Chief Justice William H. Rehnquist did not appear as planned at Supreme Court oral arguments yesterday, announcing that his pledge to do so after receiving a cancer-related tracheotomy 10 days ago was "too optimistic," and that he would remain at home while receiving radiation and chemotherapy treatment.

Rehnquist's first public comment since the court announced Oct. 25 that he had been diagnosed with thyroid cancer offered circumstantial evidence that he has the most serious form of the disease, several experts said yesterday.

"This adds up to something very bleak," said David Cooper, director of the Thyroid Clinic at the Johns Hopkins University School of Medicine. "That's very, very bad news."

The news about Rehnquist's condition came on the eve of a close election between President Bush and Sen. John F. Kerry (D-Mass.). If Rehnquist were to leave the court for any reason before Jan. 20, Bush could nominate a replacement no matter who wins today's election.

Meanwhile, the court faces a full caseload, and Rehnquist's absence from oral arguments was the first public confirmation that his illness had affected the functioning of the court.

Justice John Paul Stevens presided in the chief justice's absence, announcing that Rehnquist "reserved the right" to vote in the cases he missed based on the briefs and transcripts of the arguments.

Rehnquist, 80, said in his statement that he is working on court business at home, including some opinions in cases that have already been argued.

But Leonard Wartofsky, chairman of the department of medicine at the Washington Hospital Center, was skeptical that Rehnquist will be able to read, write and make decisions normally. "With those therapies, he is going to feel lousy," Wartofsky said. "His ability to eat, drink, speak and breathe are all in that area of the neck."

Supreme Court spokeswoman Kathy Arberg declined to comment.

A diagnosis of anaplastic thyroid cancer, the most serious of four forms of the disease, would account for Rehnquist's situation, several experts on thyroid cancer said yesterday. And no other diagnosis would adequately explain his doctors' treatment decisions, the experts said.

Rehnquist's doctors' approach, in sharp contrast to techniques usually used for milder cancers, suggested the physicians were throwing everything they had at the illness, the experts said.

"The growth rate of anaplastic leaves you a short window of time to operate, so you have to kill as many cancer cells [as possible] in a short time," said James Fagin, director of the division of endocrinology at the University of Cincinnati. "That's why you use combination treatment." Removing the thyroid -- a common treatment for patients with the other forms of thyroid cancer -- is not normally done in patients with this form because it spreads so rapidly.

"For anaplastic cancer, almost nothing works," Wartofsky said. "Radiation and chemotherapy are tried sometimes to slow the cancer and buy some time. . . . All thyroid cancers are a little more common in women than men, but men do worse."

None of the experts interviewed yesterday is part of Rehnquist's medical team. None has information beyond the facts that have been made available by the Supreme Court.

But Wartofsky, Cooper and Fagin said no other diagnosis could account for the known facts of Rehnquist's case. Steven I. Sherman, chair of the department of endocrine neoplasia at the University of Texas M.D. Anderson Cancer Center in Houston, said anaplastic cancer is "likely" but other explanations could not yet be conclusively ruled out.

Anaplastic thyroid cancer kills patients on average within six months. Looked at another way, 90 percent of patients with anaplastic thyroid cancer die within a year, Sherman said. Wartofsky said that in 35 years of treating thyroid cancer he had seen only two patients survive the anaplastic form.

"I have had patients where you can mark on the skin with a pen the edge of the tumor and watch it grow day by day -- it is that fast," Sherman said.

Wartofsky, Cooper and Fagin said the evidence pointed toward anaplastic thyroid cancer because of the clear differences between types of thyroid cancer and the differences in treating them.

Thyroid cancer comes in four forms, Wartofsky said. Papillary cancer accounts for about 80 percent of all thyroid cancers, but it is more common among younger people and is easily curable. Follicular thyroid cancer affects 10 to 12 percent of thyroid cancer patients and strikes patients in their twenties to their sixties. It is also highly curable. For both these cancers, doctors remove the thyroid gland and administer patients with radioactive iodine -- any remaining thyroid cells selectively absorb iodine, making the treatment highly effective.

Medullary thyroid cancer, the third form, has a strong genetic basis, Wartofsky said. Although it can affect patients of all ages, it accounts for only 3 to 4 percent of cases. The survival rate is around 50 percent, Wartofsky said. Surgical removal of the thyroid is often the first line of treatment. While doctors might use radiation for this cancer, the combination of radiation and chemotherapy pointed to anaplastic thyroid cancer, Fagin said.

Anaplastic thyroid cancer primarily strikes the elderly, Wartofsky said, and rapidly affects other tissues. Patients often come into treatment complaining of hoarseness -- as Rehnquist did -- after the cancer attacks nerves that control the vocal cords. It also can affect the windpipe, making it necessary for doctors to open a new airway. The cancer's rapid spread to other tissues renders both surgery and radioactive iodine less effective.

The fact that Rehnquist has had a tracheotomy, without any official confirmation that doctors removed any of his thyroid, is strongly suggestive that he has anaplastic cancer, outside medical experts said.

"To suddenly go to a tracheotomy without a thyroidectomy would most likely be anaplastic cancer," said Cooper, who also directs the division of endocrinology at Sinai Hospital of Baltimore.

Even if doctors had removed Rehnquist's thyroid without announcing it, they would be unlikely to start radiation treatment so soon, Cooper said. Besides, Rehnquist had initially announced he would be back at work yesterday, and Cooper said a thyroidectomy would have ruled out such a quick return.

One other rare cancer that usually affects the lymph nodes could theoretically explain Rehnquist's symptoms, Cooper said. But although it would explain why doctors did not remove Rehnquist's thyroid, Fagin said, "thyroid lymphoma does not fit the bill because radiation therapy would not be part of the picture."

Chief Justice William H. Rehnquist is being treated with chemotherapy and radiation.