Enhancing the Objectivity of the Japanese Classification of Peritoneal Metastases from Colorectal Cancer

Objective: The Japanese classification of peritoneal metastases from colorectal cancer is easy to use for general surgeons in routine clinical practice. However, the objectivity of this classification has not been determined. This study aimed to clarify the objectivity of the Japanese classification of peritoneal metastases from colorectal cancer. Methods: The data of patients with Stage IV colorectal cancer between 1991 and 2007 in 16 hospitals, who were members of the Japanese Society for Cancer of the Colon and Rectum, were investigated. The size, number and extent (nine areas) of peritoneal metastases according to the Japanese classification (P1, P2 and P3) were investigated using Akaike’s information criterion. Results: Of the 564 colorectal cancer patients with synchronous peritoneal metastases, 341 had hematogenous metastases. The minimum Akaike’s information criterion was obtained with the cutoff value of one area for P1 metastasis and two or more areas for P2 metastasis (P , 0.0001). When P2 metastasis was compared with P3 metastasis, the cutoff value of the number of peritoneal metastases was 10. Conclusions: The present study proposes a revision that would give objectivity to the present Japanese classification as follows: P1 is defined as peritoneal metastases 20 mm or smaller confined to one area; P2 is defined as 10 or fewer peritoneal metastases disseminated in two or more areas, or peritoneal metastases confined to one area but the size is .20 mm and P3 is defined as .10 peritoneal metastases disseminated in two or more areas.