Head and neck cancer is a term used to describe a number of different malignant tumors that can develop in or around the throat, larynx (voice box), nose, sinuses, and mouth. Most head and neck cancers are squamous cell carcinomas, meaning they begin in the flat, squamous cells that make up the thin surface layer (called the epithelium) of the structures in the head and neck. Directly beneath this lining, some areas of the head and neck have a layer of moist tissue, called the mucosa. If a cancer is limited to the squamous layer of cells, it is called carcinoma in situ. If the cancer has grown beyond this cell layer and moved into the deeper tissue, then it is called invasive squamous cell carcinoma. If the cancer starts in the salivary glands, the tumor will usually be classified as an adenocarcinoma, adenoid cystic carcinoma, or mucoepidermoid carcinoma. Mouth cancer can develop on the lip, the tongue, the floor of the mouth (under the tongue), the inside of the cheek, the roof of the mouth (the hard palate), the area behind the wisdom teeth or the gum.