Introduction

Adenocarcinoma is the commonest histological subtype of lung cancer in most of the world and accounts for almost half of all lung cancers. Over the past 20 years it has become clear that this general category of carcinoma is not nearly as monolithic as once thought. Major advances in epidemiological, radiological, histological, immunohistochemical and molecular research paint a very complex picture. These suggest a wide spectrum of different entities, when viewed from different perspectives. All agree, however, that adenocarcinoma is a malignant epithelial tumor with glandular differentiation and/or mucin production. In light of recent therapeutic advances affecting particular subgroups of adenocarcinoma, histopathologists must be aware of subtle morphological and molecular distinctions. This chapter aims to present the current level of understanding within our traditional morphological framework.

Classification and cell of origin

Tumor classifications dating back to the first World Health Organization in 1967 and progressing through to the 2004 document were conceived by and for pathologists to ensure uniform tumor reporting and to aid clinical trials. Adenocarcinoma subclassification was expanded from three entities in 1967 to five major subtypes and five variants in 2004 (Table 1). This development illustrates increased interest in this tumor and the deluge of information shaping our understanding of the carcinoma. The 2004 classification was the first to include relevant clinical and genetic information.