The international histological classification of tumours.

This article reviews the development of the WHO project on the histological classification of tumours, which has included the establishment of several collaborating centres and has involved more than 300 pathologists in over 50 countries. The project has resulted in the publication, over the last 14 years, of 25 volumes in the first series of the International Histological Classification of Tumours (IHCT), each giving a classification of tumours specific to a certain site. The classifications are based primarily on the microscopic characteristics of the tumours and are concerned with morphologically identifiable cell types and histological patterns as seen by means of light microscopy and conventional staining techniques. The article also describes the relationship between IHCT and other classification and coding systems and assesses possible future developments that may result from new approaches to diagnosis.