[Non-Hodgkin's lymphomas: principles and application of the Kiel classification]

In the Kiel classification malignant non-Hodgkin's lymphomas (NHL) are defined essentially according to their constituent cells, based on their morphologically and immunologically defined normal equivalents. For pragmatic reasons they are also divided into low and high grade malignant lymphomas. The most important condition for applying the classification is technical excellence, especially in embedding. The usual criteria of malignancy are only partially applicable to the division into reactive and malignant lymph node diseases. There are borderline cases between nonneoplastic and neoplastic lymph node diseases that cannot presently be distinguished with certainty with any method--including immunohistochemistry and molecular and classical genetics. Thus the gold standard of lymphoma diagnostics remains the subtle morphological investigation. In doubtful cases there should be close feedback between pathologists and clinicians.