Challenges in staging systems for colorectal cancer: clinical significance of metastatic lymph node number in colorectal cancer and mesorectal extension in rectal cancer.

In many countries, treatment for cancer is performed based on staging systems in which the degree of cancer development is defined objectively. A common staging system is thus needed to compare outcomes. The staging system for colorectal cancer in Japan has been made to enhance consistency with the TNM classification, and the categorization of metastatic lymph nodes and depth of invasion have been revised in recent years. Although these are important factors that determine disease stage, relationships between each factor and recurrence have shown differing prognoses. In our retrospective study, the prognosis of a group with only one metastatic lymph node was significantly better compared to a group with > or = 2 metastatic lymph nodes. In addition, rectal cancer with mesorectal extension >5 mm showed low relapse-free survival rates and high recurrence rates. The validity of staging systems should thus be inspected from various perspectives.