Douglas has said he visited doctors for months trying to determine the reason for his sore throat.
The worst survival rate is in the hypopharynx, which is a part of the throat area that is involved in the upper digestive tract. There are also three different subsites in the larynx.
Throat cancers are hard to diagnose early because the symptoms are "not specific,' according to Agrawal. "We all get sore throats and take antibiotics and there is some improvement. Patients ignore the symptoms and by the time it has developed in the lymph nodes it's stage three or four. That's when most are diagnosed."
"In general, oropharyngeal cancer is usually involved at the base of the tongue and the tonsils and can be associated with HPV, and even at stage four can have an 80 percent cure rate," he said.
The human papillomavirus, or HPV, is the most common sexually transmitted infection in the United States.
Recent studies have found connections between HPV and cancers of the mouth and throat, and recently with lung cancer. Other HPV-related cancers include vulva, vaginal, penile and anal.
HPV has long been known to result in cancers of the sex organs, particularly the cervix. Doctors say men are more prone to throat cancer after kissing or oral sex with an infected partner.
According to a National Institutes of Health website, use of tobacco or alcohol are among the factors that put people at risk of developing throat cancer. Combining tobacco and drinking increases the risk.
Douglas, 65, who has two children with Catherine Zeta-Jones, said he enjoyed the summer traveling with his family before returning to the doctor.
"The percentages are very good," he told Letterman, referring to survival rates. "I would hate to say, but right now, it looks like it should be 80 percent, and with certain hospitals and everything, it does improve."
A biopsy found he had late, stage-four cancer, "which is intense, and so they've had to go at it," he said.
Stage four means that there is lymph node involvement in the cancer. Unlike a breast cancer that has metastasized, throat cancer at this stage is not as widespread.
Cullen said that doctors likely did not operate because chemotherapy and radiation "work very well" with throat cancers. Removing a tumor at the base of the tongue could lead to speech and swallowing problems.
Others who have been diagnosed with throat cancer are writer Christopher Hitchens, who was diagnosed this summer, former Texas Gov. Ann Richards and the late actor Ron Silver.
Throat cancer forms in the tissues of the pharynx, the hollow tube inside the neck that starts behind the nose and ends at the top of the windpipe and esophagus. It can include cancer of the nasopharynx (the upper part of the throat behind the nose), the oropharynx (the middle part of the pharynx), and the hypopharynx (the bottom part of the pharynx). Cancer of the larynx (voice box) may also be included as a type of throat cancer.
Most throat cancers are squamous cell carcinomas.
According to the National Cancer Institute, there were 12,720 new cases of laryngeal and 12,660 of pharyngeal in 2010. The estimated annual deaths are 3,600 and 2,410, respectively.
Head and neck cancers account for approximately 3 to 5 percent of all cancers in the United States. These cancers are more common in men and in people over age 50.