ctDNA beyond blood: Harnessing pleural effusion circulating tumor DNA for precision lung cancer diagnostics.

e20029 Background: Tissue biopsy-derived DNA has long been the gold standard for tumor-specific genomic profiling. Similarly, genomic profiling using circulating tumor DNA (ctDNA) has been next evolving molecular companion for treatment decisions, monitoring, and escalation/de-escalation of therapy choices. In malignant pleural effusion (MPE), DNA extracted from pleural fluid cell blocks has traditionally served as the primary source for molecular testing to inform cancer treatment decisions but it fails to pass DNA isolation due to less cellular content. The ease of pleural fluid collection, its proximity to the tumor, and its utility for ongoing monitoring position it as a less invasive, practical alternative for lung cancer diagnostics. This approach holds promise for longitudinal monitoring and may reduce the need for repeat biopsies in precision oncology diagnostics. We investigated the potential of molecular testing using ctDNA obtained from pleural effusion cytology supernatants (PE-CCS) alternative to ctDNA from blood. Methods: We conducted a retrospective analysis of the genomic profiles of 15 lung adenocarcinoma patients with samples derived from both blood and malignant pleural effusion. Circulating tumor DNA (ctDNA) extracted from both pleural fluid and plasma was analyzed and compared for actionable alterations using next-generation sequencing (NGS) with the OncoIndx comprehensive demonic panel (CGP) assay. Results: Retrospectively,ctDNA was successfully isolated from both pleural fluid and peripheral blood of 15 lung cancer patients. Among the samples with successful isolation 67% (10/15) demonstrated concordance in actionable or targetable mutations (for genes such as EGFR:p.L858R, EGFR:p.S768I, TP53:p) between ctDNA derived from pleural fluid and peripheral blood. In contrast, 33% (5/15) exhibited actionable mutations detected (EGFR) in ctDNA from pleural fluid that were not identified in ctDNA from peripheral blood. In 50% of patients, variant allele frequency (VAF) (minimum highest VAF of 1% and maximum highest VAF 80%) was higher in pleural fluid than in peripheral blood. Interestingly, in one patient a resistance mutation (EGFR Amplification (13 copies) was detected in pleural fluid but not in peripheral blood. Conclusions: Proximal samples, like pleural fluid, are closer to the tumor site and thus better capture tumor-derived DNA, making them more reliable for diagnostics and therapeutic insights.Pleural ctDNA NGS offers diagnostic performance comparable to tumor biopsies and surpasses pleural cytology and plasma ctDNA in detecting oncogenic mutations in lung adenocarcinoma.