Nov. 1 (Bloomberg) -- U.S. Chief Justice William H. Rehnquist said he is receiving radiation and chemotherapy for his thyroid cancer and didn't return to the Supreme Court today as planned.

``According to my doctors, my plan to return to the office today was too optimistic,'' Rehnquist, 80, said in a statement released by the court. ``While at home, I am working on court matters, including opinions for cases already argued. I am, and will continue to be, in close contact with my colleagues, my law clerks and members of the Supreme Court staff.''

The announcement suggests that Rehnquist is suffering from anaplastic thyroid cancer, a rare and aggressive form of the disease, said Herman Kattlove, an oncologist and medical editor for the American Cancer Society. The anaplastic variety is the only type of thyroid cancer that is treated with chemotherapy.

``It's not treatable by surgery, only by chemotherapy and radiation,'' said Kattlove in a telephone interview. ``It's rarely, if ever, curable, and most patients die within a few months.''

Justice John Paul Stevens, the senior associate justice, presided over today's court session. Stevens began by saying that Rehnquist was ``unable to be present this morning but has reserved the right'' to vote on the cases being argued.

The court has released no other details about Rehnquist's treatment or his prognosis for recovery. Supreme Court spokeswoman Kathy Arberg wouldn't elaborate on the statement.

The court is scheduled to hear arguments again tomorrow and Wednesday.

Tracheotomy Performed

Rehnquist's illness, in the final days before tomorrow's presidential election, renewed interest in the future composition of the high court. Four justices are in their 70s or 80s, and the court hasn't had a vacancy in 10 years, the longest period of continuity since 1823.

On Oct. 25 the court said Rehnquist underwent a tracheotomy two days earlier at the National Naval Center in Bethesda, Maryland, ``in connection with a recent diagnosis of thyroid cancer.'' He was released from the hospital Friday, and the court had said Rehnquist was expected to be on the bench today for the next public session.

The tracheotomy administered to Rehnquist is an additional indication of anaplastic cancer, Kattlove said.

``The cancer was invading his trachea and impeding his breathing,'' Kattlove said. ``I suspect the tracheotomy will be there forever.''

Chemotherapy is used to try to slow the tumor growth and typically isn't very effective, Kattlove said.

Rehnquist was appointed to the high court by President Richard Nixon in 1972 and elevated to chief justice by President Ronald Reagan in 1986.

To contact the reporter on this story: Laurie Asseo in Washington at lasseo1@bloomberg.net.

Last Updated: November 1, 2004 12:50 EST