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Pseudomyxoma peritonei (PMP) is a rare malignant disease. Adding of the Ki67 proliferation index to the PSOGI PMP classification provided two different subcategories of the extensive HG-PMP group (HG-PMP ≤15% and HG-PMP >15%) with different survival in a previous unicentric study. This study aims to carry out an external and multicentre validation of this new proposed classification. It was a prospective analysis of samples from a historical and international cohort of patients. A representative area with higher cellular density was used to determine the Ki67%. The Ki67 proliferation index (%) was determined in all the HG-PMP patients. A Cox proportional hazard models and multivariable COX models were used. The Kaplan–Meier method and the two-tailed log-rank test were used to analyse the effect of different PSOGI-Ki67 categories on OS and DFS. Its predictive accuracy was analysed using Harrel's C-index and the ROC curve. The calibration was performed using the calibration plots matching. After exclusions, 349 patients were available for analysis. The 5-years OS were 86% for LG-PMP, 59% for HG-PMP≤15, 38% for HG-PMP>15 and 42% for SRC-PMP (p = 0.0001). The 5-years DFS were 49% for LG-PMP, 35% for HG-PMP≤15, 16% for HG-PMP>15 and 18% SRC-PMP (p = 0.0001). The discrimination capability of PSOGI-Ki67 was validated. Conclusion: the PSOGI-Ki67 classification discriminates and predicts the OS and DFS in patients with PMP dividing the HG-PMP category into two well-defined sub-categories. The Ki67 proliferation index should be incorporated routinely in the pathology report for these patients. [ABSTRACT FROM AUTHOR]

The metastasizing potential of pseudomyxoma peritonei (PMP) is largely unknown. We assessed incidence, impact on prognosis, treatments, and outcomes of systemic metastases after cytoreductive surgery and hyperthermic intraperitoneal chemotherapy (CRS/HIPEC). A prospective database of 327 patients undergoing CRS/HIPEC for PMP of appendiceal origin was reviewed. PMP was graded according to the Peritoneal Surface Oncology Group International (PSOGI) classification. Haematogenous metastases, and non-regional lymph-node involvement were considered as systemic metastases. After a median follow-up of 74.8 months (95% confidence interval [CI] = 68.0–94.8), systemic metastases occurred in 21 patients. Eleven patients were affected by low-grade PMP, and ten by high-grade PMP. Metastatic disease involved the lung (n = 12), bone (n = 1), liver (n = 4), distant nodes (n = 3), both lung and distant nodes (n = 1). Systemic metastases independently correlated with PSOGI histological subtypes (P = 0.001), and incomplete cytoreduction (P = 0.026). Median OS was 139.0 months (95%CI = 56.6–161.9) for patients who experienced systemic metastases, and 213.8 months (95%CI = 148.7-not reached) for those who did not (P = 0.159). Eight of eleven patients who had curative-intent surgery are presently alive at a median of 52.5 months (range 2.0–112.7). Seven are disease-free at a median of 27.4 months (range 2.0–110.4). At multivariate analysis, PSOGI histological subtypes (P = 0.001), completeness of cytoreduction (P = 0.001), and preoperative systemic chemotherapy (P = 0.020) correlated with poorer survival. Systemic metastases did not (P = 0.861). After CRS/HIPEC, systemic metastases occur in a small but clinically relevant number of patients, and the risk increases with incomplete cytoreduction and aggressive histology. In selected patients, surgical resection of metastatic disease can result in long survival. • A small but clinically relevant number of systemic metastases occur in both high-grade and low-grade pseudomyxoma peritonei • The risk for systemic metastasis increases with incomplete cytoreduction and aggressive histology. • In selected patients, surgical resection of metastatic disease can result in long survival • Future studies addressing the biological features related to systemic spread are warranted. [ABSTRACT FROM AUTHOR]