(USA TODAY) -- Actor Michael Douglas says he is upbeat in spite of a diagnosis of advanced throat cancer, which he discussed Tuesday on The Late Show With David Letterman.

Douglas, 65, told Letterman that his cancer is Stage IV but hasn't spread beyond the head and neck area. The cancer developed on the back of the tongue, said his spokesman, Allen Burry.

Douglas told Letterman he has just finished his first of eight weeks of combined radiation and chemotherapy treatments.

Doctors say the therapy is grueling. Many patients develop painful mouth sores that require morphine-like narcotic pain relievers, says Robert Haddad, an oncologist with the head and neck cancer program at Boston's Dana-Farber Cancer Institute.

Radiation also can burn the throat, which makes it painful to swallow. About half of patients require a feeding tube, says Haddad, who has no personal knowledge of Douglas' case.

Despite the side effects, Haddad says, Douglas' long-term quality of life "should be excellent."

Although the treatment is tough, it can cure 50% to 80% of patients, depending on the location and other details of the tumor, he says.

He notes that Stage IV throat cancer is different from other cancers. Typically, a Stage IV tumor is one that has spread to other vital organs and is usually incurable, Haddad says. Throat cancer is different, because a Stage IVa or Stage IVb cancer has spread only to the nearby lymph nodes. A Stage IVc throat cancer is one that has metastasized to other organs, Haddad says, and this most serious stage of cancer is generally incurable. Doctors typically don't subject patients with such widespread disease to chemo and radiation, because they can't cure the disease, he says.

Douglas told Letterman that he developed a sore throat in early summer and that he went through a "litany of doctors and tests" before being diagnosed with cancer.

Haddad says that's typical for many patients with throat cancers, whose most common symptom is a sore throat. Because a sore throat is commonly caused by a bacterial infection, patients are often treated with antibiotics. Patients who don't improve within three weeks or so are often referred to an ear, nose and throat specialist, who can examine the back of the tongue using a specialized camera on a thin tube threaded through the patient's nose, Haddad says. Doctors can then diagnose cancer with a biopsy.

Though sore throats are common, Haddad says, doctors treating smokers and ex-smokers are alert to the possibility of cancer. Letterman asked Douglas whether he had ever smoked, and Douglas said he smoked and drank (another risk factor), and "this particular type of cancer is caused by ... drinking."

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