WASHINGTON - U.S. Sen. Craig Thomas has been diagnosed with acute myeloid leukemia, an aggressive form of cancer of the blood and bone marrow.

Thomas, 73, who won re-election Tuesday to a six-year Senate term, has already begun treatment and will remain in the hospital for at least a month.

In a statement, the Republican said he's strong and ready to take on this challenge with grit and determination.

"I am joining all of the thousands and thousands of other cancer survivors in Wyoming," he said in a statement. "I certainly didn't expect this diagnosis, but I will be back by the January session ready for full service in the new Congress. Susan and I thank you for all of your good wishes."

The senator experienced a loss of energy in the last few weeks, his office said. He became light-headed while attending First United Methodist Church in Casper on Sunday morning and was taken by ambulance to Wyoming Medical Center.

He underwent tests while there that revealed pneumonia and showed a low white cell count in his blood but did not explain the source of the blood cell anomaly, his office said.

Thomas and his wife, Susan, flew on Sunday night back to Washington, where he expected to participate in votes and party leadership elections next week. He received a complete workup of tests, including a bone marrow biopsy, on Monday and Tuesday at Bethesda National Naval Medical Center.

Thomas received the results of the biopsy and the AML diagnosis on Tuesday, the day he won re-election, said his spokesman, Cameron Hardy. Doctors recommended that he begin treatment immediately.

Thomas told his family of the diagnosis on Wednesday, his office said. After a call with his staff Thursday morning, he made a public announcement.

Doctors have placed Thomas on induction chemotherapy, his office said. The treatments will occur over a four- to six-week period, during which the senator will remain in the Bethesda hospital in order to lower the risk of infection given his low white cell blood count, they said.

The goal of the chemotherapy is to bring the disease into remission. It will be followed by consolidation chemotherapy to kill any remaining leukemia cells, his office said.

"It is expected that Thomas will be able to perform all duties of his office following treatment," the statement said.

It also quoted Dr. Brian Monahan, director of hematology and medical oncology at the National Naval Medical Center, as saying that "the majority of people with Craig's great physical condition and general health attain remission."

There are four main types of leukemia, explained Rhonda Miller, information specialist with The Leukemia & Lymphoma Society. AML is the most common type of leukemia in adults.

The two acute types of leukemia are "very aggressive diseases," and in almost every case chemotherapy must be started immediately, she said. People of any age can get leukemia, but it's more common in older adults, she noted.

She said that younger patients can tolerate a stem cell transplant, which provides one of the best chances of a lasting remission, but that the survival rate declines with age.

From 1995 to 2002, the five-year relative survival rate for male AML patients ages 65 and up was a little under 4 percent, she said.

Miller said the treatment options and the effects it has on quality of life vary for every leukemia patient depending on what regimen the patient undergoes.

"It can go from good enough to be able to work and exercise and live a fairly normal life, to being really disabled by the disease," she said.

The cause of the disease is unknown, she said, although risk factors can possibly include very high doses of radiation, exposure to the chemical benzene, or having had treatment for another cancer years ago.

She said the total estimated number of new cases of AML for 2006 in the nation is 11,930.

Advances in the treatment of AML have resulted in substantially improved complete remission rates, according to the National Cancer Institute.

About 60 to 70 percent of adults with AML can be expected to attain complete remission following appropriate induction therapy, according to the institute's Web site. More than 15 percent of adults with AML, or about 25 percent of those who attain complete remission, can be expected to survive three or more years and may be cured, it said.

Remission rates in adult AML are inversely related to age, the institute said.

Sen. Mike Enzi, R-Wyo., whose wife, Diana, was diagnosed with colon cancer in May, asked people to send their thoughts and prayers to Thomas and his wife.

"Our recent experiences with cancer have added to our understanding," Enzi said in a statement. "There is not a word that is scarier to hear than 'cancer.' We need to remember, however, how far we've come in cancer treatment, that they found it early and that Craig is in outstanding shape."

Enzi said the Wyoming delegation will continue to work as a team come January. "When Craig wants to get something done, he gets it done," he said.

Thomas's former chief of staff, Liz Brimmer, said she had spoken with him Thursday.

"He sounded amazing - strong, even a little feisty about all the attention," she said. "What I know about Craig is the same as I've known since I first started working for him 17 years ago: No one is tougher than Craig."

Like others, she said his fighting spirit would carry him through.

"This isn't Craig's first rodeo," she said. "He's going to fight this cancer with the same determination he's had in every day. And I have no doubt that when he returns in January to be sworn into the new Congress, he'll be the same great senator for Wyoming he always has been."

Rep. Barbara Cubin, R-Wyo., said she and her husband will be praying for Thomas's full recovery.

"Craig, Susan and their family have my full support as they face this challenge," she said. "Anybody who knows Craig Thomas knows he is tough, resilient, and will soon be back working for Wyoming in the U.S. Senate."

Wyoming Gov. Dave Freudenthal also made a statement of support.

"Nancy and I - and undoubtedly thousands across Wyoming - were saddened to hear of Sen. Thomas' ill health," he said. "Our every thought and prayer is with Craig, Susan, their family and their many, many friends and supporters at this difficult time. We have absolute hope and confidence that Craig will make a full recovery."

Should any Wyoming senator have to give up his seat in Congress, state law guarantees the seat stays in his party at least until the next general election.

In case of a vacancy, the governor must notify that party's central committee, which then has to meet within 15 days to choose three people qualified to fill the vacancy. The governor then has five days to pick one of the three to temporarily fill the spot.

At the next general election - held in November every even-numbered year - a successor would be elected to fill out the remainder of the Senate term.

Star-Tribune reporter Tom Morton contributed to this report.