Malignant effusions serve as a feasible source for therapy-related biomarker testing in advanced gastric cancer

Aims Malignant effusion is a common manifestation of metastatic gastric cancer. This study compared the expression of key biomarkers (human epidermal growth factor receptor 2 (HER2), programmed cell death ligand 1 (PD-L1), claudin 18.2 (CLDN18.2) and fibroblast growth factor receptor 2b (FGFR2b)) between malignant effusion-derived cell blocks (CBs) and matched primary or metastatic tumour tissues and evaluated the longitudinal concordance of biomarker expression in serially collected CBs. Methods Biomarker status was retrospectively assessed by immunohistochemistry in CBs from malignant effusions and paired primary/metastatic gastric adenocarcinomas. HER2+ cases underwent fluorescence in situ hybridisation to confirm amplification. Results CLDN18.2 showed the highest positivity rate (34.0%). PD-L1 expression was positive in 8.6% of tumours and 13.9% of stroma. Lower rates were observed for FGFR2b (3.8%) and HER2 (3.3%). FGFR2b positivity was more frequent in CBs than in primary tumours (p=0.044) with higher expression levels (p=0.02). Conversely, CLDN18.2 positivity was lower in CBs versus primary tumours (p=0.004). Metastatic lesions showed higher CLDN18.2 positivity (p=0.04) and expression levels (p=0.002) compared with CBs. Serial CB analysis revealed high concordance rates: FGFR2b (99.1%), HER2 (98.1%), tumour PD-L1 (94.8%) and CLDN18.2 (92.2%). Conclusions Malignant effusion CBs show favourable intra-patient biomarker consistency over time, supporting their feasibility for longitudinal monitoring. Effusion-based testing shows potential value for potentially identifying candidates for FGFR2b-targeted therapies, pending further clinical validation.