"Micrometastases": the pathologist's point of view

The development of cancer screening has led to the discovert of smaller tum ours and less frequent dissemination to lymph nodes and organs that require s special techniques for detection. Numerous papers on micrometastases refl ect a considerable amount of work devoted to detection methods, technical p roblems and the prognostic value of these lesions. Apart from cytological t echniques, the pathologist can rely on two methods for the detection of mic rometastases: serial slicing of paraffin-embedded blocks and immunohistoche mistry. When these methods are combined, the detection rate is similar to t hat of biological methods and can attain levels as high as 60% for the sent inel node with the added vantage of being able to visualise cells. Despite an impressive body of studies, major disparities are found in detection rat es and the prognostic value of micrometastases is not firmly established. I n 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 t o 2 mm in size. The potential aggressiveness of micrometastases is dependen t 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 n ew pTNM classification takes into account this latter parameter and disting uishes two categories of micrometastases: "isolated tumor cells" located in the lumen of vessels or sinuses and "micrometastasis" which has already in vaded an organ. This classification warrants further analysis to determine the prognostic value of these categories. The next challenging problem cons ists in determining the key biological properties that account for distant dissemination.