Peritoneal Metastasis as a Distinct Biological Entity: Mechanisms, Microenvironment, and Therapeutic Implications

For decades, peritoneal metastases (PM) have been regarded as a terminal manifestation of advanced malignancies and managed primarily with palliative intent because of limited sensitivity to systemic therapies. Accumulating clinical, molecular, and immunological evidence now supports the view that PM is not merely an anatomic pattern of spread but a distinct metastatic niche with characteristic biological, microenvironmental, and therapeutic features. This review summarizes the major routes of PM development—transcoelomic, lymphatic, and hematologic dissemination—and emphasizes how these pathways converge through shared biological programs. Core mechanisms include epithelial–mesenchymal transition (EMT), adhesion signaling, extracellular matrix remodeling, and tumor–immune cell interactions. A central focus is the peritoneal tumor microenvironment: mesothelial-to-mesenchymal transition, cancer-associated fibroblast activity, adipocyte-derived metabolic support, macrophage polarization, and regulatory T-cell enrichment collectively shape an immunotolerant and treatment-resistant niche on the peritoneal surface. In addition, evidence from pre-metastatic niche biology suggests that primary tumor-derived exosomes and epitranscriptomic regulation can prime the peritoneal environment before overt implantation. These features provide a biological rationale for locoregional strategies such as cytoreductive surgery and hyperthermic intraperitoneal chemotherapy, as well as emerging intraperitoneal modalities and microenvironment-targeted approaches. Finally, organoid platforms, liquid biopsy-based minimal residual disease monitoring, and theranostic technologies may enable more personalized, biology-driven management of PM.