[The development of metastases and their classification]

The process of metastasis has been elucidated by thorough morphological studies on resection specimens and new findings in the field of molecular biology in recent years. The successful surgical removal of distant metastases lead to the necessity of a detailed and precise classification system of distant metastases.

The new molecular findings as well as data obtained from morphological studies and the results of a international field study on behalf of the UICC lead to the proposed system presented here as a new classification system for metastases.

A general classification of distant metastases of any primary tumor should separate 5 different categories: M1 for non-regional lymph node metastases only, M2 for liver metastases only, M3 for lung metastases only, M4 for bone metastases only and M5 for metastases in 2 or more of these organs or others. In case of liver metastases of colorectal carcinoma, 2 different classifications were proposed; a clinical classification based on locoregional tumor status, number of liver metastases and involved lobes and a post-surgical classification depending on locoregional tumor status and size of metastases removed surgically and assessed by the pathologist.

More detailed knowledge of the metastatic process and success in surgical treatment makes a detailed and exact classification system for distant metastases essential. This should be used as a fundamental base for future therapeutical and prognostic studies.