Introduction
The term “cancer of the head and neck” describes a diverse collection of cancers of varying histologies arising from a variety of anatomic sites that make up the UADT. This section, however, will deal with squamous cell carcinoma of the UADT. More than 90% of head and neck cancers of the upper aerodigestive tract are SSCs. The UADT consists of a complex mucosa-covered conduit for food and air that extends from the vermilion surface of the lips to the cervical esophagus. In common usage, this terminology has been applied primarily to those cancers arising from the mucosal surfaces of the lips, oral cavity, pharynx, larynx, and cervical esophagus. Included in this designation, however, are other important sites, such as the nose and paranasal sinuses, salivary glands (major and minor), thyroid and parathyroid, and skin (melanoma and nonmelanoma skin cancers). Some cancers arising in this region are typically excluded from the generic designation of head and neck cancer. Examples are tumors of the central nervous system, ocular neoplasms, primary tumors of lymphatic origin, and neural and endocrine malignancies.

Because of the diversity of sites and tissues of origin, the biology of tumor growth, patterns of metastases, natural boundaries for tumor extension, and signs and symptoms of disease are quite varied. The anatomy of the region has also dictated that optimal evaluation, diagnosis, and treatment require specific multidisciplinary expertise that frequently crosses traditional training backgrounds to include neurosurgery, otolaryngology, head and neck surgery, oral surgery, cosmetic and reconstructive disciplines, and specialized radiology, pathology, radiation therapy, and chemotherapy.

The clinical manifestations of disease are varied and have a significant impact on the cosmetic and functional integrity of the head and neck region. Although the anatomic structures are only millimeters apart, the low metastatic potential and high curability of vocal cord cancers stand in extreme contrast to the early dissemination and grim prognosis of stage-matched pyriform sinus cancers. Clinical differences between cancers in different sites are not explained solely by anatomic factors but by major biologic differences. Regrettably, the relatively small number of head and neck cancer patients often requires grouping many different types in each HNSCC therapy trial. Associated morbidities of disease and treatment involve all of the special senses to varying degrees, notably speech, swallowing, smelling, breathing, and mastication functions critically important for social interaction, a good quality of life, and survival.