The fact that Mayor Bob O'Connor had to have fluid drained from his brain yesterday does not necessarily mean that his cancer has worsened, a cancer specialist said last night.

The treatment he is receiving, even if effective, could cause inflammation of brain tissue that could block tiny channels in the brain that carry cerebrospinal fluid, resulting in a fluid buildup that needed to be removed, said Dr. Estil Vance, a specialist in oncology, infectious disease and medicine at the Baylor University Medical Center in Dallas.

Yesterday, the mayor's office and UPMC Shadyside, where Mr. O'Connor is being treated, issued a statement saying that the mayor underwent a 30-minute surgical procedure to drain fluid buildup from his brain.

The statement said his condition remains stable and that his medical team is performing tests to determine the cause of the buildup.

Dr. Vance emphasized that he does not have direct knowledge of the mayor's condition or treatment.

But he said it is possible for patients with primary central nervous system lymphoma, the type of cancer the mayor is battling, to develop a blockage in the brain in response to treatment that could cause a fluid buildup.

While it could signal that the cancer has worsened, the buildup is "not necessarily the result of the tumors advancing," he said.

It is possible, he said, "that the cancer could be improving and he could still have a complication like this."

The brain is bathed in cerebrospinal fluid that escapes through absorption, he said, and also flows toward the back of the brain and out the base of the spine through small spaces no wider than a few millimeters.

Tumors, prior surgery or other factors, including inflammation resulting from treatment, can block those spaces and cause fluid to build in the brain, a condition known as obstructive hydrocephalus, he said.

A buildup is also possible if an outright blockage does not exist, but fluid is not being absorbed normally, he said.

Chemotherapy can irritate tissues and cause a blockage, Dr. Vance said. And tumors can swell and block the flow even if treatment is progressing.

Tests that can be performed in response, he said, could include spinal fluid analysis, which could show evidence of inflammation or tumors involving the spinal fluid, or brain scans that could indicate blockages.

Tubes known as shunts can be inserted surgically or holes opened in areas of the brain to drain excess fluid.

Dr. Vance said the buildup and surgery to relieve it is not necessarily a serious complication.

Doctors at UPMC are expected to provide more details on the mayor's condition and treatment at a news briefing tentatively scheduled for early this afternoon, said spokeswoman Clare Collins.

A brief written update also is planned later this morning, she said.

Since his cancer was diagnosed July 10, the mayor has been undergoing treatment with the chemotherapy drug methotrexate, which doctors have said has been effective in treating his cancer. This month, doctors also have added another chemotherapy agent, Temodar.

He also has had physical rehabilitation and has been treated with other drugs, including a steroid to reduce his brain swelling and the drug leucovorin to counteract potential side effects.

In news conferences, doctors have detailed his diagnosis, treatment and symptoms, from hiccups to extreme fatigue. They also have been overly optimistic about when he might be discharged, most recently suggesting that he could have been released to a rehabilitation facility a week ago.