INTRODUCTION

Carcinoma is defined as a malignant tumour of epithelial tissue. A variety of types are recognised, reflecting the tissue of origin. Broadly, malignant tumours of squamous epithelial origin, or squamous carcinomas, are differentiated from the tumours of glandular epithelia – the adenocarcinomas. Carcinomas constitute the most prevalent human malignancy, accounting for approximately 85% of tumours worldwide.

AETIOLOGY AND PATHOGENESIS

Certain crucial concepts have long been identified that, in part, serve to explain the pathogenetic mechanisms of this group of tumours. Firstly, carcinomas are a phenomenon associated with ageing. They are uncommon before 50 years, but thereafter increase exponentially with increasing age. Furthermore, there are often distinct geographical trends associated with carcinomas of different organ systems. It was therefore postulated early on that the aetiology of carcinoma has its origin in the environment. This is now largely established. General aspects of the pathogenesis of carcinomas have been covered in Chapter 17. There are a number of additional factors of special relevance to carcinomas. These include:

Many chemicals are now identified as causing malignant disease. Probably the best example is the relationship between cigarette smoking and bronchogenic carcinoma, one of the commonest visceral malignancies worldwide. Early epidemiological studies identified a strong association between most carcinomas of the lung and chronic tobacco usage. At a histological level, the simple columnar epithelium that lines most of the respiratory tract is subjected to constant irritation by smoke inhalation and responds by changing its phenotype to that of a mature squamous epithelium, a process known as metaplasia. Thereafter a percentage of these metaplastic epithelia undergo further cytological changes with increasing abnormality in numbers, shape, size and orientation of the epithelial layer, the process of dysplasia, considered a pre-neoplastic lesion. Once a highgrade dysplasia, sometimes loosely termed carcinoma in situ, is identified, this abnormal tissue may invade across the basement membrane of the epithelium, thus progressing to invasive carcinoma.