["Micrometastases": the pathologist's point of view]

The development of cancer screening has led to the discovery of smaller tumours and less frequent dissemination to lymph nodes and organs that requires special techniques for detection. Numerous papers on micrometastases reflect a considerable amount of work devoted to detection methods, technical problems and the prognostic value of these lesions. Apart from cytological techniques, the pathologist can rely on two methods for the detection of micrometastases: serial slicing of paraffin-embedded blocks and immunohistochemistry. When these methods are combined, the detection rate is similar to that of biological methods and can attain levels as high as 60% for the sentinel node with the added vantage of being able to visualise cells. Despite an impressive body of studies, major disparities are found in detection rates and the prognostic value of micrometastases is not firmly established. In order to facilitate comparisons and analyses, it is essential to adopt a common terminology with precise definitions. The UICC advocates the use of the term micrometastasis which denotes a metastasis smaller than or equal to 2 mm in size. The potential aggressiveness of micrometastases is dependent on other poorly explored parameters such as the number of cells detected in the bone marrow or lymph node and the location of micrometastases. The new pTNM classification takes into account this latter parameter and distinguishes two categories of micrometastases: "isolated tumor cells" located in the lumen of vessels or sinuses and "micrometastasis" which has already invaded an organ. This classification warrants further analysis to determine the prognostic value of these categories. The next challenging problem consists in determining the key biological properties that account for distant dissemination.