The aim of our study was to provide population-based data on incidence and prognosis of synchronous peritoneal carcinomatosis and to evaluate predictors for its development. Diagnosed in 1995-2008, 18,738 cases of primary colorectal cancer were included. Predictors of peritoneal carcinomatosis were analysed by multivariable logistic regression analysis. Median survival in months was calculated by site of metastasis. In the study period, 904 patients were diagnosed with synchronous peritoneal carcinomatosis (4.8% of total, constituting 24% of patients presenting with M1 disease). The risk of peritoneal carcinomatosis was increased in case of advanced T stage [T4 vs. T1,2: odds ratio (OR) 4.7, confidence limits 4.0-5.6), advanced N stage [N0 vs. N1,2: OR 0.2 (0.1-0.2)], poor differentiation grade [OR 2.1 (1.8-2.5)], younger age [<60 years vs. 70-79 years: OR 1.4 (1.1-1.7)], mucinous adenocarcinoma [OR 2.0 (1.6-2.4)] and right-sided localisation of primary tumour [left vs. right: OR 0.6 (0.5-0.7)]. Median survival of patients with peritoneum as single site of metastasis remained dismal [1995-2001: 7 (6-9) months; 2002-2008: 8 (6-11) months], contrasting the improvement among patients with liver metastases [1995-2001: 8 (7-9) months; 2002-2008: 12 (11-14) months]. To conclude, synchronous peritoneal metastases from colorectal cancer are more frequent among younger patients and among patients with advanced T stage, mucinous adenocarcinoma, right-sided tumours and tumours that are poorly differentiated. The prognosis of synchronous peritoneal carcinomatosis remains poor with a median survival of 8 months and even worse if concomitant metastases in other organs are present. Copyright © 2010 UICC.
