Purpose: We devised a simple dichotomous classification system and showed sufficient reproducibility to indicate treatment strategies for peritoneal metastasis of colorectal cancer. Methods: We included 67 patients with peritoneal metastasis of colorectal cancer and classified them according to the largest lesion size, number of lesions and number of regional peritoneal metastases. The oncological data were recorded and compared. Results: According to the univariate analyses, the prognoses were significantly better in patients with ≤3 disseminated lesions than in those with ≥4, and in patients with disseminated lesions in only one region than in those with ≥2 lesions. A multivariate analysis showed that primary tumor resection and the presence of peritoneal metastases in only one region were favorable factors for the patient survival. Patients with disseminated lesions in only one region (localized group) and those with nonlocalized lesions had three-year survival rates of 45.6 and 12.2 %, respectively. Finally, primary tumor resection improved the prognoses in both the localized and nonlocalized groups. Conclusions: Colorectal cancer patients were categorized into localized and nonlocalized groups according to the number of regions with peritoneal metastasis, and significant prognostic associations were demonstrated. Subsequent analyses of the oncological data suggested that primary tumor resection contributes to an improved prognosis in all patients with synchronous peritoneal metastases.
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