Are "micrometastases" of the peritoneum equivalent to distant metastases?

Tumor progression after curative resection of gastrointestinal cancer is probably caused by disseminated tumor cells that can be detected in different body compartments, e.g. the peritoneum. The clinical importance and prognostic significance of gross peritoneal metastasis is well known whereas the prognostic relevance of disseminated tumor cells in the peritoneum of patients with gastrointestinal cancer is still unclear. Disseminated tumor cells in the peritoneal cavity are generally detected by cytology, immunohistochemistry or polymerase chain reaction-based molecular methods. A consensus on the most adequate detection method has not yet been found making the comparison of different data difficult. The prognostic relevance of tumor cell dissemination has, at least in part, been shown for gastric, pancreatic and colon cancer, and the prognostic data regarding gastric cancer are most convincing. Peritoneal "micrometastases" are obviously not equivalent to distant metastases as the evidence for their prognostic significance is clearly less than that for gross peritoneal metastases. This article gives a critical review of the detection and prognostic significance of disseminated tumor cells in the peritoneum of patients with gastrointestinal cancer.