Washington ? Chief Justice William Rehnquist, ailing from thyroid cancer, defended lifetime appointments for judges as necessary to insulate them from pressures as they deal with politically sensitive issues.

Rehnquist used his year-end report today to address concerns about so-called activist judges and Congress’ move to strip judges of some of their authority.

The 80-year-old wrote the report from home, where he has been recovering since announcing in October that he has cancer.

Few details have been released about Rehnquist’s illness, except that it is being treated with chemotherapy and radiation, a combination commonly used for an aggressive type of cancer. Rehnquist missed arguments in about 25 Supreme Court cases during November and December, but has said that he plans to swear in President Bush on Jan. 20.

The chief justice mentioned his condition only briefly at the close of the 18-page report. “On a personal note, I also want to thank all of those who have sent their good wishes for my speedy recovery,” he said.

Rehnquist, who marks his 33rd anniversary on the court next week, said there had been “mounting criticism” recently of judges accused of interpreting the law to fit their politics.

Bush and Republican congressional leaders have been particularly outspoken about activist judges, especially those in gay marriage cases. But Democrats also have also accused conservative judges of stretching the law.

Rehnquist said judges should not be punished by Congress because of their decisions and that their lifetime tenure protected their independence.

“It is not a perfect system — vacancies do not occur on regular schedules, and judges do not always decide cases the way their appointers might have anticipated. But for over 200 years it has served our democracy well and ensured a commitment to the rule of law,” Rehnquist said.

Speculation has been rampant about when the Supreme Court will have its next vacancy. Justices typically retire during the court’s summer recess, but Rehnquist’s health could force him to step aside sooner.

Supreme Court justices are appointed for life by the president and confirmed by the Senate. The average age of the current nine justices is 70.