Jennings, ABC's chief anchorman since 1983, planned to be at his desk Tuesday — a day after getting his diagnosis.
A former smoker who quit several years ago, the 66-year-old anchor was too ill to work Saturday during the network's special report on Pope John Paul II's death. He hasn't been feeling well the past few months, and didn't travel under doctor's orders after December's tsunami because of what was described then as an upper respiratory infection. He did go to Iraq in January for the elections.
Lung cancer is the leading cancer killer in the United States, and roughly four out of five people diagnosed with the disease die within five years, said Dr. Cliff Connery, chief of thoracic surgery at St. Luke's-Roosevelt Hospital in Manhattan.
"Overall, lung cancer is the number one mortality for cancer-related deaths in this country, even combining breast, colon and prostate together doesn't add up to lung cancer," Dr. Robert Ashton with St Luke's Roosevelt Medical Center tells CBS News Correspondent Elizabeth Kaledin.
Despite the high mortality rate associated with lung cancer, Kaledin reports, the key to fighting it is catching it as early as possible. If it is found early, the cure rate jumps to 50 percent.
Doctors said most lung cancer patients can continue to work throughout treatment, but need flexibility to take it easy on days they are not feeling well.
With his very visible position on television each night, Jennings could be an inspiration for many Americans going through a similar fight, said Dr. David Johnson, chief of oncology and hematology at the Vanderbilt University School of Medicine.
"I think it sets the right example," Johnson said. "I think it says you shouldn't stop your life if you have cancer. It may take your life, but you shouldn't let it control your life."
There are effective ways to treat lung cancer, but its mortality rate is so high because so many patients aren't diagnosed until their disease is in an advanced stage, Connery said.
Charles Gibson, Elizabeth Vargas and others will substitute for Jennings from time to time, said ABC News President David Westin. Gibson is in Rome for coverage of the pope's funeral, an assignment that Jennings, a former Rome and London correspondent for ABC News, normally would have taken.
"He's already bringing to this new challenge the courage and strength we've seen so often in his reporting from the field and in anchoring ABC News," Westin told ABC staffers. "I know that all of us will give him every bit of support that he needs and asks for."
Jennings is the last of the anchor troika that dominated broadcast network news divisions over the last two decades. NBC's Tom Brokaw stepped down last year and CBS' Dan Rather left last month.
"Peter is an old friend," Brokaw said Tuesday. "I'm heartbroken, but he's also a tough guy. I'm counting on him getting through this very difficult passage."
While still in his 20s, Jennings anchored ABC's evening news for two years in the 1960s. He returned to the desk in 1978 when third-place ABC tried a multi-anchor format, which was abandoned in 1983 when Frank Reynolds died from cancer. Jennings has been ABC's sole evening anchor ever since.
A Canadian who proudly became a U.S. citizen in 2003, the urbane Jennings dominated the ratings from the late 1980s to the mid-'90s, when Brokaw surpassed him.
"Jennings has served American broadcast news with a graceful internationalist intelligence," said Rich Hanley, director of graduate programs at Quinnipiac University's school of communications. "His manner of news delivery stood in contrast to but not in opposition of the gentle Midwestern aesthetic expressed by Tom Brokaw and the don't-mess-with-Texas eccentricity of Dan Rather."
ABC is part of the media and entertainment company The Walt Disney Co.
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