NEW YORK — More than a dozen rectal cancer patients in the United States have seen their cancer disappear after undergoing experimental immunotherapy, in what doctors are calling an astonishing result.

The patients, who were part of a small clinical trial led by researchers from New York’s Memorial Sloan Kettering (MSK) Cancer Center, saw their tumors vanish after being treated with an experimental drug called dostarlimab.

Details of the trial were published on Sunday in the New England Journal of Medicine.

The paper described the results of 12 patients with rectal cancer, all of whom saw their cancer vanish after treatment with dostarlimab.

Participants received a dose of dostarlimab every three weeks for six months, with the idea being that they would need to undergo standard treatments of chemotherapy, radiation therapy and surgery following treatment.

However, researchers found that in every case, the cancer was cleared through the experimental treatment alone.

The trial has been hailed as a first in cancer treatment, with one of the paper’s authors, Dr. Luis Diaz Jr. of Memorial Sloan Kettering, telling the New York Times that 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,” he said.

Immunotherapy harnesses the body’s own immune system to identify and destroy cancer cells.

The trial focused on a subset of rectal cancer patients whose cancer had a specific mutation, MSK said in a statement.

This sort of rectal cancer, known as "mismatch repair-deficient" (MMRd) rectal cancer, tends to respond poorly to standard chemotherapy regimens. In the trial, researchers wanted to investigate if immunotherapy alone could beat rectal cancer that had not spread to other tissues, the organization said.

The research, which is ongoing, has seen at least 14 patients “and counting” have their tumors disappear, with none of them experiencing significant side effects, it added.

There was no need for standard treatments of radiation, surgery, or chemotherapy, and the cancer has not returned in any of the patients, who have been cancer-free for up to two years, it said.

“It’s incredibly rewarding to get these happy tears and happy emails from the patients in this study, who finish treatment and realize, ‘Oh my God, I get to keep all my normal body functions that I feared I might lose to radiation or surgery’,” said Dr. Andrea Cercek of Memorial Sloan Kettering, who co-led the trial.

The results have provided “what may be an early glimpse of a revolutionary treatment shift”, Dr. Hanna Sanoff, an oncologist at the Lineberger Comprehensive Cancer Center at the University of North Carolina, who was not involved in the trial, wrote in an editorial accompanying the paper.

However, she added that although the results are “cause for great optimism”, such an approach “cannot yet supplant our current curative treatment approach”.

“Whether the results of this small study conducted at Memorial Sloan Kettering Cancer Center will be generalizable to a broader population of patients with rectal cancer is also not known,” she said.

“In order to provide more information regarding which patients might benefit from immunotherapy, subsequent trials should aim for heterogeneity in age, coexisting conditions, and tumor bulk”.

The clinical trial is continuing to enroll patients and is growing, the MSK researchers said. They are also investigating to see if the same method can beat other cancers, and are looking at patients with gastric (stomach), prostate, and pancreatic cancers. — Euronews