Lung cancer is currently the most frequently diagnosed major cancer in
the world and the most common cause of cancer mortality world- wide.
This is largely due to the carcinogenic effects of cigarette smoke. Over
the coming decades, changes in smoking habits will greatly influ- ence
lung cancer incidence and mortality throughout the world. These changes
may also impact upon the histological types of lung cancer. Tumour
classification is important for consistency in patient treat- ment, and
because it provides a basis for epidemiologic and biologi- cal studies.
The previous WHO classification was published in 1981 and since then
considerable progress has been made in our understanding of cer- tain
lung tumours 1. The concept of neuroendocrine tumours of the lung has
been refined with recognition of large cell neuroendocrine carci- noma
and modification of criteria for atypical carcinoid. Atypical ade-
nomatous hyperplasia is now recognized as a potential precursor to
adenocarcinoma. Studies have documented the histological hetero- geneity
of lung carcinomas, particularly among adenocarcinomas and poorly
differentiated carcinomas. Molecular studies have also shown that
hamartomas and sclerosing hemangiomas are true neoplasms rather than
tumour-like lesions. This classification is based on histological
characteristics of tumours seen in surgical or needle biopsy and autopsy
material. Though a large percentage of lung carcinomas are now diagnosed
on cytology specimens, the classification does not address cytology.