Squamous cell carcinoma of the lung

Introduction Although squamous cell carcinoma (SCC) of the lung has been supplanted by adenocarcinoma as the most common histological type of non-small cell carcinoma (NSCLC) in pockets of the developed world, this carcinoma is by no means a rare entity. In fact, incidence rates for SCC in men and women still surpass those of adenocarcinoma in several European countries. The tobacco-related malignancy probably represents at least 30% of newly diagnosed lung cancers worldwide, but exact figures are difficult to ascertain. Since SCC has higher resectability and perhaps cure rates than other lung carcinomas, this subtype is overrepresented in surgical series and underrepresented in autopsy series. Basic pathogenetic mechanisms have been known for decades and our ability to monitor high-risk patients has greatly improved (see Chapters 23 and 24). Given the recent emphasis on distinguishing SCC from adenocarcinoma for therapeutic reasons, this chapter will present our current understanding of this carcinoma and highlight recent developments in diagnosis. Classification and cell of origin The 2004 World Health Organization (WHO) Classification is the globally recognized lung tumor classification scheme. The classification is descriptive and almost entirely based on light microscopic features. Squamous cell carcinoma is a malignant epithelial tumor with keratinization and/or intercellular bridges. Although many morphological variants have been described, the WHO limits the classification to papillary, clear cell, small cell, and basaloid subtypes (Table 1). Squamous cell carcinoma may also be combined with adenocarcinoma or small cell carcinoma (SCLC) in addition to other malignancies. A diagnosis of adenosquamous carcinoma requires that at least 10% of the SCC tumor features either an unequivocal glandular component or alternatively at least 10% of an obvious adenocarcinoma contains unequivocal keratinization or intercellular bridges. Of note, rare mucin vacuoles can be seen in typical SCC. Squamous cell carcinoma seen in association with SCLC is termed combined SCLC (see Chapter 31).