Abstract 708: Limited release of circulating tumor DNA into the systemic circulation by peritoneal metastases from colorectal cancer

Introduction: Circulating tumor DNA (ctDNA) from plasma is a promising biomarker. Patients with metastatic colorectal cancer (CRC) often have high ctDNA levels compared to other cancer types. However, it is unclear whether this is also the case for patients with peritoneal metastases from CRC. Aim: To compare the propensity of ctDNA shedding by isolated liver metastases and isolated peritoneal metastases from CRC. Methods: Plasma was collected from 100 CRC patients (64% male, mean age of 60 [SD=10] years) with isolated unresectable liver metastasis (CRC-LM). Plasma and ascites were obtained from 20 CRC patients (60% male, mean age of 63 [SD=9.8] years) with isolated unresectable peritoneal metastases (CRC-PM). All liquid biopsies were obtained prior to treatment and cell-free DNA was isolated using the QIAsymphony (Qiagen, Germany) and assessed by droplet digital PCR (ddPCR; Bio-Rad, Hercules, USA). Patients with a KRAS or BRAF tumor tissue mutation were suited for plasma ctDNA ddPCR analysis, and were detected in 57 CRC-LM (57%) and 11 CRC-PM (55%) patients. The ability to shed ctDNA into the circulation was evaluated by comparing KRAS/BRAF mutation status in plasma and ascites with the mutation status based on tumor tissue. Regarding liquid biopsies, mutant allele fraction (MAF) and mutant copies per ml input (MTc/ml) were reported. Results: Tissue mutations could be confirmed in plasma in 93% of CRC-LM and only 20% of CRC-PM patients, whereas mutations were detected in ascites in 100% of CRC-PM patients. The MAF and MTc/ml were both significantly lower in CRC-PM plasma ctDNA (median MAF=0.28% and MTc/ml=21) compared to CRC-LM plasma ctDNA (median MAF=18.9% and MTc/ml= 1758; P