151751-Najiba-Chargi

12 CHAPTER 1 GENERAL INTRODUCTION The inside of the human mouth, throat and airway ducts is composed of epithelial cells who form a protective network of cells. The outer lining of these cells are squamous cells. Epi - thelial cells are continually replaced. A mutation during replication of squamous cells leads to mutated cells who divide uncontrollably and invade surrounding tissues. This marks the start of head and neck squamous cell carcinomas (HNSCC). The vast majority of head and neck cancers are HNSCC. Despite the fact that HNSCC arise from one cell type in the head and neck region, HNSCC are remarkably heterogeneous. This heterogeneity is in part caused by the different etiologies, complexity of the anatomical sites in which it develops and the large molecular heterogeneity. 1 Head and neck cancer is the ninth most common cancer type worldwide with an estimated 835.000 new cases and 428.000 deaths in 2018. 2 In the Netherlands, approximately 3000 pa- tients are newly diagnosed with head and neck cancer and approximately 900 patients die annually due to head and neck cancer. 3 Men are more frequently diagnosed with head and neck cancer. The incidence of head and neck cancer increases with age. Head and neck cancer is frequently located in the oral cavity, nasopharynx, oropharynx, hypopharynx and larynx (figure 1). Less frequent locations are the paranasal sinuses and salivary glands. Figure 1. Major anatomical sites of squamous cell carcinoma of the head and neck. The inset shows the typical histological features of squamous cell carcinoma of the head and neck. 4