**1. Introduction**

Head and Neck Cancer (HNC) is the seventh most common type of cancer worldwide [1]. The regions of HNC include cancers of the nasal cavity, oral cavity, nasopharynx, oropharynx, hypopharynx, larynx, and paranasal sinuses (see **Figure 1**). Risk factors are tobacco use, alcohol consumption [2], and viral infections with the Human Papilloma Virus (HPV) (for oropharyngeal cancers) [3] and Epstein-Barr Virus (EPV) (for nasopharyngeal cancers) [4].

#### **1.1 TNM classification**

HNC tumors can be classified using the TNM stage classification published by the American Joint Committee on Cancer and International Union for Cancer

**Figure 1.** *Regions of head and neck cancer, source: https://www.intechopen.com/chapters/67124*

Committee (AJCC/UICC) [5]. This classification is based on the anatomic tumor extent and includes three different aspects. Firstly, the size of the primary tumor (T), secondly the presence or absence and extent of involved regional lymph nodes (N), and lastly the presence or absence of distant metastasis (M). With the TNM classification, it is possible to give an estimate on cancer prognosis and it is helpful for treatment selection and proper communication. An example of a TNM classified advanced oropharynx carcinoma with one lymph node involved and diagnosed with no distant metastasis is T3N1M0.
