It was a small trial, just 18 rectal cancer patients, every one of whom took the same drug.

But the results were astonishing. The cancer vanished in every single patient, undetectable by physical exam, endoscopy, PET scans or M.R.I. scans.

Dr. Luis A. Diaz Jr. of Memorial Sloan Kettering Cancer Center, an author of a paper  published Sunday in the New England Journal of Medicine describing the results, which were sponsored by the drug company GlaxoSmithKline, said he knew of no other study in which a treatment completely obliterated a cancer in every patient.

“I believe this is the first time this has happened in the history of cancer,” Dr. Diaz said.

Dr. Alan P. Venook, a colorectal cancer specialist at the University of California, San Francisco, who was not involved with the study, said he also thought this was a first.

A complete remission in every single patient is “unheard-of,” he said.

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