To predict clinical outcome by classification of peritoneal metastases (PM) of colorectal or appendiceal origin.

This study investigates whether standardized histological classification can predict outcome for PM of colorectal or appendiceal origin treated with cytoreduction and hyperthermic intraperitoneal chemotherapy (HIPEC).

Histology of PM (n = 269) was evaluated by analysis of mitotic activity, atypia, cellularity, and mucinous component. For overall survival (OS) and disease-free survival (DFS) Cox proportional-hazard models were constructed. Covariates included tumor, patient, and treatment characteristics.

PM could be categorized into four groups: low-grade, well-differentiated mucinous tumor (DPAM); intermediated-grade mucinous carcinoma (PMCA-i); high-grade mucinous carcinoma (PMCA); and high-grade nonmucinous carcinoma (PCA). Multivariate analysis showed that histological classification, gender, number of segments affected, completeness of cytoreduction, and HIPEC as primary treatment were significant related to OS and DFS. The 5-year OS was 64% in the DPAM group, 36% in the PMCA group, and 24% in the PCA group. Of PM originating from an appendix tumor, 29% were of non-DPAM type. Of primary colorectal tumors, 37% resulted in mucinous PM, and another 26% of PM of colorectal origin had partly mucinous histology.

Histology is a significant predictive factor of OS and DFS in PM treated with surgical cytoreduction and HIPEC. Low-grade PM (DPAM) should be regarded as a separate entity because of its clearly different clinical course. High-grade mucinous PM has significant better prognosis than nonmucinous PM and should thus be distinguished.