But the drugs have run into an obstacle that so far has been impassable. Because they are radioactive, they are almost always administered in hospitals, not doctors’ offices. As a result, doctors are not paid by Medicare and private insurers for prescribing them, as they are when they give patients a more common treatment, chemotherapy.

In addition, most oncologists outside academic hospitals treat many different cancers and may be only vaguely familiar with the drugs, said Dr. Andrew D. Zelenetz, chief of the lymphoma service at Memorial Sloan-Kettering Cancer Center. “There are a number of barriers,” Dr. Zelenetz said.

Dr. Press and Dr. Zelenetz acknowledge that they have their own financial incentives to support the drugs. Dr. Press has been paid to speak at medical education seminars sponsored by the makers of the drugs. Dr. Zelenetz has been paid when the companies sponsor clinical trials at Memorial Sloan-Kettering. But both said the money was a small part of their total income and had not colored their views.

Some patients say they would not have received Bexxar and Zevalin if they had not demanded them. Mr. Wheeler of Kalamazoo, Mich., said he received Bexxar in April 2004 only after insisting on it when his lymphoma recurred. “I told my local oncologist, I want Bexxar, you give me a referral,” Mr. Wheeler said. “I’ve been a real pain.”

Mr. Wheeler, whose lymphoma was diagnosed in 2000 and recurred in 2003, has been cancer-free since receiving Bexxar. His cancer was growing when he received the infusion. He thinks he would be dead by now if he had not received the drug.

Ms. Stephens feels similarly. She was diagnosed with lymphoma in December 1998, and chemotherapy proved both difficult and ineffective. By August 1999, her disease was spreading. “Every lymph node in my body was involved,” she said. She received Bexxar as part of a clinical trial in January 2000 and quickly began gaining strength. She has remained in remission since, she said.

Zevalin and Bexxar are the first in a new class of drugs called radioimmunotherapies. Essentially, they deliver radioactive particles directly to cancerous cells to kill them. Idec, now part of Biogen Idec, invented Zevalin. Corixa, a Seattle company bought by GlaxoSmithKline, developed Bexxar. Both drugs are very expensive, costing about $25,000 per treatment. But one dose is usually enough. The cost of the drugs is similar to a full four-month regimen of chemotherapy and Rituxan, another lymphoma treatment.