The Classification of Pre-invasive Lesions

Abstract

This chapter discusses the three pre-invasive lesions recognised in the WHO classification of lung tumours. Squamous dysplasia and carcinoma in situ is a group of lesions recognised in the epithelium of bronchi. These lesions probably originate from hyperplasia of basal cells of the bronchial epithelium and ultimately may progress to invasive squamous cell carcinoma and possibly other bronchogenic tumours. Squamous dysplasia is classified in three grades; mild, moderate and severe dysplasia, with carcinoma in situ a separate, more advanced lesion. Distinct from central bronchial squamous carcinogenesis, atypical adenomatous hyperplasia (AAH) and adenocarcinoma in situ (AIS) are the pre-invasive lesions which define peripheral lung adenocarcinogenesis. These lesions arise from peripheral airway bronchiolo-alveolar epithelium, sometimes referred to as the terminal respiratory unit. AAH may be regarded as ``dysplasia'' arising in this epithelium and is a precursor of AIS, which then gives rise to peripheral type pulmonary adenocarcinoma. The least common, by far, of the pulmonary pre-invasive lesions is diffuse idiopathic pulmonary neuroendocrine cell hyperplasia. A rare condition with an associated asthma-like syndrome, this condition is associated, in most examples, with the development of peripheral, spindle cell-type carcinoid tumours, hence its inclusion in the WHO classification.