Frank R. Lautenberg of New Jersey, the second-oldest member of the United States Senate, has a curable form of stomach cancer and will undergo chemotherapy, his office and doctor said on Friday.
His doctor said he expected the senator, who turned 86 in January, to be back at work between treatments, which could last four to six months.
Any deterioration in Mr. Lautenberg’s prognosis would be of grave concern to Democrats in the Senate, who have lost their veto-proof majority and whose 59-to-41 margin is threatened by a raft of unexpected retirements and stiff election challenges.
Gov. Christopher J. Christie of New Jersey, a Republican, would choose a new senator if Mr. Lautenberg were unable to complete his term.
Mr. Lautenberg has said he intends to run for re-election in 2014. But his advanced age and Mr. Christie’s election had prompted an effort by Democrats to pass legislation that would have prevented the governor from appointing a Republican to the Senate — whether by specifically requiring him to choose a Democrat or by keeping the seat vacant until a special election could be held.
That attempt failed in early January after Democrats could not agree on the details. Mr. Christie denounced it as a “political power play by the party that’s losing power.”
On Friday, Mr. Christie called Mr. Lautenberg “a tenacious fighter” and wished him a speedy recovery.
Among current senators, only Robert C. Byrd of West Virginia, 92, is older than Mr. Lautenberg. Representative Ralph M. Hall of Texas, who turns 87 in May, is the oldest House member.
When Mr. Lautenberg was admitted to the hospital on Monday, his office said that he had grown lightheaded and had fallen at his condominium in Cliffside Park, N.J., across the Hudson from Manhattan.
Dr. James F. Holland, an internationally known oncologist at Mount Sinai Medical Center who is treating the senator there, said in an interview on Friday that Mr. Lautenberg’s stomach had ulcerated, causing “massive” gastrointestinal bleeding that required “several transfusions.”
He described Mr. Lautenberg’s cancer as a diffuse large B-cell lymphoma, a type of non-Hodgkin’s lymphoma. He said a PET scan showed the lymphoma was localized to one area of the stomach. “There was nothing in nodes, spleen or liver,” he added.
Dr. Holland said that he expected Mr. Lautenberg to make a “full and complete recovery” and that the senator should be back at work between chemotherapy treatments.
“I anticipate he’ll be back in the Senate very quickly,” Dr. Holland said. “He’s a vigorous man.”
Dr. Holland said Mr. Lautenberg would receive the standard chemotherapy regimen for his cancer, known as R-CHOP, a combination of rituximab, cyclophosphamide, doxorubicin, vincristine and prednisolone. The treatment would take a few hours for intravenous injections the first day of each cycle, he said, but should not hinder him from returning to work thereafter. Side effects could include hair loss and a reduced white blood cell count.
No decision has been made about whether the senator will be treated in New York or Washington, the doctor said.
Non-Hodgkin’s lymphoma afflicts more than 65,000 people a year, and kills more than 19,000, according to the National Cancer Institute. But Dr. Holland said that R-CHOP had been “highly successful in treating large cell lymphomas,” and added that he expected Mr. Lautenberg’s cancer to “go away quickly.”
A former chairman and chief executive of Automatic Data Processing, the payroll-processing company, Mr. Lautenberg, a multimillionaire, was elected to the Senate in 1982. He retired in 2000, saying that he wanted to devote more time, among other things, to the Lautenberg Center for General and Tumor Immunology, a research and instruction unit in immunological science that he founded at the Hebrew University-Hadassah Medical School in Jerusalem. His father, grandfather and uncle all died of cancer.
In 2002, he became a candidate again, and easily won, after the incumbent, Senator Robert G. Torricelli, withdrew from his re-election campaign amid a federal corruption investigation.
Despite his age, Mr. Lautenberg has taken pride in his busy schedule. He visited Haiti on Feb. 12, three days before being hospitalized. And on Thursday, while he was undergoing tests, his office released a letter he signed pushing the Senate majority leader, Harry Reid, to include more money for public works in a forthcoming jobs bill.