarkers for malignant pleural and ascitic effusion.

BACKGROUND: Malignant pleural and ascitic effusion (PAE) represents a prevalent complication of advanced cancer with significant diagnostic challenges. Current cytological examination lacks optimal sensitivity, necessitating novel diagnostic targets. Cell-in-cell (CIC) structures, characterized by one cell engulfing another, represent an emerging morphological biomarker with potential diagnostic and therapeutic implications. This study evaluates the diagnostic efficacy of CIC and its homotypic subtype (HoCIC) as independent morphological targets for malignant PAE detection. METHODS: A total of 584 PAE samples were prepared as cell blocks from consecutive patients at a tertiary center (June 2020-May 2024). CIC structures were quantified in hematoxylin and eosin (H&E)-stained sections (per 10 high-power fields, 40× objective). Homotypic CIC (HoCIC) was identified using immunohistochemical (IHC) profiling (CK-pan, CD68) to distinguish tumor cell-tumor cell engulfment. Diagnostic performance was assessed by sensitivity, specificity, accuracy, and receiver operating characteristic (ROC) analysis with Youden's index optimization. RESULTS: Among 357 malignant and 227 benign PAE cases, CIC was detected in 87.39% (312/357) versus 8.37% (19/227) of benign cases (p < 0.001). HoCIC demonstrated higher specificity, present in 77.31% (276/357) malignant but only 1.32% (3/227) benign cases (p < 0.001). Diagnostic metrics for CIC: sensitivity 87.4%, specificity 91.6%, accuracy 89.0%, NPV 92.6%; for HoCIC: sensitivity 77.3%, specificity 98.7%, accuracy 85.6%, PPV 98.9%. Optimal cut-off values were 0.5/10 HPF or 1/20 HPF (Youden's index 0.79 for CIC, 0.76 for HoCIC). Both markers showed significant independent discriminatory value. CONCLUSION: CIC and HoCIC represent validated, readily implementable morphological biomarkers with complementary diagnostic utilities-CIC optimized for sensitive screening and HoCIC for specific confirmation. Integration of this tiered morpho-molecular approach into routine cytopathology workflows may enhance diagnostic precision and guide therapeutic decision-making in advanced malignancies.