Histological classification of lung cancer.

In 1924, when lung cancer was an uncommon disease , Marchesani proposed a classification of lung cancer into four histological groups. This was later expanded in the World Health Organisation's his-tological classification of lung tumours of 1967.' Further modification was found necessary and a second edition was produced by the WHO in 1981.2 This includes, under the heading "Malignant epithelial tumours," eight major groups, 12 variants or subtypes, and additional recommended subgroups based on the degree of differentation. Whimster in a volume reviewed on page 178 of this issue refers to some 70 tumours or tumour like conditions which have been reported as occurring in the lungs or pleura.3 How can we reconcile such apparent pathological complexity with the need for a histological classification of lung tumours appropriate for day to day clinical practice? Most clinicians and some pathologists ignore the extended pathological classifications and manage with a condensed version of four headings: squamous carcinoma, adenocar-cinoma, small cell undifferentiated carcinoma, and large cell undifferentiated carcinoma without further subdivision-shades of Marchesani. Such a shorthand summary of lung cancer classification is valuable for day to day discussion but is it valid as a serious classification given the complexity of the lat-est WHO classification? Is there evidence that the numerous subtypes detailed in this classification have any clinical significance? There are some features in the natural history of tumours which appear to be related to their histolog-ical type. The rapid growth and early metastasis of small cell carcinoma is well recognised. Squamous carcinomas tend to grow slowly and metastasise late and a large proportion have not extended beyond the thorax at the time of death, whereas adenocar-cinomas and large cell undifferentiated carcinomas fall in between these two extremes. When groups of tumours of comparable stage are compared, differences in prognosis can be shown to be related to the cell type.4 Differences in tumour behaviour are also revealed by response to treatment. For example, EH8 9AG. among patients undergoing resection who had diseased but resectable mediastinal nodes, those whose tumours had a squamous pattern of differentiation had a significantly better prognosis than those with either adenocarcinomas or large cell undifferenti-ated carcinomas5 and similar, better results for squamous carcinoma are seen for radiotherapy.6 When adenocarcinomas and squamous carcinomas are subdivided by degree of differentiation the better differentiated tumours have a less aggressive natural history than the poorly differentiated.7 The response of small cell carcinoma to …