Five said they offer it, with a variety of provisos, offering comments like, “If I was a patient, I want my doc to do everything.”

Dr. Daniel George, at Duke University, said he does not offer immunotherapy to every man who is dying of prostate cancer. But, he said, “for those patients who want to do everything they possibly can, that’s the group where we try checkpoint inhibitors,” a type of immunotherapy.

To the others — the majority of his patients with metastatic prostate cancer — he does not mention immunotherapy.

“We have to balance between hope and reality,” he said. “The most difficult conversation we have with patients is when we have to tell them that more treatment is actually hurting them more than the cancer.”

Dr. Daniel Petrylak, a prostate cancer specialist at Yale, said his inclination was to offer immunotherapy only to those rare patients whose tumors have a genetic marker indicating the immune system is trying to attack — already an approved indication for prostate cancer, he noted. But this strategy gives him a rationale for trying the drugs on patients with other cancers.

With the possibility of a dramatic and prolonged response, he said in an interview, “how can you ethically deny this to patients?”

At the Dana-Farber Cancer Institute in Boston, Dr. Christopher Sweeney said he petitions an insurance company to get an immunotherapy drug when the patient has a genetic marker predicting a possible response — an indicator the drug might work even if there is as yet no clinical trial evidence that it will — and is strong enough to tolerate the treatment.