Involvement of the peritoneum by metastatic colorectal cancer cells is commonly defined as peritoneal carcinomatosis. However, this term is now considered obsolete and should be replaced by the term peritoneal metastases (PMs), in analogy to the other main sites of metastasis (liver, lung and non-locoregional lymph nodes in primis). Approximately one quarter of patients with metastatic CRC have PM at onset, and even after radical surgery, CRC PM are estimated to develop in 15% of cases, an incidence that is probably underestimated.

Colorectal peritoneal metastases (CRC PMs) are an insidious site of metastasis because of their impact on bowel function (stenosis and dysmotility) and quality of life of affected patients and are often the cause of death in most of them (so-called peritoneal cancer syndrome).

Patients with CRC PM have significantly worse survival in comparison with patients with CRC at other metastatic sites. Even after optimal treatment, survival in CRC PM can be prolonged up to 24 months with modern systemic chemotherapy regimens. The reasons why CRC PM is difficult to treat are somewhat intuitive but remain largely unknown and relatively understudied. Intrinsic adverse factors, such as the higher incidence of mucinous/signet ring cell histology and BRAF status in patients with CRC PM, are one of the possible explanations. Moreover, very few studies have investigated the role of epithelial-mesenchymal transition, angiogenesis and inflammation, and the microenvironment of the peritoneal nodules in terms of chemoresistance.