Principles of lymph node and distant metastasis and metastasis classification

The process of lymphatic and hematogenous metastasis has been elucidated in recent years by thorough morphological studies on resection specimens and new findings in the field of molecular biology. However, this process is far from being completely understood. Metastasis is a multistep process requiring numerous interactions of tumor cells with the surrounding matrix. Especially, adhesion, invasion of basement membranes and passage through extracellular matrix are active processes taking place in the tissue of the primary tumor and later on in host tissues. Metastases to distant organs can be the result of a venous invasion of the primary tumour as well as lymphovenous short-circuits or by the thoracic duct. Using immunohistological methods, tumor cells can be detected in lymph nodes, blood and bone marrow. This is not termed metastasis, these cells are classified as isolated tumor cells (M1(i)). The frequency of lymph node metastasis depends on the intensity of histological examination. In colorectal carcinomas the risk of metastatic lymph nodes can be estimated by conventional parameters such as pT category of the primary tumor, histological grade or invasion of lymphatic vessels. The benefit of applying molecular markers (such as nm23-H1) is unclear. The successful removal of hepatic metastasis of colorectal carcinomas requires a more precise classification. A proposal of a more detailed and clarified classification will be discussed.